Increasing dietary fat elicits similar changes in fat oxidation and markers of muscle oxidative capacity in lean and obese humans

Mitochondrial puzzle in muscle: Linking the electron transport system to overweight

Human skeletal muscle mitochondria regulate energy expenditure. Research has shown that the functionality of muscle mitochondria is altered in subjects with overweight, as well as in response to nutrient excess and calorie restriction. Two metabolic features of obesity and overweight are (1) incomplete muscular fatty acid oxidation and (2) increased circulating lactate levels. In this study, I propose that these metabolic disturbances may originate from a common source within the muscle mitochondrial electron transport system. Specifically, a reorganization of the supramolecular structure of the electron transport chain could facilitate the maintenance of readily accessible coenzyme Q pools, which are essential for metabolizing lipid substrates. This approach is expected to maintain effective electron transfer, provided that there is sufficient complex III to support the Q-cycle. Such an adaptation could enhance fatty acid oxidation and prevent mitochondrial overload, thereby reducing lactate production. These insights advance our understanding of the molecular mechanisms underpinning metabolic dysregulation in overweight states. This provides a basis for targeted interventions in the quest for metabolic health.

FAT/CD36 Participation in Human Skeletal Muscle Lipid Metabolism: A Systematic Review

Fatty acid translocase/cluster of differentiation 36 (FAT/CD36) is a multifunctional membrane protein activated by a high-fat diet, physical exercise, fatty acids (FAs), leptin, and insulin. The principal function of FAT/CD36 is to facilitate the transport of long-chain fatty acids through cell membranes such as myocytes, adipocytes, heart, and liver. Under high-energy expenditure, the different isoforms of FAT/CD36 in the plasma membrane and mitochondria bind to the mobilization and oxidation of FAs. Furthermore, FAT/CD36 is released in its soluble form and becomes a marker of metabolic dysfunction. Studies with healthy animals and humans show that physical exercise and a high-lipid diet increase FAT/CD36 expression and caloric expenditure. However, several aspects such as obesity, diabetes, Single Nucleotide polymorphisms (SNPs), and oxidative stress affect the normal FAs metabolism and function of FAT/CD36, inducing metabolic disease. Through a comprehensive systematic review of primary studies, this work aimed to document molecular mechanisms related to FAT/CD36 in FAs oxidation and trafficking in skeletal muscle under basal conditions, physical exercise, and diet in healthy individuals.

Breastfeeding duration modifies the association between maternal weight status and offspring dietary palmitate oxidation

BACKGROUND

Offspring of obese rodents develop a metabolic phenotype that favors fat deposition. Data regarding the impact of maternal obesity programing of offspring fuel usage in humans is scarce.

OBJECTIVE

The objective of this study was to explore the association between maternal weight status and dietary palmitate oxidation (DPO) in 2-y-old offspring, taking into consideration potential confounders and modifiers.

METHODS

Women (n = 56) were enrolled by the first trimester of gestation. Maternal physical activity (PA; measured with accelerometers) at enrollment and gestational weight gain (GWG) were measured. Offspring sex, race, and breastfeeding (BF) duration were self-reported. Human milk (HM) composition was determined at 6 mo postpartum. At age 2 y, dietary quality [healthy eating index (HEI)] and parental feeding practices [Child Feeding Questionnaire (CFQ)] were assessed. DPO in 2-y-olds (2-yo-DPO) was measured using deuterated palmitic acid. Generalized linear regression analysis was used to model the associations of 2-yo-DPO with maternal weight status [normal weight (NW), BMI <25 (in kg/m2) compared with excessive weight (EW), BMI ≥25].

RESULTS

DPO was higher in offspring of women with EW compared with NW (2.1 ± 1.2%/h compared with 1.4 ± 0.7%/h, P = 0.03). Maternal weight status interacted with BF duration in association with 2-yo-DPO (log ß: 0.05, P = 0.04). Specifically, 2-yo-DPO was higher in the EW compared with NW group if BF duration was ≥9 mo. HM insulin (log ß: 0.35, P = 0.002) and HM leptin (log ß: 0.81, P = 0.001) concentrations directly associated with 2-yo-DPO. PA (log ß: 0.06, P = 0.013), parental feeding restriction (log ß: 0.05, P < 0.0001), and male sex (log ß: 0.54, P < 0.001) were positively associated with 2-yo-DPO. HEI was negatively associated with 2-yo-DPO (log ß:-0.03, P < 0.0001).

