After treatment with forskolin and epinephrine, adipocytes were washed with phosphate-buffered saline PBS and lysed using mammalian protein extraction reagent containing a protease inhibitor cocktail.

Protein concentration was measured using a Coomassie Plus assay kit. Blots were washed 3 times with TBS and then treated with ECL Plus western blotting substrate. Finally, the blots were exposed to X-ray films and developed.

Expression levels were quantified and normalized to GAPDH. Changes with respect to the control expression were computed and the normalized fold change in expression was compared to the change of fluorescence lifetime. Oxygen consumption rate and extracellular acidification rate were determined in murine adipocytes, using a XF24 analyzer Seahorse Bioscience.

At day 6 of differentiation, cells were plated into V7-PS plates coated with collagen at 20, cells per well. Growth medium was replaced before treatment for both control and experimental groups.

The same mixing and measurement protocol was used for the glycolysis stress test to measure glycolytic function. Injections consisted of glucose, oligomycin and 2-deoxy-D-glucose 2-DG.

The SRS detector was placed downstream of the sample in the trans-direction. The PMT gain and laser power were kept constant and were measured for each image. Fluorescence images of the mitochondria stains were performed on living cells using an inverted confocal microscope FV, Olympus.

Correlations between cytoplasmic and mitochondrial compartment images determined with FLIM, and mitochondrial stain images were calculated using the CORR2 function in MATLAB which implements the Pearson correlation to 2-D arrays For 3T3-L1 preadipocytes FLIM data was compared to OCR and ECAR data after applying mitochondria and glycolysis stress tests.

Cell culture dishes that were imaged were treated with the same stress test protocol as cell cultures that were analyzed by the extracellular flux analyzer. The fluorescence lifetime decay was spatially binned over the pixel of interest and its nearest neighboring pixels, which was then individually fitted with a double-exponential model.

Spatial binning of the fluorescence decay defines how many immediate adjacent pixels are combined before the lifetime is calculated. This results in an increased accuracy of fluorescence lifetime decay times at the cost of decreased spatial resolution.

Fluorescence lifetime analysis was performed using commercially available software SPCImage, v5. where α i and τ i are the relative amplitude and lifetime of the i th fluorescence component, respectively.

The relative concentration of NADH associated with a given lifetime is directly related to the fluorescence decay coefficients α i for a given pixel We assume that NADH is the only fluorophore contributing to the signal, where NADPH fluorescence can safely be neglected, as explained earlier Relative fluorescence contributions were calculated for each FLIM image to produce the average value and standard deviations of specific parameters such as lifetime components, corresponding coefficients and their ratios.

To reduce background noise and disturbing factors, only pixels with a total photon count higher than and χ 2 values between 0. Fluorescence lifetime histograms based on intensity were fit with a series of Gaussian functions suitable to the number of peaks required to acquire a satisfying fit:.

where a is the height of the i th lifetime component peak, b is the position of the center of the peak and c controls the width. The area of each individual Gaussian function was computed and were used to determine individual lifetime components and ratios, and the peak positions were utilized to define the mean lifetime for each of the NADH pools.

test and lm function in R, respectively. The phasor plot is a graphical and fit-free representation of raw FLIM data. Phasor plots were generated by a custom designed software developed in Matlab based on the source code of Lanker et al.

The intensity I t of a time-resolved fluorescence decay recorded at each pixel location can be plotted as a single point in the phasor plot by applying the sine and cosine transforms to the measured decay data.

This is equivalent to the real and imaginary components of the Fourier transform of the decay data:. The universal semicircle is a lifetime ruler in the phasor plot, where the lifetimes increase counterclockwise from right 1,0 lifetimes approaching zero to left 0,0 lifetimes approaching infinity.

A phasor population falling on the semicircle indicates a mono-exponential decay, while multi-exponential decays fill the area inside the semicircle as linear combinations of their contributing mono-exponential components.

Principal component analysis was used to create fitted linear functions on the universal circle to determine fluorescence lifetimes. As shown in Fig. Fluorescein fluorescence was used as a reference to correct for the instrumentation response function Qian, S.

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Then, the mean weight of the visceral WAT tissue was divided by the calculated adipocyte weight. After termination of the mice organs were collected and cleaned from surrounding fat or connective tissue and their weight was determined using an analytical balance.

Tissue pieces were homogenized using the Precellys 24 system Peqlab in the presence of 1 ml QIAzol reagent QUIAGEN. The RNA was isolated using the RNeasy Lipid Tissue Mini kit QUIAGEN according to the protocol of the manufacturer, incubated with RQ1 RNase-free DNase Promega for 30 min at 37 °C and purified further using the RNeasy Plus Mini kit QUIAGEN starting from step 4.

One μg RNA was reverse transcribed into cDNA with Oligo d T primers using the Transcriptor First Strand cDNA Synthesis kit Roche. Tissues were lysed in RIPA buffer as described in Müller et al. Equal amounts of protein were separated by SDS-PAGE and transferred to a PDVF membrane.

For detection, Lightning Plus ECL reagent Perkin Elmer or ECL prime reagent GE Healthcare was used. For re-probing, the membranes were incubated for 15 min with Restore Western Blot Stripping buffer Thermo Fisher. All graphs show average ± standard error of the mean SEM. Single mice were excluded when results indicated technical failure of the experimental performance.

Furthermore, extreme outliers were excluded from the analysis. In the interests of transparency, eLife publishes the most substantive revision requests and the accompanying author responses. Your article has been reviewed by 2 peer reviewers, and the evaluation has been overseen by a Reviewing Editor and Matt Kaeberlein as the Senior Editor.

The reviewers have opted to remain anonymous. The reviewers have discussed the reviews with one another and the Reviewing Editor has drafted this decision to help you prepare a revised submission. However, all of the reviewers shared the major concern that it appears that only male mice were studied here, and that this fact — or the rationale for using only male mice — was not clearly articulated within the manuscript.

This makes interpretation quite challenging, especially given that the authors previously published that lifespan extension in the uORF KO mice is much more pronounced in female compared to male mice. There was consensus that this is a substantial weakness to the current manuscript which limits its overall impact.

It's possible the authors already have this data, and we would need to see inclusion of data supporting similar outcomes for the key experiments in female mice to recommend publication in eLife.

If the outcomes are different in males and females, this is likely quite interesting and would need to be developed further. The other significant concern was related to the RT-qPCR data, which is indicative but not conclusive support for the authors' conclusions, especially since many of the changes are small in magnitude.

It was noted that most of the relevant proteins have ELISAs available, and they all have antibodies which could be used to support the robustness and importance of the small but plausibly important differences observed. IHC against CD68 in the fat depots could be performed and the authors could strengthen their claims about adipose tissue inflammation by measuring the expression levels of inflammatory cytokines in adipose depots.

We now included data of female mice complementary to most of the originally performed experiments for males. Bar graphs: in the various figures we now grouped genotypes instead of diet type for — in our opinion — easier assessment.

The body weight of female mice was obtained at the end of the experiment upon termination of the mice Figure 1E. Due to our move to a different institute, we could not perform body composition analysis of females by micro-CT as we did with males and therefore used the weights of visceral and subcutaneous fat obtained from isolated fat tissue from terminated mice at the end of the experiment Figure 1F, G.

We could not include results of food intake and energy efficiency from female mice. Figure 2: panel A shows male data from previous Figure 1E and panel B show new data from females. Figure 3: Panel A was shown in previous Figure 1F.

The panels B and C shows new data for males. IHC against CD68 in the visceral fat depot were performed that strengthen the claim about macrophage infiltration in the visceral adipose tissue B. In addition, we included qPCR analyses of the inflammatory cytokines TNFα, MCP1, IL-1β an IL-6 in visceral fat from males C.

Figure 4: Shows new data from females complementary to the male data in Figure 3. Also here, qPCR analysis of CD68 expression C , IHC against CD68 B and qPCR analyses of the inflammatory cytokines TNFα, MCP1, IL-1β an IL-6 in the visceral fat depot were performed.

Figure 5: Panel A and C show male data previously shown in Figure 2A and figure 2 supplement 1A. Panels B and D show new data for females. The lipid staining in livers from female was performed using Oil-Red-O Figure 5B instead of Sudan III that was used for males.

Figure 5 supplement 1: Panels A, B and C show male data previously shown in Figure 2 B, C and Figure 2 supplement 1 B.

Panel D shows new data from females. Figure 6: Panel A and C show data from males previously shown in Figure 3. Panels B and D show new data from females.

Figure 7: Panel A shows data from males previously shown in Figure 4. Panel B shows data from females. We show immunoblot analysis of genes whose expression was different between the diets as determined by qPCR analysis.

Figure 8: shows all new data for males and females. Discussion: The section has been extended based on added results and suggestions by the reviewers and parts of the discussion on the connection of our findings to the ageing process was removed to limit the extend of this section and to focus more on HFD feeding and obesity.

Material and methods: experimental details have been supplemented based on added data. The funders had no role in study design, data collection and interpretation, or the decision to submit the work for publication.

We thank Susanne Klaus and Susanne Keipert DIfE, Potsdam for help with bomb calorimetry and Maaike Oosterveer UMCG for providing the SREBP1 antibody. At the FLI, Verena Kliche for technical assistance, the staff of the animal house facility for embryo transfer and advice on mouse experiments, and Maik Baldauf for help with histology.

This article is distributed under the terms of the Creative Commons Attribution License , which permits unrestricted use and redistribution provided that the original author and source are credited.

Article citation count generated by polling the highest count across the following sources: Crossref , PubMed Central , Scopus. Apicomplexans are ubiquitous intracellular parasites of animals.

These parasites use a programmed sequence of secretory events to find, invade, and then re-engineer their host cells to enable parasite growth and proliferation. The secretory organelles micronemes and rhoptries mediate the first steps of invasion.

After invasion, a second secretion programme drives host cell remodelling and occurs from dense granules. The site s of dense granule exocytosis, however, has been unknown.