CONCLUSIONS

Higher 2-yo-DPO in offspring of women with EW compared with NW were driven by BF duration. Higher HM insulin and leptin concentrations in women with EW may explain these finding. More studies are needed to confirm these results. This trial was registered at clinicaltrials.gov as NCT03281850.

Association Between Adipose Tissue Characteristics and Metabolic Flexibility in Humans: A Systematic Review

Adipose tissue total amount, distribution, and phenotype influence metabolic health. This may be partially mediated by the metabolic effects that these adipose tissue characteristics exert on the nearby and distant tissues. Thus, adipose tissue may influence the capacity of cells, tissues, and the organism to adapt fuel oxidation to fuel availability, i.e., their metabolic flexibility (MetF). Our aim was to systematically review the evidence for an association between adipose tissue characteristics and MetF in response to metabolic challenges in human adults. We searched in PubMed (last search on September 4, 2021) for reports that measured adipose tissue characteristics (total amount, distribution, and phenotype) and MetF in response to metabolic challenges (as a change in respiratory quotient) in humans aged 18 to <65 years. Any study design was considered, and the risk of bias was assessed with a checklist for randomized and non-randomized studies. From 880 records identified, 22 remained for the analysis, 10 of them measured MetF in response to glucose plus insulin stimulation, nine in response to dietary challenges, and four in response to other challenges. Our main findings were that: (a) MetF to glucose plus insulin stimulation seems inversely associated with adipose tissue total amount, waist circumference, and visceral adipose tissue; and (b) MetF to dietary challenges does not seem associated with adipose tissue total amount or distribution. In conclusion, evidence suggests that adipose tissue may directly or indirectly influence MetF to glucose plus insulin stimulation, an effect probably explained by skeletal muscle insulin sensitivity. Systematic Review Registration: PROSPERO [CRD42020167810].

Dietary fatty acids as nutritional modulators of sirtuins: a systematic review

CONTEXT

The sirtuins (SIRT1 to SIRT7) constitute a family of highly conserved nicotinamide adenine dinucleotide-dependent proteins. When activated, sirtuins control essential cellular processes to maintain metabolic homeostasis, while lack of expression of sirtuins has been related to chronic disease.

OBJECTIVE

The aim of this systematic review is to analyze the role of fat consumption as a modulator of human sirtuins.

DATA SOURCES

This review was conducted according to PRISMA guidelines. Studies were identified by searches of the electronic databases PubMed/MEDLINE, Scopus, and Web of Science.

STUDY SELECTION

Randomized clinical trials assessing the effect of fatty acid consumption on sirtuin mRNA expression, sirtuin protein expression, or sirtuin protein activity were eligible for inclusion.

DATA EXTRACTION

Two authors screened and determined the quality of the studies; disagreements were resolved by the third author. All authors compared the compiled data.

RESULTS

Seven clinical studies with 3 different types of interventions involving healthy and nonhealthy participants were selected. Only SIRT1 and SIRT3 were evaluated. Overall, the evidence from clinical studies to date is insufficient to understand how lipid consumption modulates sirtuins in humans. The best-characterized mechanism highlights oleic acid as a natural activator of SIRT1.

CONCLUSION

These results draw attention to a new field of interest in nutrition science. The possible activation of sirtuins by dietary fat manipulation may represent an important nutritional strategy for management of chronic and metabolic disease.

SYSTEMATIC REVIEW REGISTRATION

PROSPERO registration number CRD42018114456.

Tuning fatty acid oxidation in skeletal muscle with dietary fat and exercise

Both the consumption of a diet rich in fatty acids and exercise training result in similar adaptations in several skeletal muscle proteins. These adaptations are involved in fatty acid uptake and activation within the myocyte, the mitochondrial import of fatty acids and further metabolism of fatty acids by β-oxidation. Fatty acid availability is repeatedly increased postprandially during the day, particularly during high dietary fat intake and also increases during, and after, aerobic exercise. As such, fatty acids are possible signalling candidates that regulate transcription of target genes encoding proteins involved in muscle lipid metabolism. The mechanism of signalling might be direct or indirect targeting of peroxisome proliferator-activated receptors by fatty acid ligands, by fatty acid-induced NAD⁺-stimulated activation of sirtuin 1 and/or fatty acid-mediated activation of AMP-activated protein kinase. Lactate might also have a role in lipid metabolic adaptations. Obesity is characterized by impairments in fatty acid oxidation capacity, and individuals with obesity show some rigidity in increasing fatty acid oxidation in response to high fat intake. However, individuals with obesity retain improvements in fatty acid oxidation capacity in response to exercise training, thereby highlighting exercise training as a potential method to improve lipid metabolic flexibility in obesity.

Fiber Intake Predicts Weight Loss and Dietary Adherence in Adults Consuming Calorie-Restricted Diets: The POUNDS Lost (Preventing Overweight Using Novel Dietary Strategies) Study

BACKGROUND

The effects of dietary composition on weight loss are incompletely understood. In addition to energy intake, fiber intake, energy density, macronutrient composition, and demographic characteristics have all been suggested to contribute to weight loss.

OBJECTIVE

The primary aim of this analysis was to assess the role of dietary fiber as a predictor of weight loss in participants who consumed calorie-restricted diets (-750 kcal/d from estimated energy needs) for 6 mo, using data from the POUNDS Lost (Preventing Overweight Using Novel Dietary Strategies) Study-a randomized trial that examined the effects of calorie-restricted diets varying in macronutrient composition on weight loss in adults.

METHODS

Data were randomly partitioned to a training data set (70%) in which the effects of fiber and other weight-loss predictors were identified using adjusted Least Absolute Shrinkage and Selection Operator and model averaging. The retained predictors were then fit on the testing data set to assess predictive performance.

RESULTS

Three hundred and forty-five participants (53.9% female) provided dietary records at baseline and 6 mo. Mean ± SD age and BMI for the full sample was 52.5 ± 8.7 y and 32.6 ± 3.9 kg/m2, respectively. Mean ± SD (99% CI) weight change at 6 mo for the full sample was -7.27 ± 5.6 kg (-8.05, -6.48 kg). The final, best fit model (R2 = 0.41) included fiber, energy density, fat, age, adherence, baseline weight, race, and changes from baseline in carbohydrate, fiber, PUFA, and MUFA intake, but the most influential predictor was fiber intake ($\hat{\beta }$ = -0.37; P < 0.0001). In addition, fiber was strongly associated with adherence to the macronutrient prescriptions (P < 0.0001). Interactions between race and adherence, age, baseline weight, carbohydrate, energy density, and MUFAs were also retained in the final model.

CONCLUSION

Dietary fiber intake, independently of macronutrient and caloric intake, promotes weight loss and dietary adherence in adults with overweight or obesity consuming a calorie-restricted diet. This trial was registered at clinicaltrials.gov as NCT00072995.

Genetic Evidence That Carbohydrate-Stimulated Insulin Secretion Leads to Obesity

BACKGROUND

A fundamental precept of the carbohydrate-insulin model of obesity is that insulin secretion drives weight gain. However, fasting hyperinsulinemia can also be driven by obesity-induced insulin resistance. We used genetic variation to isolate and estimate the potentially causal effect of insulin secretion on body weight.

METHODS

Genetic instruments of variation of insulin secretion [assessed as insulin concentration 30 min after oral glucose (insulin-30)] were used to estimate the causal relationship between increased insulin secretion and body mass index (BMI), using bidirectional Mendelian randomization analysis of genome-wide association studies. Data sources included summary results from the largest published metaanalyses of predominantly European ancestry for insulin secretion (n = 26037) and BMI (n = 322154), as well as individual-level data from the UK Biobank (n = 138541). Data from the Cardiology and Metabolic Patient Cohort study at Massachusetts General Hospital (n = 1675) were used to validate genetic associations with insulin secretion and to test the observational association of insulin secretion and BMI.