In Toxoplasma gondii , small subapical annular structures that are embedded in the IMC have been observed, but the role or significance of these apical annuli to plasma membrane function has also been unknown.

Here, we determined that integral membrane proteins of the plasma membrane occur specifically at these apical annular sites, that these proteins include SNARE proteins, and that the apical annuli are sites of vesicle fusion and exocytosis.

Specifically, we show that dense granules require these structures for the secretion of their cargo proteins. When secretion is perturbed at the apical annuli, parasite growth is strongly impaired. The apical annuli, therefore, represent a second type of IMC-embedded structure to the apical complex that is specialised for protein secretion, and reveal that in Toxoplasma there is a physical separation of the processes of pre- and post-invasion secretion that mediate host-parasite interactions.

Cellular metabolism plays an essential role in the regrowth and regeneration of a neuron following physical injury. Yet, our knowledge of the specific metabolic pathways that are beneficial to neuron regeneration remains sparse.

Previously, we have shown that modulation of O-linked β-N-acetylglucosamine O-GlcNAc signaling, a ubiquitous post-translational modification that acts as a cellular nutrient sensor, can significantly enhance in vivo neuron regeneration.

Here, we define the specific metabolic pathway by which O-GlcNAc transferase ogt-1 loss of function mediates increased regenerative outgrowth. Performing in vivo laser axotomy and measuring subsequent regeneration of individual neurons in C. elegans , we find that glycolysis, serine synthesis pathway SSP , one-carbon metabolism OCM , and the downstream transsulfuration metabolic pathway TSP are all essential in this process.

Testing downstream branches of this pathway, we find that enhanced regeneration is dependent only on the vitamin B12 independent shunt pathway.

These results are further supported by RNA sequencing that reveals dramatic transcriptional changes by the ogt-1 mutation, in the genes involved in glycolysis, OCM, TSP, and ATP metabolism. Strikingly, the beneficial effects of the ogt-1 mutation can be recapitulated by simple metabolic supplementation of the OCM metabolite methionine in wild-type animals.

Taken together, these data unearth the metabolic pathways involved in the increased regenerative capacity of a damaged neuron in ogt-1 animals and highlight the therapeutic possibilities of OCM and its related pathways in the treatment of neuronal injury.

Mitochondrial membrane potential directly powers many critical functions of mitochondria, including ATP production, mitochondrial protein import, and metabolite transport.

Its loss is a cardinal feature of aging and mitochondrial diseases, and cells closely monitor membrane potential as an indicator of mitochondrial health. Given its central importance, it is logical that cells would modulate mitochondrial membrane potential in response to demand and environmental cues, but there has been little exploration of this question.

We report that loss of the Sit4 protein phosphatase in yeast increases mitochondrial membrane potential, both by inducing the electron transport chain and the phosphate starvation response.

Indeed, a similarly elevated mitochondrial membrane potential is also elicited simply by phosphate starvation or by abrogation of the Phodependent phosphate sensing pathway. We also demonstrate that this connection between phosphate limitation and enhancement of mitochondrial membrane potential is observed in primary and immortalized mammalian cells as well as in Drosophila.

These data suggest that mitochondrial membrane potential is subject to environmental stimuli and intracellular signaling regulation and raise the possibility for therapeutic enhancement of mitochondrial function even in defective mitochondria.

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For instance, one research study in pediatric burn patients demonstrated elevated levels of catecholamines for 2 years in comparison to healthy individuals Kulp et al.

In another study, norepinephrine levels were noted to be fold higher in urine samples and remained elevated for more than a year Jeschke et al. Adipocytes are sensitive to insulin action and halt lipolysis to maintain systemic energy homeostasis Ghaben and Scherer, Insulin binds to the insulin receptor substrate IRS α subunit on the external cell surface leading to conformational changes in the β-subunit for binding of ATP.

Binding of ATP triggers the auto-phosphorylation of IRS β-subunit inside the cell that activates tyrosine kinase activity and enables further cellular activity of insulin Wolever, ; Burks and White, ; Kido et al. There are four known types of IRS 1—4 Burks and White, IRS-3 is present only in adipose tissue.

Phosphorylation of IRS protein by insulin activates a variety of signaling pathways. Intracellular transport of glucose is mediated by insulin action via glucose transporter 4 GLUT4 to promote glucose uptake, lipogenesis and suppress lipolysis that reduces FFA flux in blood stream Smith, Therefore, in an insulin deficient state, adipose tissue liberates FFA to maintain energy homeostasis by fatty acid oxidation and for direct utilization by organs such as the liver, heart, and muscle.

In addition, the liver converts FFA to ketone bodies which is an alternative substrate for the brain during fasting periods Wilcox, Severe burn injury is often associated with systemic insulin resistance, hyperglycemia and hyperlipidemia that significantly contributes to the morbidity and mortality in burn patients Jeschke et al.

Therefore, insulin therapy and similar analogs are considered an important therapeutic measure against the hypermetabolic response and have shown promising results in critical burn patients Jeschke et al. Under normal circumstances, a postprandial elevation in blood glucose levels stimulates the release of insulin from pancreatic β-cells.

Released insulin thereby promotes glucose uptake in skeletal muscle and adipose tissue, and inhibits glycolysis, glycogenolysis and lipolysis to maintain euglycemia Belfort et al. In critical circumstances such as severe burn injuries, the release of stress mediators causes metabolic derangements and significantly affects energy homeostasis.

To meet the enhanced energy demand in stress conditions, the stress mediators oppose the anabolic action of insulin inhibiting glucose production causing a state of insulin resistance.

Catecholamines inhibit insulin action during stress or exercise through α2 adrenoreceptors Wilcox, , suggesting the role of adrenergic stress signaling in promoting the insulin resistance state. These hypermetabolic conditions activate alternative energy sources such as adipose tissue lipolysis, hepatic glucose release, and skeletal muscle proteolysis Jeschke et al.

Thus, hyperglycemia and insulin fail to inhibit lipolysis and glucose production. Enhanced systemic energy circulation results in lipotoxicity, insulin resistance, and associated ectopic fat deposition in vital organs, thus affecting their function.

Therefore, adipose tissue lipolysis and insulin resistance plays an intriguing role in contributing to a burn-induced hypermetabolic state. Recent studies using PET-CT scan have shown the metabolic activation of BAT in response to different stress stimulators such as cold exposure, burn injury, and cutaneous wound Carter et al.

Intriguingly, mice subjected to burn injury had reduced lipid content, and enhanced UCP1 content in BAT Carter et al. Furthermore, Yo et al. Upon targeting mitochondrial activity using SS31, Yo et al.

More recently, Bhattarai et al. Overall, studies assessing the role of BAT when challenged with burn injury have revealed that BAT activates afterward with evidence of enhanced Ucp1 expression.

However, the insight into the role of BAT when challenged with burn injury is very limited. Further studies are required to determine if this depot contributes to systemic dysfunction via enhanced lipolysis or inflammatory mediators. The concept of WAT browning implies triggering WAT to transition into brown-like adipose tissue known as beige adipose tissue.

The latter is characteristic of an intermediate state between WAT and BAT, having a mixture of unilocular and multilocular adipocytes, enhanced mitochondrial content and exhibiting UCP1 expression Bartelt and Heeren, ; Peirce et al. Research shows that WAT and BAT originate from different mesenchymal stem cell lineages and have different myogenic signatures Timmons et al.

However, beige and white adipocytes share the same cell lineage and white adipocytes can transdifferentiate into beige adipocytes upon stimulation Park et al. In theory, beige adipocytes can originate from progenitor cells residing within WAT in response to external stimuli Wang et al.

Alternatively, they can also transdifferentiate from WAT or BAT that involves direct conversion of WAT or BAT into beige adipocytes and vice versa Barbatelli et al. Both WAT and BAT are highly innervated and sensitive to energy demand transmitted by the sympathetic nervous system that regulates the transcriptional circuit of the browning process.

In response to cold exposure or catecholamines, WAT browning is initiated by peroxisome proliferator-activated receptor γ Ppar γ that further activates thermogenic gene Pgc1 α, Prdm16 , and Ucp1 expression.

In addition, WAT browning can be activated by several hormones via crosstalk between tissues. For instance, FGF21 and bone morphogenetic protein 4 BMP4 that are produced in response to adrenergic signaling can promote WAT browning Fisher et al.

Other WAT-released hormones such as leptin and insulin also promote WAT browning through proopiomelanocortin POMC neurons and the sympathetic nervous system Dodd et al.

Furthermore, although catecholamines are required for the activation of WAT browning, M2 macrophages resident in WAT are also a source of catecholamines Nguyen et al.

Overall, WAT browning can be triggered by energy sensing, metabolic demand, hormonal activation and adrenergic signaling, all of which suggest a crucial metabolic role for WAT in energy homeostasis. Evolutionarily, WAT acts as an excellent thermal insulator that maintains body temperature and energy balance in humans.

Also, activating WAT browning can potentially help obese or overweight individuals aiming to reduce body weight and become metabolically healthy Bartelt and Heeren, However, stress conditions such as severe burn injury also trigger WAT browning that results in enhanced REE, enhanced mitochondrial content, UCP1 expression and abundant nutrient supply by triggering WAT lipolysis for fatty acid oxidation.

WAT browning is seemingly a common attribute in severe burn-injured patients and is a major factor that fuels the hypermetabolic response in critically injured patients Abdullahi and Jeschke, Therefore, WAT browning can be perceived as a dysfunctional trait in adipose and novel challenge for clinicians to deal with while treating severe burn-injured patients.

Although significant success has been achieved in identifying the critical precursors triggering WAT browning, research is ongoing to identify measures to inhibit WAT lipolysis and browning in severe burn-injured patients Abdullahi and Jeschke, In fact, a study conducted in pediatric and adult burn patients has demonstrated that WAT browning is a major driver of the hypermetabolic response and associated metabolic dysfunction Patsouris et al.

Furthermore, rodent studies have revealed the critical role of cytokines such as IL6 and macrophages in activating WAT browning and causing hepatic steatosis post burn injury Abdullahi et al.