RESULTS

Higher genetically determined insulin-30 was strongly associated with higher BMI (β = 0.098, P = 2.2 × 10^-21), consistent with a causal role in obesity. Similar positive associations were noted in sensitivity analyses using other genetic variants as instrumental variables. By contrast, higher genetically determined BMI was not associated with insulin-30.

CONCLUSIONS

Mendelian randomization analyses provide evidence for a causal relationship of glucose-stimulated insulin secretion on body weight, consistent with the carbohydrate-insulin model of obesity.

Taurine supplementation increases irisin levels after high intensity physical training in obese women

BACKGROUND

Irisin is a myokine/adipokine that under stimulus of physical exercise is able to improve thermogenic capacity in adipose tissue. Likewise, taurine supplementation has demonstrated similar effects on energy metabolism. Therefore, we hypothesized that taurine supplementation combined with physical training may induce an increase in irisin concentrations, optimizing energy metabolism in obese individuals.

OBJECTIVE

To evaluate if taurine supplementation associated with a high intensity physical training program increases irisin levels in obese women.

METHODS

double-blind study with 22 obese women (BMI 32.4 ± 2.0 kg/m², 36.6 ± 6.4 years and sedentary) who were randomly divided into two groups, control group (GC, n = 14), exercised and supplemented with placebo (3 g of starch), and taurine group (GTAU, n = 8), exercised and supplemented with taurine (3 g). The subjects performed high intensity physical training, Deep Water Running (DWR), for 8 weeks, 3 times/week, for 50 min per training session, at 70-85% maximum heart rate. Resting metabolic rate (RMR) was evaluated by indirect calorimetry, body composition by deuterium oxide, plasma taurine by HPLC, plasma irisin by Multiplex Kit, and food consumption by food records. The results were analyzed by an ANOVA two way repeated measures mixed model, with the Sidak post hoc (p < 0.05).

RESULTS

No changes were observed in body composition. DWR increased RMR independent of supplementation (p < 0.001) and irisin levels (pg/mL) showed a significant difference only in the GTAU in 1 h after exercise (p < 0.001).

CONCLUSION

DWR associated with taurine supplementation resulted in increased plasma irisin concentrations after physical training in obese adult women.

Evaluation of Energy Expenditure and Oxidation of Energy Substrates in Adult Males after Intake of Meals with Varying Fat and Carbohydrate Content

Obesity is a result of positive energy balance. The aim of this study was to measure (in crossover trials) the energy expenditure and oxidation of glucose and lipids, both at the fasting state and after an intake of meals with a varying macronutrient content, in normal-weight and overweight/obese people. In the study, 46 healthy adult males (23 with normal body weight and 23 overweight/obese), aged 21–58, were examined. During two consecutive visits, subjects received isocaloric standardized meals (450 kcal) with different content of basic nutrients. Resting metabolic rate and carbohydrate and fat utilization were evaluated during the fasting state and postprandially, using an indirect calorimetry method. Energy expenditure was higher in people with normal body weight and slightly higher after the high-carbohydrate meal. In overweight/obese people, increased expenditure was noted after normo-carbohydrate meal intake. The high-fat meal induced lower postprandial thermal response compared to a high-carbohydrate meal, both in people with normal body weight and in overweight/obese men. Glucose utilization was higher after the high-carbohydrate meal, and it was higher in the normal body weight group than in overweight/obese people. In addition, overweight/obese people showed a lower level of fatty acid oxidation under fasting conditions which, together with limited ability to oxidize energy substrates, depending on their availability, indicates that these people are characterized by lower metabolic flexibility.