In most of the cases, inflammatory triggers vanish as soon as the problem is resolved. However, in some cases, when the trigger is constant, the ongoing acute inflammation can turn into chronic inflammation Barrett et al.

In stressed conditions, adipocytes can actively produce and recruit other inflammatory cells and mediators that are capable of activating and recruiting immune cells such as macrophages and T-lymphocytes T-cells Abdullahi et al. WAT acts as a reservoir from which a myriad of metabolic signals can originate via the secretion of a plethora of cytokines and hormones.

For instance, TNFα can directly disrupt insulin signaling Hotamisligil, and IL6 can activate WAT browning Abdullahi et al. Severe burn-injured patients often experience chronic inflammation and associated metabolic dysfunction Jeschke et al. Such chronic hyper-inflammation often affects wound healing, triggers WAT browning, lipolysis, lipotoxicity, sepsis, and associated multi-organ failure complications.

Research over the past two decades has shown that inflammation in WAT is a major contributing factor in the hypermetabolic response observed in burn patients. WAT acts as an endocrine organ and plays an active role in secreting inflammatory moieties such as cytokines, hormones, and other growth factors.

Under critical stress, adipocytes recruit inflammatory mediators, chemo-attractants such as monocyte chemoattractant protein-1 or MCP-1 and cytokines that activate macrophage polarization as well as multiple metabolic signaling pathways.

It is established that burn injury results in structural, functional, and morphological changes in WAT, with enhanced levels of circulating WAT-derived adipokines, inflammatory mediators and hormones that are known to regulate WAT inflammation and metabolism Jeschke et al.

Studies in human patients and rodents have shown that neutralization of TNFα accelerates wound healing Ashcroft et al. Assessment of WAT collected from burn patients revealed the enhanced leukocyte infiltration, macrophages, and activation of Nod-like inflammasome receptor-3 NLRP3 protein that plays a crucial role in multiple signaling pathways Stanojcic et al.

Furthermore, studies elucidating the role of NLRP3 in WAT after burn injury shows that NLRP3 has an anti-browning effect and that genetic deletion of this inflammasome augments WAT browning and the hypermetabolic response Vinaik et al.

Macrophage recruitment in WAT, on the other hand, undergoes alternate polarization and activation leading to the secretion of catecholamines and cytokines which induce multiple signaling cascades Abdullahi et al.

For example, alternatively activated macrophages secrete IL6 that plays a crucial role in activating WAT browning and associated dysfunction post-burn injury Ashcroft et al.

Moreover, inhibition of alternatively activated macrophages impairs metabolic adaptation and the thermogenic response of adipose tissue. Furthermore, administration of interleukin-4 reinstates thermogenic gene expression, systemic fat mobilization and energy homeostasis in response to cold Nguyen et al.

Therapeutic interventions targeting β-adrenergic receptors, cytokines, WAT lipolysis, and browning mediators after burn injury have shown promising results in improving REE, hepatic steatosis, reducing hyperglycemia, hyperlipidemia, and chronic inflammation.

Propranolol, a non-selective β-adrenergic signaling blocker, has shown promising benefits in reducing REE and reduction in the expression of browning markers Ucp1 , Cox-iv in WAT, suggesting the importance of regulating catecholamines and stress hormones to mitigate WAT-associated dysfunction Sidossis et al.

Propranolol has higher affinity toward β1 and β2 receptors Barbe et al. However, studies conducted by an independent research group has shown that chronic adrenergic stress post-burn injury upregulates β3 receptor expression in WAT albeit its role in WAT browning remains elusive Saraf et al.

Also, studies in pediatric burn patients treated with propranolol by an independent group has shown a decrease in cardiac workload accompanied by reduced lipolysis, muscle catabolism, hepatosteatosis, and ultimately REE Finnerty and Herndon, However, the non-selective nature of propranolol also exposes patients to a significant risk of cardiac failure Unpublished clinical data.

Furthermore, systemic IL6 levels were found upregulated in burn patient samples soon after burn injury and persisted for more than a month Patsouris et al.

Subsequent studies assessing the role of IL6 in WAT using the global IL6 knockout mice model has shown that deletion of IL6 prevents WAT browning after burn injury Abdullahi et al. Also, IL6 deletion reduces infiltration of macrophages and inhibits alternative activation and polarization of macrophages Abdullahi et al.

Metformin, a successful clinical drug for use against diabetes has shown promising results in improving insulin resistance without causing hypoglycemia in phase II randomized clinical trials conducted in burn patients Jeschke et al.

Mechanistic studies in a murine model of thermal injury assessing the action of metformin has demonstrated that metformin induces protein phosphatase 2A PP2A activity, thus dephosphorylating key enzymes in the WAT lipolytic pathway [such as acetyl-CoA carboxylase ACC and HSL] and promoting fat storage in adipocytes Auger et al.

In fact, metformin treatment also reduces mitochondrial respiration and enhances mitochondrial coupling control in WAT, suggesting an indirect protective effect of metformin in reducing WAT browning and REE Auger et al. While these changes are independent of adenosine monophosphate kinase AMPK activation, the canonical mechanism of metformin, the authors postulate that higher concentrations of this biguanide would be necessary to activate this signaling pathway in highly energetic beige adipose Auger et al.

Additionally, another clinical study assessing metformin has shown promising results against skeletal muscle catabolism and insulin resistance following severe burn injury Gore et al.

Furthermore, studies assessing the impact of metformin on inflammation in adipocytes has revealed that metformin administration suppresses pro-inflammatory cytokines such as TNFα and IL-1β and also, indirectly enhances the anti-inflammatory effect of metformin Qi et al.

Moreover, in burn patients there is evidence that this biguanide can decrease inflammatory mediators in the serum such as IL-1β and MCP-1 Jeschke et al. To date, the potential benefits of other biguanides or PPARγ agonists such as thiazolidinediones on glucose control and systemic dysfunction post-burn have not been adequately explored.

Acipimox, a niacin derivative that targets WAT lipolysis, has shown effective results in a murine model when challenged with severe burn injury. Acipimox not only reduced systemic lipid levels, in fact, it also attenuated WAT browning and hepatic fat infiltration after burn injury Barayan et al.

Additionally, acipimox has shown promising results in 3-month clinical trials in HIV-infected patients 23 with hyperlipidemia, and abnormal fat distribution. Acipimox treatment resulted in reduced systemic lipid levels, decreased WAT lipolysis and enhanced insulin sensitivity in HIV-infected patients Hadigan et al.

However, the impact of acipimox on WAT inflammation and systemic glucose metabolism in these adverse events has yet to be elucidated. Adipose tissue has an enormous buffering capacity for release, storage, and dissipating energy in times of need. Research over recent years has made it clear that adipose tissue function and dysfunction has a major role to play in burn injury and its associated hypermetabolic response which often progresses to multi-organ dysfunction Figure 2.

Being an endocrine organ, the adipose tissue secretes a myriad of adipokines and maintains energy homeostasis in humans. Although significant success has been achieved in understanding the factors that trigger adipocyte dysfunction, our knowledge is still limited to the tip of an iceberg.

Much of the research in the field has focused on identifying the key markers being altered when challenged by burn injury. Although the role of insulin has been thoroughly covered, there are a plethora of cytokines, adipokines leptin, adiponectin and stress hormones that are still not fully understood and how they affect insulin action and WAT morphology is still a matter of debate.

Figure 2. Adipose dysfunction and associated multi-organ damage after burn injury. Elevated levels of systemic FFA flux, inflammatory mediators and adipokines collectively contribute to a feed-forward loop, hypermetabolism, and multi-organ damage Jeschke et al. Macrophage infiltration and polarization in adipose tissue after burn injury is known, however, the exact role of inflammatory mediators is still not clear.

Studies elucidating the role of IL6 and its inhibition have revealed the detrimental role of this cytokine in the processes of WAT browning and hepatic steatosis Abdullahi et al. However, its role in macrophage recruitment and polarization is not clear Abdullahi et al.

Similarly, studies conducted to understand the role of TNFα in burn patients have revealed that it is upregulated after burn injury Yeh et al.

Research studies conducted in the NLRP3 murine model have demonstrated that deleting NLRP3 augments WAT browning, lipolysis, hepatic steatosis and impairs wound healing Stanojcic et al. However, further research studies are required to understand the protective role and mechanistic action of NLRP3 when challenged with burn injury.

Research over the past decade and advances in clinical burn care have significantly advanced our knowledge and greatly improved the survival of burn patients Figure 3. Interventions such as metformin have shown promising safety and efficacy in phase II clinical trials.

Moreover, metformin protects against WAT lipolysis, browning, and helps in maintaining euglycemia. Further detailed clinical investigation is, however, required to elucidate its effect on adipose tissue function after severe burn injury.

Recently, the WAT lipolysis inhibitor acipimox has shown promising results in rodent studies when challenged with burn injury, suggesting the possible benefits of targeting WAT dysfunction in the future.

However, further mechanistic studies are required to elucidate the action of acipimox and its possible impact on insulin sensitivity and WAT inflammation. The strong correlation of these drugs targeting WAT dysfunction suggests that reducing WAT lipolysis and browning is an important therapeutic strategy for the treatment of the hypermetabolic response in burn patients.

Other potentially relevant strategies could be understanding the role of macrophage recruitment in WAT and mechanisms involved in activation and polarization of macrophages.

To that effect, much remains to be uncovered with regards to the interactions of macrophages with themselves and the interaction of macrophages with adipocytes when challenged with burn injury.

Lastly, understanding the role of adipokines and their impact on signaling pathways in vital target organs such as the brain, central nervous system, heart, liver, and skeletal muscle, can possibly reveal novel therapeutic strategies in reducing the WAT-associated hypermetabolic response in burn patients.

Figure 3. Summary of the therapeutic advances targeting adipocyte lipolysis and browning post-burn injury. Metabolic impact of drug treatment A Propranolol B Tocilizumab C Metformin D Acipimox post-burn injury. All authors listed have made a substantial, direct and intellectual contribution to the work, and approved it for publication.

The authors received no particular funding for this work. The grants supporting the research work are Canadian Institute of Health Research , NIH R01GM and R01GM and Ontario Institute of regenerative medicine. The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Abdullahi, A. Alternatively activated macrophages drive browning of white adipose tissue in burns. doi: PubMed Abstract CrossRef Full Text Google Scholar. IL-6 signal from the bone marrow is required for the browning of white adipose tissue post burn injury.

Shock 47, 33— Taming the flames: targeting white adipose tissue browning in hypermetabolic conditions. Browning of white adipose tissue after a burn injury promotes hepatic steatosis and dysfunction. Cell Death Dis. Ahima, R. Adipose tissue as an endocrine organ. Trends Endocrinol.

CrossRef Full Text Google Scholar. Alberti, K. I, Donato, K.

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Financial Assistance Documents — Arizona. Once thought to be an inert energy storage depot, adipose tissue is now known to be a critical endocrine organ. The term "adipocytokines" or "adipokines" has been used to describe the numerous adipocyte secretory products which include: adiponectin, adipsin, estrogen, angiotensin II, angiotensinogen, leptin, plasminogen activator I PAI-1 , agouti protein, resistin [ 56 ], acylation stimulating protein ASP , bone morphogenic protein BMP , prostaglandins, IGF-1, and various IGF binding proteins, tumor necrosis factor alpha TNFα , interleukins ILs , transforming growth factor TGF -B [ 57 ], and fibroblasts, as well as FFAs themselves.

Adipokines such as IL-6 and PAI-1 are more highly secreted by VAT than abdominal SCAT [ 58 , 59 ], while leptin is more highly secreted by SCAT [ 60 ]. Adipokines from VAT can be delivered via the portal system directly to the liver where they can affect hepatic, and ultimately systemic, inflammation.

In an ex vivo study, VAT released greater amount of IL-6 and PAI-1 compared with abdominal SCAT [ 58 , 61 ]. Adiponectin has many beneficial vascular and metabolic effects, e.

Ironically, although produced by adipose tissue, adiponectin levels are lowered with greater degrees of obesity and with overfeeding.

Decreased concentrations of adiponectin are associated with type 2 diabetes, hypertension, elevated glucose levels, insulin and TGs, and cardiovascular disease CVD.

It has been suggested that adiponectin is under feedback inhibition in obesity and reduced in patients with metabolic syndrome [ 66 ]. Adiponectin mRNA and protein levels have been found to be reduced in omental VAT compared with SCAT [ 67 ], and VAT may also produce an as-yet-identified factor that destabilizes adiponectin mRNA [ 66 , 68 ].

The strong inverse correlation between serum adiponectin levels and VAT mass may in part explain the link between VAT and metabolic syndrome [ 66 ].

Over 90 percent of the adipokines released by adipose tissue, except for adiponectin and leptin, could be attributed to non-fat cells, e.

Fat mass can expand in one of two ways: individual adipocytes can increase in volume or they can increase in number as more are derived from preadipocytes. As adipocytes grow larger, they become dysfunctional. The total number of adipocytes is increased with increasing fat mass, but it is the increased number and percentage of large adipocytes, compared to the smaller ones, that may partially account for the inability of adipose tissue to function properly [ 69 ].

While the smaller adipose cells tend to be more insulin sensitive, large adipocytes become insulin resistant and contribute more to the metabolic problems associated with obesity [ 69 ].

Preadipocytes from the SCAT depots have a greater differentiation capacity than those from the VAT depots [ 70 , 71 ]. The differentiation of preadipocytes into lipid-storing adipocytes is regulated in part by the nuclear hormone receptor, peroxisome proliferators activated receptor PPAR.

Activation of this receptor by natural ligands, such as prostaglandin metabolites, or synthetic ligands, such as thiazolidinediones TZDs , leads to stimulation of the differentiation pathway [ 71 ]. This increases the number of smaller adipocytes in SCAT with a high avidity for FA and TG uptake.

These increased adipose stores made up of new, smaller, more insulin sensitive adipocytes act as a sink or powerful 'buffers,' avidly absorbing circulating fatty acids and triglycerides in the postprandial period. This prevents their diversion to non-adipose tissues, thereby protecting against ectopic fat syndrome and metabolic syndrome.

It has been proposed that an inability to differentiate new adipocytes to accommodate and store excess energy, underlies the development of type 2 diabetes [ 72 , 73 ]. TZDs can increase the number of new fat cells, and because obesity is a major cause of insulin resistance, this represents an apparent paradox.

Ex-vivo studies of human preadipocytes from SCAT and VAT depots have demonstrated that TZD-stimulated differentiation is much greater in SCAT than VAT preadipocytes [ 71 ].

Since TZDs selectively promote adipogenesis in SCAT and not VAT, this would encourage the redistribution of body fat away from "harmful" VAT sites and toward "safer" SCAT ones [ 74 — 76 ]. Thus, in this way, TZDs could allow for pushing the patient to below his CVATT. Paradoxically, the TZDs can lead to weight gain while improving insulin sensitivity as the new SCAT adipocytes continue to trap FA and as fat storage continues, eventually the new adipocytes will enlarge, become less insulin sensitive, and ultimately contribute to insulin resistance [ 77 ].

TZDs may also exert anti-inflammatory effects on adipocytes by reducing the production of serum amyloid A SAA and preventing the TNFα-mediated expression of adiponectin production [ 69 ]. Macrophages increase their accumulation within fat depots in direct proportion to increases in adipose tissue and adipocyte size.

The increased macrophage activity observed in the adipose tissue of the obese may reflect a combination of conversion of local preadipocytes to macrophages and activation and recruitment of resident macrophages and circulating monocytes.

This seems to occur after the onset of adiposity but prior to insulin resistance, and supports the notion that pathophysiological consequences of obesity involve macrophages and inflammation that contribute to insulin resistance and metabolic syndrome [ 78 , 79 ].

Evidence suggests that macrophages and adipocytes not only express overlapping sets of genes and serve similar functions, but also commingle in the same part of the body — the fat tissue [ 80 ]. There are numerous inherent differences between VAT and SCAT. VAT is a major predictor for insulin resistance [ 81 ] and metabolic syndrome [ 11 ].

Compared to SCAT, VAT adipocytes have a higher rate of lipolysis, which is more readily stimulated by catecholamines and less readily suppressed by insulin [ 82 ]. VAT also produces more IL-6 and plasminogen activator inhibitor-1 PAI-1 [ 81 ]. The "Portal Theory" suggests that insulin resistance and many of its related features could arise from VAT delivering free fatty acids FFAs at a high rate to the liver via the portal vein into which VAT directly drains [ 83 , 84 ].

This, in turn, would increase hepatic glucose production, reduce hepatic insulin clearance, and ultimately lead to insulin resistance, hyperinsulinemia, hyperglycemia as well as non-alcoholic fatty liver disease NAFLD.

FFA flux could also lead to enhanced production of triglycerides TGs and apolipoprotein B-rich lipoproteins, which are features of the insulin resistance syndrome [ 55 , 85 ]. Delivery of VAT derived pro-inflammatory cytokines may contribute to hepatic pathology such as non-alcoholic steatohepatitis NASH.

VAT also releases a large amount of glycerol which enters the liver where it can be converted to glucose, thereby contributing to hyperglycemia [ 86 ]. It is likely that the relationship observed between VAT and metabolic complications may not exclusively result from FFA flux from VAT into the portal vein and the portal theory does not adequately hold up as the sole explanation of the role of VAT in metabolic syndrome [ 7 ].

Recently, omental VAT cells have been shown to have an approximately two-fold higher rate of insulin-stimulated glucose uptake compared with SCAT adipocytes, and this could be explained by a higher GLUT-4 expression [ 87 ]. Perhaps in situations with a high intake of dietary glycemic load, a higher rate of glucose uptake and subsequently lipogenesis might be one mechanism by which TGs are stored preferentially in the VAT depot.

VAT is highly lipolytic and resistant to insulin's lipogenic effects yet apparently can remain insulin sensitive to glucose uptake. This efficiency in glucose uptake may reflect VAT's ability to accumulate and maintain its activity.

Enhanced glucose utilization in VAT would be accompanied by less lipid oxidation, which would indirectly promote TG storage [ 87 ]. VAT has a high density of androgen receptors and testosterone which can amplify its own effect by up-regulation of androgen receptors, inhibiting the expression of lipoprotein lipase LPL and FA uptake [ 5 , 88 ].

In men, VAT is strongly negatively correlated with plasma total and free testosterone and sex-hormone binding globulin SHBG concentrations. Thus, in young men whose plasma total testosterone and free testosterone are high, the amount of VAT is low.

As men age, exceed their 20s, and reach middle age, their total and free testosterone decline, more fat is deposited in VAT stores, they often develop the "pot belly," and their risk for CHD increases [ 5 , 89 ].

The effects of testosterone on insulin resistance and metabolic syndrome risk factors are opposite in men and women [ 5 , 88 , 90 , 91 ]. Testosterone production often declines in women as they age, but VAT obesity in women is associated with elevated levels of total testosterone, free testosterone.

Hyperandrogenicity can also occur in polycystic ovary syndrome, where hyperinsulinemia can stimulate ovarian androgen production and suppress serum SHBG [ 88 , 93 ]. While weight loss in both sexes has been consistently shown to reverse the abnormalities in testosterone levels [ 94 — 97 ], a number of placebo controlled studies have consistently demonstrated that administering testosterone to obese men resulted in a significant reduction in VAT.

This occurred without significantly altering amounts of total body fat or lean body mass [ 89 , 98 — ]. However, the use of testosterone for VAT obesity is left open to debate [ 90 ]. Patients with type 2 diabetes and metabolic syndrome often appear Cushingoid, yet they invariably do not have elevated plasma cortisol [ ].