A shift toward a high-fat diet in the current metabolic paradigm: A new perspective

OBJECTIVE

Investigations into the relationship between dietary carbohydrate restriction and health are mixed. Current guidelines for nutrition promote low-fat foods and higher carbohydrate consumption for optimal health and weight loss. However, high-fat, low-carbohydrate diets are revealing both intra- and extracellular adaptations that have been shown to elicit favorable cardiometabolic changes associated with obesity. Moreover, dietary fat is associated with higher satiety levels from the hormones adiponectin, leptin, and cholecystokinin. Additionally, insulin responses from high-glycemic carbohydrates are known to alter these pathways, potentially leading to an increase in energy consumption and a possible mechanism for obesity.

CONCLUSION

There is convincing evidence of beneficial effects of controlled trials implementing high-fat, low-carbohydrate diets in both sedentary and obese individuals, but longer duration clinical trials are required to confirm this hypothesis.

Obesity Energetics: Body Weight Regulation and the Effects of Diet Composition

Weight changes are accompanied by imbalances between calorie intake and expenditure. This fact is often misinterpreted to suggest that obesity is caused by gluttony and sloth and can be treated by simply advising people to eat less and move more. Rather various components of energy balance are dynamically interrelated and weight loss is resisted by counterbalancing physiological processes. While low-carbohydrate diets have been suggested to partially subvert these processes by increasing energy expenditure and promoting fat loss, our meta-analysis of 32 controlled feeding studies with isocaloric substitution of carbohydrate for fat found that both energy expenditure (26 kcal/d; P <.0001) and fat loss (16 g/d; P <.0001) were greater with lower fat diets. We review the components of energy balance and the mechanisms acting to resist weight loss in the context of static, settling point, and set-point models of body weight regulation, with the set-point model being most commensurate with current data.

Impact of Phytochemicals and Dietary Patterns on Epigenome and Cancer

Cancer remains one of the leading causes of death around the world. Initially it is recognized as a genetic disease, but now it is known to involve epigenetic abnormalities along with genetic alterations. Epigenetics refers to heritable changes that are not encoded in the DNA sequence itself, but play an important role in the control of gene expression. It includes changes in DNA methylation, histone modifications, and RNA interference. Although it is heritable, environmental factors such as diet could directly influence epigenetic mechanisms in humans. This article will focus on the role of dietary patterns and phytochemicals that have been demonstrated to influence the epigenome and more precisely histone and non-histone proteins modulation by acetylation that helps to induce apoptosis and phosphorylation inhibition, which counteracts with cells proliferation. Recent developments discussed here enhance our understanding of how dietary intervention could be beneficial in preventing or treating cancer and improving health outcomes.

Oral exposure of pubertal male mice to endocrine-disrupting chemicals alters fat metabolism in adult livers

The potential for the exposure of humans and wildlife to environmental endocrine-disrupting chemicals (EDCs) has been increasing. Risk assessment for such EDCs is primarily based on detecting the main endpoints related to the endocrine and reproductive systems, while the effects on glucose and fat metabolism have only received limited attention. In this study, pubertal male C57BL/6J mice were orally administered 10 mg/kg body weight cypermethrin (CYP), 100 mg/kg body weight atrazine (ATZ), and 0.1 mg/kg body weight 17α-ethynyestradiol (EE2) for 4 weeks and then switched to a high-energy diet (HD) for 8 weeks. The body weight gain in the EDC-treated groups was lower than that in the control group during exposure and then tended to show values similar to the HD group. The epididymal fat weight, cell size and serum triacylglycerol (TG) and total cholesterol (TCH) levels in the EDC-HD groups were lower than those in the HD group. The transcription of genes related to glycolytic and gluconeogenic processes in the liver was affected by EDC exposure. Furthermore, the expression levels of transcriptional factors including PPARα, PPARγ, and SREBP1C and their target genes related to fatty acid synthesis and oxidation in the liver were also influenced by early life EDC administration. The results showed that early-life-stage exposure to high doses of various environmental EDCs affected the homeostasis of glucose and fatty acid metabolism in the livers of adult male mice.