Compared to SCAT, VAT has more glucocorticoid receptors [ 88 ]. The enzyme β hydroxysteroid dehydrogenase type 1 β HSD1 converts inactive cortisone to the active compound cortisol, and, if overexpressed, may cause increases in local cortisol concentrations [ ].

Local production of active cortisol from inactive cortisone driven by β-HSD-1 activity is very high in VAT and barely detectable in SCAT.

Therefore it is likely that the VAT depot actively contributes to the production of high local concentrations of cortisol, which might not be reflected by plasma levels. These, in turn, might contribute to an increase in VAT accumulation [ ].

The amount of fat deposited within skeletal muscle intramyocellular lipid — IMCL and the ability of muscle to oxidize fat are important determinants of weight gain,[ ] weight regain following weight loss [ ], and the development of insulin resistance syndrome [ ].

IMCL and the VAT depot might not be independent from each other. Furthermore, the relationship between IMCL and insulin sensitivity is independent of percent total body fat and SCAT but not of VAT [ ]. In individuals with type 2 diabetes, among the depots of regional and overall adiposity, VAT was the depot of adipose tissue that was most strongly related to skeletal muscle insulin resistance [ ].

The researchers found that insulin sensitivity as well as postabsorptive rates of FFA utilization or oxidation by muscle were diminished in relation to VAT. Women with increased VAT did not have lower plasma FFA levels or lower rates for appearance of FFA, yet they had an impaired or reduced uptake of plasma FFA by the skeletal muscle in the leg [ ].

Together, this supports a role for VAT, IMCL lipid deposition, and perhaps impaired oxidation of nonadipose tissue lipid in insulin resistance and metabolic syndrome.

Mauriege et al found that adrenoreceptor sensitivity was increased in SCAT cells of individuals who have a higher VAT accumulation compared to those with a low VAT deposition [ ]. SCAT adipocytes from women with visceral obesity exhibit higher lipolysis rates in vitro than those obtained from women with little VAT [ ].

Mauriege et al also demonstrated that among men with high levels of VAT, SCAT adipocytes are more sensitive to β-adrenergic lipolysis which may further exacerbate an impaired insulin action, a potentially important factor in the etiology of metabolic syndrome associated with visceral obesity [ ].

Moreover, an increased truncal SCAT mass and an increased amount of VAT mass can independently predict insulin resistance [ ]. Together, these findings support that VAT may enhance central SCAT lipolysis and accelerate release of peripheral FFAs.

The PPARs are important transcription factors that play an important role in the induction of adipose-specific genes, the proliferation and differentiation of adipocytes, and the development of mature adipose tissue.

A number of transcription factors are involved, including PPARγs. Giusti et al suggest that in VAT, the expression of PPARγ2 is controlled by local transcription factors RXRα, αSREBP1, and SREBP1c promoting fat storage in adipocytes. Given that the fat storage capacity is limited in VAT, RXRα induces the expression of PPARγ2 in SCAT to increase its overall capacity [ ].

These data also suggest that the signal to promote fat storage may occur in VAT and that other metabolic and hormonal factors are involved in the control and modulation of adipogenesis in visceral fat [ ].

Perhaps the above can be explained as follows. SCAT cells may act as a buffer or sink for circulating FAs and TGs but once they reach their capacity they lose their protective benefits.

Initially, VAT may influence SCAT to expand and act as a buffer. However, once the critical VAT threshold CVATT is achieved and metabolic syndrome has begun to develop, then VAT may influence central SCAT to become more VAT-like, i.

As discussed earlier, preadipocytes from SCAT depots have a greater capacity than VAT to differentiate into numerous, small, insulin-sensitive, adipocytes [ 70 , 71 ]. These lipid-storing cells act as a buffer or sink for circulating FAs and TGs, thereby preventing their deposition in non-adipose tissues, e.

In defending the role of VAT accumulation in individuals with metabolic syndrome, we must postulate a high rate of lipid turnover, with high rates of lipolysis at certain times matched by high rates of lipid deposition at other times.

Otherwise, as Frayn points out, the hyperlipolytic VAT would ultimately disappear [ ]. He also suggests that if SCAT were to become insulin resistant, and therefore resistant to fat storage, then fat might tend to be deposited in VAT depots. Another possibility is that the usually larger SCAT depot has a greater potential to contribute to insulin resistance through release of FFA into the systemic circulation.

However, this would not adequately explain the subset of individuals who demonstrate metabolic profiles consistent with insulin resistance but are in fact lean, healthy-appearing with normal BMIs, excess VAT, little SCAT, and are referred to as "metabolically obese, normal weight MONW [ 26 ].

As described above, perhaps once VAT expands and SCAT depots reach their capacity for storing FAs, then do SCAT adipocytes become insulin resistant, release FFAs, and contribute to systemic insulin resistance and metabolic syndrome. While some studies cast doubt on the portal theory and its implications for VAT's direct delivery of FFA to the liver [ , ], they leave open other mechanisms via which VAT could induce insulin resistance and other metabolic disturbances, e.

These will be discussed below. If trunk fat is taken into account, accumulation of fat in the hips and legs is an independent predictor of lower cardiovascular and diabetes-related mortality, and it seems to protect against impaired glucose metabolism, especially in women [ — ].

In a study of 1, women ages 60—85, those with excessive peripheral fat had less atherosclerosis determined by aortic calcification scores , and the quartile with both the highest amount of central fat and peripheral fat seemed to be partially protected by the high percentage of peripheral fat mass as reflected in a number of measured risk factors [ ].

These findings corroborate similar findings by the same group who followed postmenopausal women for 7. In yet another study, Tanko et al demonstrated that peripheral fat mass SCAT in generally obese, post-menopausal women is associated with increased adiponectin and higher insulin sensitivity [ ].

Together, these support protective roles for peripheral fat. In addition to fat trapping, these might include possible influences on adipokines, e. One must interpret these results with caution because the measuring technique of dual-energy X-ray absorptiometry DXA does not allow separate quantification of intermuscular and subcutaneous fat in the arms and legs as well as SCAT in the trunk [ ].

While VAT is a major predictor of insulin sensitivity in overweight and lean individuals [ , ], others have found abdominal SCAT to contribute to insulin resistance independently of VAT [ , ]. When there is an inability to store fat, due to lipodystrophy, the adipocytes' storage capacity is exceeded and lipids accumulate and cause lipotoxicity in liver, muscle, and other organ tissues [ 7 ].

A counterpart of lipodystrophy may be illustrated by patients with multiple symmetric lipomatosis MSL , a condition characterized by regional excess of subcutaneous adipose tissue.

These patients have higher adiponectin levels, a high degree of insulin sensitivity and glucose tolerance, very low lipid levels in liver and muscle cells, and markedly little VAT [ ].

In this case, SCAT may be protective and beneficial. This may be analogous to thiazolidinedione action, which also promotes SCAT deposition while improving insulin sensitivity and glucose tolerance [ 74 , 75 ]. Estrogen promotes the accumulation of peripheral gluteo-femoral SCAT, which may be protective [ ].

The abundant presence of peripheral fat mass in generally obese women is associated with increased plasma adiponectin, and the loss of estrogen with menopause is associated with an increase in central fat [ ].

This accounts for the progression in many overweight women after menopause from a predominantly pear-shape or "gynoid" habitus to the apple or "android" shape. Contrary to popular belief, menopause does not seem to independently cause a gain in total body weight; the increases in BMI that often accompany menopause are usually consistent with normal aging [ ].

However, even without weight gain, body fat distribution changes; postmenopausal obese women tend to accumulate abdominal fat along with deterioration of risk factors, even if total body weight and BMI do not change during menopause transition. After menopause, when ovarian function declines, adipocytes become the primary source of endogenous estrogens [ ], and compared to "gynoid" or pear-shaped women, those with central obesity apple- or "android-" shaped have lower plasma SHBG and higher estradiol [ , ].

This suggests regional differences in the enzymatic conversion of steroid hormones in VAT versus SCAT [ , — ].

In ovarian hormone-deficient women, SCAT adipocyte size, lipoprotein lipase LPL activity, and basal lipolysis were not found to be significantly greater compared to regularly cycling premenopausal women.

For a given amount of total body fat, men have been found to have about twice the amount of VAT than what is found in premenopausal women but this may change after menopause when VAT storage becomes predominant [ , ].

Along with an increase in VAT, a decline in estrogen is also associated with reduced lean body mass as well as other features of the metabolic syndrome including: dyslipidemia with elevation in Lp a , triglycerides, and an increase in small, dense, LDL particles.

Estrogen deficiency also may influence cardiac risk by its effects on the insulin action, the arterial wall, and fibrinolysis. Park et al showed that postmenopausal women lost less VAT compared with the premenopausal women during a weight reduction program The reasons behind this are presently unclear.

As mentioned above, in menopause, adipocytes are primary sources of endogenous estrogens in women [ , ], and estrogens are known inhibitors of IL-6 secretion [ ]. It is worth noting that the relationship between BMI and serum IL-6 was observed only in postmenopausal women, and this relationship was lost among those women receiving hormone replacement [ ].

Adipose tissue-derived estrogens in postmenopausal women would not be sufficient to reduce IL-6 in a similar way as endogenous estrogens do in premenopausal women [ ]. Perhaps in premenopausal women, endogenous estrogen from the ovaries helps keep VAT volume relatively low and is thereby protective.

Estrogen by itself seems to protect postmenopausal women receiving replacement therapy from VAT accumulation, and in women with type 2 diabetes, estrogen replacement may protect against the risk of cardiac events [ , ].

Compared to men of similar age, premenopausal women appear to be significantly protected from CHD. However, by age 70 the incidence of CHD is equal in men and women, suggesting that estrogen deficiency causes a rapid acceleration in CHD risk [ ].

Yet, in elderly, postmenopausal women, Tanko et al showed that those women with higher amounts of central versus peripheral obesity had significantly higher levels of estradiol and lower adiponectin. This suggests that prolonged and increased exposure of SCAT cells to estradiol may eliminate the protective effect of SCAT by affecting SCAT's ability to release adiponectin thereby promoting the atherogenic effects of IL-6 [ ].

Perhaps future research will help clarify whether central obesity has any implication for increased susceptibility to the adverse cardiovascular effects of hormone replacement therapy HRT in diabetic patients early after initiation of therapy [ ]. Obesity, particularly visceral obesity, as well as insulin resistance and hyperinsulinemia are associated with breast cancer [ ].

Insulin may increase estrogen action by increasing bioavailable estrogen due to a decrease in sex hormone-binding globulin, by influencing estrogen receptors, and by increasing aromatization of androgen to estrogen at the tissue level, a phenomenon which has been demonstrated in breast tissue.

Estrogen upregulates the IGF-1 receptor and IGFBP-1 and -2 and may directly activate the IGF-1 receptor, thereby increasing insulin signaling [ ]. Around , most women died soon after menopause. The average lifespan of persons in the United States has since lengthened by greater than 30 years [ ], which means that women, and men, too, are now spending 30 or more years with hormonal and physiological states that society and medicine has not had to deal with previously.

These, combined with significant dietary and lifestyle changes since , must be considered as critical contributing factors to the world's current epidemic of metabolic syndrome.

When one consumes too many calories, especially in the form of excessive carbohydrates, the liver converts excess glucose to fatty acids. First, glucose that is not oxidized or stored as glycogen is metabolized to acetyl CoA, which then enters the lipogenic pathway.

Acetyl CoA is catalyzed to form malonyl CoA, which in turn inhibits carnitine palmitoyl transferase 1 CPT-1, the enzyme responsible for fatty acid transport into the mitochondria [ 42 ]. The net effect is that malonyl CoA from excess carbohydrates, glucose, and insulin reduces the oxidation of FAs [ ].

This results in increased accumulation of intracellular fat in the form of long chain fatty acids and their derivatives, e. Cellular TG accumulation is not initially toxic and may actually be protective by diverting excess FAs from pathways that lead to cytotoxicity [ ].

While glucose is being preferentially utilized, the FAs are metabolized by pathways other than their preferred β oxidation, leading to toxic products, e.

The subsequent development of the cell's resistance to insulin-mediated glucose uptake, which prevents further influx of glucose, may be viewed as being protective in that it limits the amount of intracellular glucose to be preferentially metabolized over the β oxidation of intracellular FAs [ 29 , 37 , ].

The cell can be insulin resistant with respect to glucose uptake and metabolism but remain sensitive to insulin's lipogenic effects and the de novo synthesis of fat.

Overconsumption of calories, especially in the form of carbohydrates, also stimulates hyperinsulinemia that can then upregulate SREBP-1c and increase de novo lipogenesis [ 43 ]. The first adipocyte-specific hormone to be characterized, leptin is produced predominantly by SCAT adipocytes compared to VAT.

Females produce leptin at about twice the rate in males [ ], and leptin secretion increases with enlarged adipocyte cell size. Circulating leptin rises by 40 percent after acute overfeeding and more than three-fold after chronic overfeeding, whereas fasting is associated with decreased leptin levels [ ].

The increase in leptin concentration after meals is not simply a result of a caloric load, but is in response to a signal that is not present following a fat load without carbohydrate [ ].

Leptin circulates in a free form and is also bound to a soluble leptin receptor — sOBR, which is positively associated with energy intake from carbohydrates and negatively associated with energy intake from dietary fat [ ]. Excess caloric consumption and fat deposition results in newly synthesized FAs that are transported as VLDLs and stored as TG in adipocytes.

Initially, these expanding adipocytes secrete leptin in proportion to their growing fat accumulation. Leptin also crosses the blood brain barrier, stimulates its receptor in the hypothalamus, and causes the release of neuropeptide-Y NP-Y , which reduces feeding behavior [ 85 ].

This, in turn, suppresses appetite and stimulates thyroid function. Leptin affects peripheral tissues, and is a determinant of insulin sensitivity. The ensuing hyperleptinemia increases fat oxidation in skeletal muscle [ — ], and also keeps de novo lipogenesis in check by lowering the involved transcription factor, i.

It promotes cholesterol ester synthesis in macrophages in a hyperglycemic environment, an important process in the formation of foam cells in atherosclerosis which may suggest a protective role of relative leptin resistance [ ].

Leptin also possibly increases sympathetic nervous system SNS activity with subsequent decreased FFA oxidation and thermogenesis [ ]. All of these effects of leptin tend to limit further weight gain. As the process progresses, inefficient leptin action can lead to the opposite of leptin's protective effects, e.

Subsequently, plasma leptin levels rise. The majority of obese individuals with high leptin levels show a leptin insensitivity or "resistance [ ]," which occurs at the leptin receptor level. In animal models, leptin-resistance and leptin-deficiency increases, and upregulates the hepatic expression of SREBP-1c mRNA, which may stimulate an increase in fat production via de novo lipogenesis.

Together, all of these features suggest a state of "leptin resistance" which may ultimately lead to obesity and metabolic syndrome [ 29 , ]. It is quite possible that hyperleptinemia in diet-induced obesity serves to protect nonadipose tissues e.

muscles, liver, pancreatic β cells, and myocardium from the toxic effects resulting from the spillover of full adipose stores and subsequent ectopic deposition of FFAs.

In defense of this paradigm, Unger points out that normally rats can tolerate a 60 percent fat diet because 96 percent of the surplus fat is stored in an enlarging adipose tissue mass, in which leptin gene expression increases proportionally [ ].

However, when leptin is congenitally absent or inactive, or ineffective due to resistance, even on a normal or low-fat diet, excess dietary fat is deposited in nonadipose tissues.

This causes dysfunction lipotoxicity , and possible cell death lipoaptosis [ 29 ]. Acquired leptin resistance occurs in aging, obesity, Cushing's syndrome, and acquired lipodystrophy, a condition associated with protease inhibitor therapy of AIDS.

Preliminary evidence suggests that patients with these conditions have increased ectopic fat, i. The relation between cerebrospinal fluid and serum levels of leptin in obese humans suggests that defective blood brain barrier BBB transport accounts for a great deal of leptin resistance in the CNS.

Banks et al showed in mice that serum TGs directly inhibit the transport of leptin across the BBB and so could be a major cause of leptin resistance across the central nervous system CNS. Thus they suggest that serum TGs are likely a major cause of the leptin resistance seen in both obesity and starvation [ ].

This hypothesis explains why lowering TGs may be therapeutically useful in enhancing the effects of leptin. Compared to VAT, SCAT is the predominant source of leptin [ 60 ], yet patients with VAT obesity may tend to have higher leptin levels than normal, lean individuals but lower than those with predominantly SCAT or subcutaneous obesity [ 29 ].

This suggests that the hyperleptinemia of predominantly VAT obesity is not high enough to overcome a leptin resistance due to the accumulation of ectopic fat in nonadipose tissues, which leads to lipotoxicity and ultimately the metabolic syndrome [ 29 ].

A number of clinical states exhibit evidence of leptin insufficiency, either leptin deficiency or resistance, and they all have in common the metabolic syndrome. These include rare genetic diseases known as lipodystrophies, which are characterized by a redistribution of fat.

Ironically, in the more severe cases, e. There is hyperleptinemia along with hyperphagia and a predominance of intra-muscular fat [ ]. Dunnigan-type familial partial lipodystrophy is a rare autosomal dominant condition characterized by markedly reduced plasma leptin levels along with gradual loss of SCAT from the extremities, trunk, and gluteal region, commencing at the time of puberty, as well as hyperinsulinemia, glucose intolerance, dyslipidemia high TGs with low HDL , and diabetes [ , ].

These individuals do maintain central obesity and VAT [ ], which supports a relatively protective role for SCAT and implicates VAT as being more pathogenic. The aforementioned potential role of TGs in leptin resistance may have implications for patients with lipodystrophy and lipoatrophy who have little or no fat mass, and as a result, have very little or no leptin.

They also have severe hypertriglyceridemia that is reversed by treatment with leptin [ , ]. The elevated plasma level of TGs in these patients is likely inducing leptin resistance that is preventing the leptin from inducing TGs to be used as an energy source.

Thus the TGs in these patients are not oxidized, and they are unable to settle into fat stores that would normally act as a TG sink and prevent their diversion to non-adipose tissues where they contribute to lipotoxicity and insulin resistance.

Transplantation of adipose tissue grafts in animal models of congenital lipoatrophy reverses the signs of the metabolic syndrome in a dose-dependent fashion [ ].

Furthermore, leptin treatment in humans and animals with lipodystrophies also reverses fatty liver and insulin resistance. These support the notion that insufficient leptin action may be a cause of metabolic syndrome, and that adequate leptin derived from SCAT is protective.

Like leptin, adiponectin secretion increases early on in obesity and plays a role in reducing the expression of lipogenic enzymes and increases FA oxidation in peripheral tissues thus limiting ectopic fat accumulation [ ]. The fact that adiponectin is secreted initially by fat but levels are reduced as fat depots increase, may help resolve the paradox of both lipodystrophy and obesity both being insulin-resistant states [ 73 ].

The CVATT has tremendous individual variation; thus a relatively "thin" individual with a normal BMI and an excess of VAT for him, may be metabolically obese, normal weight MONW [ 26 ].

Meanwhile, another individual with a large "pot belly" may have a great capacity to store fat as SCAT with relatively little VAT or he may have a high threshold for VAT.

This may explain the finding that while some individuals weighing even up to kg do not show any signs of type 2 diabetes or dyslipidemia, while in others, diabetes or dyslipidemia either develop or deteriorate with an increase in body weight of only one kg [ ] — perhaps just enough to exceed the CVATT.

A number of studies have looked at a possible CVATT [ — ]. Using CT scans to measure VAT volume, Williams et al found that a value of above cm 2 was associated with an increased risk of CHD in pre-and postmenopausal women [ ]. Similarly, Despres and Lamarche observed a VAT cutoff of cm 2 was associated with increased CHD risk in young adult men and premenopausal women mostly of French Canadian descent [ ], and a cutoff range of — cm 2 has also been observed by others [ , ].