Calories, carbohydrates, and cancer therapy with radiation: exploiting the five R's through dietary manipulation

Aggressive tumors typically demonstrate a high glycolytic rate, which results in resistance to radiation therapy and cancer progression via several molecular and physiologic mechanisms. Intriguingly, many of these mechanisms utilize the same molecular pathways that are altered through calorie and/or carbohydrate restriction. Furthermore, poorer prognosis in cancer patients who display a glycolytic phenotype characterized by metabolic alterations, such as obesity and diabetes, is now well established, providing another link between metabolic pathways and cancer progression. We review the possible roles for calorie restriction (CR) and very low carbohydrate ketogenic diets (KDs) in modulating the five R's of radiotherapy to improve the therapeutic window between tumor control and normal tissue complication probability. Important mechanisms we discuss include (1) improved DNA repair in normal, but not tumor cells; (2) inhibition of tumor cell repopulation through modulation of the PI3K-Akt-mTORC1 pathway downstream of insulin and IGF1; (3) redistribution of normal cells into more radioresistant phases of the cell cycle; (4) normalization of the tumor vasculature by targeting hypoxia-inducible factor-1α downstream of the PI3K-Akt-mTOR pathway; (5) increasing the intrinsic radioresistance of normal cells through ketone bodies but decreasing that of tumor cells by targeting glycolysis. These mechanisms are discussed in the framework of animal and human studies, taking into account the commonalities and differences between CR and KDs. We conclude that CR and KDs may act synergistically with radiation therapy for the treatment of cancer patients and provide some guidelines for implementing these dietary interventions into clinical practice.

Glucose homeostasis during short-term and prolonged exposure to high altitudes

Most of the literature related to high altitude medicine is devoted to the short-term effects of high-altitude exposure on human physiology. However, long-term effects of living at high altitudes may be more important in relation to human disease because more than 400 million people worldwide reside above 1500 m. Interestingly, individuals living at higher altitudes have a lower fasting glycemia and better glucose tolerance compared with those who live near sea level. There is also emerging evidence of the lower prevalence of both obesity and diabetes at higher altitudes. The mechanisms underlying improved glucose control at higher altitudes remain unclear. In this review, we present the most current evidence about glucose homeostasis in residents living above 1500 m and discuss possible mechanisms that could explain the lower fasting glycemia and lower prevalence of obesity and diabetes in this population. Understanding the mechanisms that regulate and maintain the lower fasting glycemia in individuals who live at higher altitudes could lead to new therapeutics for impaired glucose homeostasis.

Insulin sensitivity is independent of lipid binding protein trafficking at the plasma membrane in human skeletal muscle: effect of a 3-day, high-fat diet

The aim of the present study was to investigate lipid-induced regulation of lipid binding proteins in human skeletal muscle and the impact hereof on insulin sensitivity. Eleven healthy male subjects underwent a 3-day hypercaloric and high-fat diet regime. Muscle biopsies were taken before and after the diet intervention, and giant sarcolemmal vesicles were prepared. The high-fat diet induced decreased insulin sensitivity, but this was not associated with a relocation of FAT/CD36 or FABPpm protein to the sarcolemma. However, FAT/CD36 and FABPpm mRNA, but not the proteins, were upregulated by increased fatty acid availability. This suggests a time dependency in the upregulation of FAT/CD36 and FABPpm protein during high availability of plasma fatty acids. Furthermore, we did not detect FATP1 and FATP4 protein in giant sarcolemmal vesicles obtained from human skeletal muscle. In conclusion, this study shows that a short-term lipid-load increases mRNA content of key lipid handling proteins in human muscle. However, decreased insulin sensitivity after a high-fat diet is not accompanied with relocation of FAT/CD36 or FABPpm protein to the sarcolemma. Finally, FATP1 and FATP4 protein was located intracellularly but not at the sarcolemma in humans.

Twenty-four hour total and dietary fat oxidation in lean, obese and reduced-obese adults with and without a bout of exercise

Background

It has been hypothesized that obese and reduced-obese individuals have decreased oxidative capacity, which contributes to weight gain and regain. Recent data have challenged this concept.

Objective

To determine (1) whether total and dietary fat oxidation are decreased in obese and reduced-obese adults compared to lean but increase in response to an acute exercise bout and (2) whether regular physical activity attenuates these metabolic alterations.