De Nino et al found that insulin resistance did not appear until women were older than 60 years and had accumulated levels of VAT that approximated the levels seen in men, suggesting a possible threshold effect of VAT on insulin resistance [ ]. As discussed below with MONW, genetic and ethnic factors play a role.

Brochu et al were unable to demonstrate that obese postmenopausal women who reduced their weight and attained a level of VAT below cm 2 would show greater improvement in their metabolic profile compared to those who also lost weight but remained above the cm 2 VAT threshold [ ].

However, there were only 25 total subjects and the women had relatively normal metabolic profiles at baseline. Perhaps due to the relatively small number of subjects, only five lost less than 20 percent of their baseline VAT value.

Thus it is unclear whether even smaller losses of VAT than those observed improved metabolic outcomes. It should also be noted that in postmenopausal women, peripheral SCAT may be protective, even in the face of large amounts of VAT, and this needs to be accounted for [ , , ].

VAT accumulation contributes to metabolic risk factors in nonobese individuals [ , ]. Ruderman et al have shown that normal weight individuals may also have insulin resistance and the disorders of the metabolic syndrome [ 26 ].

They designated such individuals as "metabolically obese normal weight — MONW [ , ]. As pointed out earlier, the development of insulin resistance may limit further weight gain [ 34 , 38 — 41 , ].

A rapid and early development of insulin resistance prior to significant weight gain would explain that a significant number of the normal-weight population have insulin resistance [ 26 ].

The prevalence of MONW could be as high as 13 — 18 percent [ 26 , , ]. Both low birthweight LBW [ ] and lowest weight at one year of age have been linked to, VAT accumulation[ ] insulin resistance and cardiovascular risk factors in middle-aged and elderly individuals, many of whom could be classified as MONW with metabolic syndrome.

While some data suggest that LBW babies have central adiposity in middle age, definitive measurements of VAT in these individuals are still lacking [ 26 ]. One should consider ethnic differences when attempting to identify MONW subjects.

Lean appearing individuals, especially in certain ethnic groups such as the Japanese, may have significant amounts of VAT that surpass their CVATT but appear with what, for the general population, would be considered a normal BMI and waist circumference [ ].

In another study, relatively lean Japanese patients with newly diagnosed type 2 diabetes had increased VAT. Through diet and without medication for three months, the amount of VAT in these patients became comparable to that in normal-weight control subjects. Therefore, a three-month dietary treatment regimen with small to moderate weight loss was very effective in decreasing excess VAT in this population [ ].

This illustrates the importance of early recognition of an individual's approaching or exceeding his CVATT. Park et al were among the first to demonstrate that healthy, non-obese Asian American women may have higher amounts of VAT, and that normative values or standards for VAT derived from Caucasians may not be applicable to Asians [ ].

On the other side of the spectrum, a year prospective study studied increased BMI in Micronesian Nauruans an ethnic group from the central Pacific Ocean with rapidly increase in prevalence of obesity and Melanesian- and Indian-Fijians.

Overall, there was little evidence to suggest that obesity was a risk factor for total or cardiovascular mortality in these populations [ ]. McGarry found that one of his most obese patients in a series BMI Conversely, another subject, with a BMI of only This supports that insulin sensitivity appears to correlate more with where the fat is located rather than the total amount in the body [ 42 ].

This has implications for the phenomena of the metabolically obese normal weight MONW and the metabolically normal obese MNO individuals. Like some of the Micronesian Nauruans and Indian-Fijians above, there are individuals who are obese and who nevertheless are metabolically normal — "metabolically normal obese; MNO.

They often share an onset of obesity early in childhood, normal VAT, lower TGs, and increased HDL. The actively competitive Japanese wrestlers maintain their gross obesity by consuming a 5, to 6, calorie diet.

They are MNO, and their VAT is normal in amount, i. On retirement, when they discontinue their rigorous training regimen, they markedly develop increased insulin resistance and metabolic syndrome.

It is likely that that their VAT increases concomitantly [ 26 , 27 ] and exceeds their CVATT. MNO could account for as much as 20 percent of the obese population [ ].

In another study using HOMA to determine insulin resistance, Bonora et al showed that 11 percent of the entire group of overweight individuals fit the criteria of MNO [ ].

Brochu et al extensively studied 43 sedentary, obese, postmenopausal women and found that 17 were MNO, while 26 had reduced insulin sensitivity estimated by clamp [ ].

The two groups were similar in total body fat mass, SCAT amount, as well as waist circumference, and total daily energy expenditure. However, lean body mass was significantly greater in the metabolically abnormal subjects.

Unlike SCAT, VAT measured by CT was inversely related to the insulin sensitivity and to a classification of MNO. In fact, despite similar levels of total body fatness, MNO individuals showed 49 percent less VAT as measured by CT. However, the level of VAT was still significant.

Furthermore, using doubly labeled water and indirect calorimetry, Brochu et al were unable to demonstrate a meaningful difference between resting metabolic rate and daily physical energy expenditure between MNO and obese individuals at risk [ ]. Several investigators have found that there has been a positive association between insulin sensitivity and duration of obesity, i.

In one study 48 percent of the MNO women presented with a history of an earlier age-related onset of obesity between 13 and 19 years of age and less VAT compared with 29 percent of the metabolically abnormal obese [ ].

Insulin sensitivity seems to be dependent upon adipose cell size; as adipocytes within tissue grow larger, they become more insulin resistant [ ]. Normal-sized, more insulin-sensitive adipocytes have been associated with early onset of obesity [ ].

Perhaps today we are beginning to see that with the marked increase in overfeeding and extent of obesity at younger ages, hypertrophy of fat cells may occur earlier and hence metabolic syndrome is now occurring with greater frequency in children. During puberty, a certain degree of insulin resistance is normal, and children who are more insulin resistant have decreased SCAT fat gain [ ].

Early in the development of juvenile obesity, increased VAT, hyperinsulinemia, and insulin resistance are closely linked [ ]. Adrenal androgens are elevated in obese children and have been associated with early pubertal development in these children[ , ] Sex differences in VAT begin to emerge during puberty, with boys having more VAT than girls.

Some studies suggest that the rate of VAT accumulation can be slowed in children by using exercise interventions [ , ]. VAT is strongly associated with fitness even within individuals of the same weight. This is illustrated by the earlier mentioned example of the active Sumo wrestler in his prime who has relatively little VAT [ 91 ].

Regular exercise can selectively reduce VAT with minimal change in weight [ — ]. This could especially add to the frustration level of the middle-aged or post-menopausal woman who regularly exercises moderately without inducing measurable reduction in body weight or fatness.

She may still benefit from reducing her VAT or attenuating the gain of VAT "normally" experienced by sedentary women as they age. It should be emphasized that the lower VAT level associated with increased fitness is modest but nonetheless clinically important.

Reduced morbidity is likely explained by factors in addition to a reduced VAT, and VAT likely explains morbidity independent of fitness [ ].

Sumo wrestlers tend to have most of their central adiposity stored subcutaneously as SCAT , and, perhaps a shift toward more VAT accompanies their contracting metabolic syndrome upon their retirement — with premature death to follow [ 26 , 91 ].

This may also explain the body of work showing that overweight or "fat" individuals who are fit according to cardiorespiratory testing on a treadmill are at less risk for a cardiac event or developing type 2 diabetes than a "leaner" individual who is unfit [ , ].

Thus, the former could be considered "fit and fat. CRF is also associated with lower abdominal fat independent of BMI, and for a given BMI or waist circumference WC , individuals with moderate CRF had lower levels of total fat mass and abdominal SCAT and VAT than individuals with low CRF for a given BMI or WC value [ , ].

Low CRF is an independent risk factor for mortality in healthy-appearing and diseased populations, and is associated with elevated CRP and reduced fasting glucose control in women with type 2 diabetes [ ].

It is likely that compared to the fit and fat, the unfit and lean-appearing individual may have greater amounts of "hidden" VAT. In obese patients, increasing physical activity can enhance fat oxidation, reduce IMCL and improve insulin sensitivity [ ].

Exercise training may reduce waist size, independent of changes in BMI, and exercise without weight loss is effective in reducing VAT and preventing further increases in obesity [ , ]. Ross et al showed that either modality, caloric restriction alone or daily exercise without calorie restriction, is an effective strategy for reducing obesity in moderately obese men.

Their findings also suggest that exercise without weight loss is a useful method for reducing VAT and preventing further increases in obesity [ ]. Irwin et al studied overweight, postmenopausal, previously sedentary women in a randomly controlled trial of exercise versus no exercise.

While the body weight lost at 12 months among the exercisers was modest, the amount of intra-abdominal fat lost was considerable 8. Exercise may counteract the abnormal metabolic profiles associated with abdominal obesity by reducing VAT along with other independent mechanisms.

It promotes adaptive responses including those causing muscles to increase their use of lipid stores rather than relying primarily on carbohydrate reserves. Even a single bout of exercise can reduce triglyceride levels, increase HDL levels, reduce resting blood pressure, increase glucose tolerance, and reduce insulin resistance [ ].

While evidence supports that CRF may be associated with a lower VAT, this is certainly not proven. However, study results suggest that individuals with moderate to high CRF levels have lower WC than men with low CRF independent of BMI [ , ].

This is reinforced by the finding that reductions in VAT alone were related to improvements in glucose tolerance and insulin sensitivity [ , ]. Therefore, it would seem reasonable to infer that the combination of high CRF and low abdominal fat, especially VAT, would be associated with reductions in metabolic risk compared with those with the same BMI, but low CRF and high VAT [ ].

Adding resistance training to aerobic exercise may add to an improvement in insulin sensitivity related to a loss of VAT and an increase in muscle density [ , ].

Surgical removal of VAT in animals and humans dramatically improves insulin resistance and diabetes. In a Swedish, single-center, randomized and controlled pilot trial of 50 severely obese adults, Thorne et al compared 25 patients who underwent adjustable gastric banding AGB alone with AGB plus surgical removal of the total greater omentum.

At two-year follow-up there were no statistical differences between groups with regard to weight loss, changes in WHR or sagittal diameter. However, the improvements in oral glucose tolerance insulin sensitivity and fasting plasma glucose and insulin were 2—3 times greater in omentectomized as compared to control subjects, which was statistically independent of the loss in BMI [ 52 ].

More recently, this has led to a study of another experimental procedure performed by surgeons at Boston's Beth Israel Deaconess Medical Center working in conjunction with Joslin Diabetes Center.

Using a two-hour laparoscopic procedure that involves extracting strips of only the omentum through tiny incisions, this will be the first study to examine the possible health benefits of removing only the omentum [ ].

Recently Klein et al. demonstrated that liposuction conferred no benefits with regard to metabolic profile [ 53 ]. Together with the findings above, these support a pathologic role for VAT and a possible protective role for SCAT.

Removing SCAT might actually increase risk as one removes a buffer or sink for peripheral TGs [ ]. Since our genes have not changed significantly in the past 10, years, the rise in obesity can be attributed to the environment, including what we are exposed to in the way of food as well as the level of physical activity.

While the main focus has been on diet and activity, what may be overlooked is the tremendous increase in exposure to synthetic organic and inorganic chemicals, which can damage many of the mechanisms involved in weight control. Most of us have been exposed to organochlorines found in pesticides, dyes, solvents, etc.

Thus, the obese tend to have increased organochlorine concentrations compared to lean individuals [ ]. During body weight loss, a decrease in fat mass results in lipid mobilization, and organochlorine concentrations increase both in plasma and remaining adipose tissue.

Even after adjustment for weight loss, the related increase in organochlorine concentration has been correlated with decreases in triidothyronine T3 concentration and resting metabolic rate [ ].

This is also associated with a reduction in activity of the skeletal muscle oxidative enzymes that normally are involved in fat oxidation [ ]. The net effect could prevent further weight gain and might even encourage weight regain beyond the initial baseline [ ], which could contribute to VAT.

Frisancho points out that an important contributing factor for obesity in modern as well as developing nations is a reduced fat oxidation and increased metabolism of carbohydrate. This has been brought about by a shift toward the body's preference toward oxidizing carbohydrate rather than fat — resulting in an increased deposition of body fat.

In developing nations, obesity can co-exist with developmental undernutrition, which can result in obesity with short stature [ ]. A solution to reducing the ectopic fat, as well as VAT, burden would be to enhance its oxidation in nonadipose tissues, e.

This will push the system toward below the CVATT and improve insulin sensitivity. Such diets will reduce muscle glycogen content and carbohydrate oxidation, even in well-trained athletes who already demonstrate increased oxidation [ 37 , ].

The authors' paradigm suggests that, under these conditions, insulin resistance could improve by reducing glucose appearance and cellular influx, resulting in a preferential fat oxidation and protection against lipotoxicity. In an elegant study, Bisschop et al support this by showing that high-fat, low-carbohydrate diets do not affect the action of insulin on total glucose disposal but decrease basal endogenous glucose production and improve insulin-stimulated nonoxidative glucose disposal [ ].

Sharman et al demonstrated short term improvements of a ketogenic diet on lipids in normal weight men. These benefits occurred without total weight loss but there was evidence of a change in body composition toward more lean body mass [ ].

One would also expect a reduction in VAT as he moves to the left or below his CVATT See figure 1. Weight loss does not appear to be necessary to reduce mortality rates in overweight or obese men who increase their aerobic fitness or level of physical activity [ ]. Similarly, in overweight, postmenopausal women, exercise may lead to improved metabolic profiles and VAT loss without total weight loss [ ].

Optimizing macronutrients and food preparation can have beneficial effects in those with visceral obesity. A number of recent reviews support the metabolic benefits of controlling glycemic index GI [ ] and glycemic load GL [ ].

In a month pilot study of teens, compared to a conventional diet, a lower GI diet led to greater total weight and fat loss without regain from months 6— While insulin resistance as measured by HOMA increased in the conventional group possibly in part attributable to puberty , the lower GI group showed no change [ ].

Recently, Silvestre et al showed that compared to an energy-restricted low-fat diet, a short-term, very low-carbohydrate diet was associated with greater weight and fat loss with an apparent preferential loss of central fat [ ].

VAT cells have a two-fold higher glucose uptake rate compared with SCAT cells [ 87 ]. It may follow that reducing glucose exposure by reducing glycemic load may reduce the supply of glucose to the VAT depot and possibly impair its accumulation.

Glucose raises insulin concentration, which can stimulate β-HSD1, increase active cortisol in VAT, and enhance VAT accumulation [ ].

Feeding rats a high-GI starch diet over five weeks resulted in higher VAT and larger adipocyte volume than did feeding low-GI starch ad libitum. Replacing this with a low-GI starch diet increased insulin -stimulated glucose oxidation, decreased glucose incorporation into total lipids and decreased VAT adipocyte diameter [ , ].

Together, these add to the evidence supporting the benefits of lowering GI to reduce and maintain lower volumes of VAT. Feeding rats a high sucrose diet increases both VAT and muscle insulin resistance [ ].

Keno et al. demonstrated in rats that a high sucrose diet compared to a lab chow diet led to a significantly greater fat cell volume in VAT depots [ ]. Although fat cell number did not change, the ratio of VAT weight to SCAT weight was also significantly increased in the rats fed a high sucrose diet, providing further evidence for controlling the dietary GI and GL.

A number of studies have demonstrated an association between glycemic load GL and levels of CRP [ , ], which is a powerful predictor for diabetes and CHD, and is positively associated with both insulin resistance and the prevalence of the metabolic syndrome [ ].

O'Brien et al showed that compared to a high carbohydrate diet, a low carbohydrate diet reduced SAA and CRP, both markers of inflammation and risk factors for metabolic syndrome [ ].

Relative to fat cream and protein casein , a glucose challenge elicits the greatest production of radical oxygen species ROS by polymorphonuclear and mononuclear white cells [ , ]. Chronic carbohydrate ingestion with a high GL diet can lead to hyperinsulinemia, as well as hypertrophy, functional dysregulation, and overresponsiveness of the pancreatic β cell and hepatic production of newly synthesized fatty acids via de novo lipogenesis [ 43 ].

A Johns Hopkins study examined intra-operative liver biopsies obtained from 74 consecutive morbidly obese patients undergoing bariatric surgery. Compared with patients with the lowest carbohydrate intake [ ], a high-carbohydrate diet was associated with an odds ratio of 7.

A high fat diet appeared to be protective, with those in the highest fat intake group having an OR of 0. This is consistent with the findings of others who found that dietary fat explained only two percent of the variance in general adiposity and dietary fat appears to play only a minor role in determining general adiposity and is not related to VAT when measured in cross-sectional studies [ ].

Apparently, GL may be more significant in this regard. Compared to SCAT, VAT both adipose and non-adipose cells within VAT is associated more with PAI-1 — a powerful risk factor for CHD [ 58 , ]. The results support the potential benefit of lowering dietary GI in patients with metabolic syndrome, especially those with VAT and elevated PAI This is also supported by the observation of hyperglycemia induces PAI-1 gene expression in adipose tissue of rats [ ].

Esposito et al demonstrated in both diabetics and non-diabetics that after consuming a high carbohydrate high-fiber meal, IL a potent pro-inflammatory cytokine concentrations increased [ 49 ].

Adiponectin concentrations decreased after the high-carbohydrate, low-fiber meal in diabetic patients. The fiber content of complex carbohydrates seemed to affect circulating IL and adiponectin concentrations in response to the same carbohydrate load.

The pizza that was made with whole flour and was rich in fiber was associated with reduce serum IL concentrations and unchanged serum adiponectin concentrations. Meanwhile, the pizza prepared with refined flour and was low in fiber raised circulating IL concentrations.

Serum glucose and TG concentrations were not significantly different between the two types of pizza. The study did not completely resolve the mechanism by which the fiber content of meals influences IL and adiponectin. However, it appears that while the GL of each pizza was the same, the GI of the whole wheat pizza would be much less and may be more beneficial.

Recently, dietary TGs have been demonstrated to contribute to CNS leptin resistance by impairing the transport of leptin across the blood brain barrier where it would usually stimulate the release of neuropeptide-Y and reduce feeding behavior [ ].

Reducing dietary carbohydrates lowers serum TGs, which theoretically should protect against this form of leptin resistance [ ]. Leptin may enhance fatty acid oxidation and protects against fat deposition and lipotoxicity. As mentioned earlier, normally, rats can tolerate a 60 percent fat diet because 96 percent of the surplus fat is stored in an enlarging adipose tissue mass, in which leptin gene expression increases proportionally [ ].

However, when leptin is congenitally absent or inactive, or ineffective due to resistance, even on a normal or low-fat diet, unutilized dietary fat is deposited in nonadipose tissues, causing dysfunction lipotoxicity , and possible cell death lipoaptosis [ 29 ]. Acute overfeeding can cause circulating leptin levels to rise by 40 percent and more than three-fold after chronic overfeeding, whereas fasting is associated with a decreased leptin levels.

This may suggest that overfeeding leads to leptin resistance. Dietary carbohydrates may influence leptin action. Some investigators have suggested that the increase in plasma leptin concentration observed after meals is not simply a result of an energy load but is in response to a signal that is not present following a fat load without carbohydrate [ ].

SCAT-derived leptin which circulates in a free form and is bound to a soluble leptin receptor — sOB-R plays a key role in regulating energy homeostasis and metabolism, sOB-R is positively associated with energy intake from carbohydrates and negatively associated with energy intake from dietary fat [ ].

While this suggests that dietary fat and carbohydrates regulate free leptin levels, the implications of this are not yet completely clear. There is an association with lifestyle, worry, cortisol levels, and abdominal girth.

Those who were found to have the highest levels of chronic stress had the highest levels of cortisol and VAT [ — ].

This is supported by evidence that a number of medications, including prednisone, may cause an excess of cortisol and insulin resistance.