Design

We measured 24-hr total (whole-room calorimetry) and dietary fat (¹⁴C-oleate) oxidation in Sedentary Lean (BMI = 21.5±1.6; n = 10), Sedentary Obese (BMI = 33.6±2.5; n = 9), Sedentary Reduced-Obese (RED-SED; BMI = 26.9±3.7; n = 7) and in Physically Active Reduced-Obese (RED-EX; BMI = 27.3±2.8; n = 12) men and women with or without an acute exercise bout where energy expended during exercise was not replaced.

Results

Although Red-SED and Red-EX had a similar level of fatness, aerobic capacity and metabolic profiles were better in Red-EX only compared to Obese subjects. No significant between-group differences were seen in 24-hr respiratory quotient (RQ, Lean: 0.831±0.044, Obese: 0.852±0.023, Red-SED: 0.864±0.037, Red-EX: 0.842±0.039), total and dietary fat oxidation. A single bout of exercise increased total (+27.8%, p<0.0001) and dietary (+6.6%, p = 0.048) fat oxidation across groups. Although exercise did not impact RQ during the day, it decreased RQ during sleep (p = 0.01) in all groups. Red-EX oxidized more fat overnight than Red-SED subjects under both resting (p = 0.036) and negative energy balance (p = 0.003) conditions, even after adjustment for fat-free mass.

Conclusion

Obese and reduced-obese individuals oxidize as much fat as lean both under eucaloric and negative energy balance conditions, which does not support the hypothesis of reduced oxidative capacity in these groups. Reduced-obese individuals who exercise regularly have markers of metabolic health similar to those seen in lean adults. Both the acute and chronic effects of exercise were primarily observed at night suggesting an important role of sleep in the regulation of lipid metabolism.

Total daily energy expenditure is increased following a single bout of sprint interval training

REGULAR ENDURANCE EXERCISE IS AN EFFECTIVE STRATEGY FOR HEALTHY WEIGHT MAINTENANCE, MEDIATED VIA INCREASED TOTAL DAILY ENERGY EXPENDITURE (TDEE), AND POSSIBLY AN INCREASE IN RESTING METABOLIC RATE (RMR: the single largest component of TDEE). Sprint interval training (SIT) is a low-volume alternative to endurance exercise; however, the utility of SIT for healthy weight maintenance is less clear. In this regard, it is feasible that SIT may evoke a thermogenic response above and beyond the estimates required for prevention of weight gain (i.e., >200-600 kJ). The purpose of these studies was to investigate the hypotheses that a single bout of SIT would increase RMR and/or TDEE. Study 1: RMR (ventilated hood) was determined on four separate occasions in 15 healthy men. Measurements were performed over two pairs of consecutive mornings; each pair was separated by 7 days. Immediately following either the first or third RMR measurement (randomly assigned) subjects completed a single bout of SIT (cycle ergometer exercise). RMR was unaffected by a single bout of SIT (7195 ± 285 kJ/day vs. 7147 ± 222, 7149 ± 246 and 6987 ± 245 kJ/day (mean ± SE); P = 0.12). Study 2: TDEE (whole-room calorimeter) was measured in 12 healthy men, on two consecutive days, one of which began with a single bout of SIT (random order). Sprint exercise increased TDEE in every research participant (9169 ± 243 vs. 10,111 ± 260 kJ/day; P < 0.0001); the magnitude of increase was 946 ± 62 kJ/day (∼10%). These data provide support for SIT as a strategy for increasing TDEE, and may have implications for healthy body weight maintenance.

Mitochondrial dysfunction in insulin resistance: differential contributions of chronic insulin and saturated fatty acid exposure in muscle cells

Mitochondrial dysfunction has been associated with insulin resistance, obesity and diabetes. Hyperinsulinaemia and hyperlipidaemia are hallmarks of the insulin-resistant state. We sought to determine the contributions of high insulin and saturated fatty acid exposure to mitochondrial function and biogenesis in cultured myocytes. Differentiated C2C12 myotubes were left untreated or exposed to chronic high insulin or high palmitate. Mitochondrial function was determined assessing: oxygen consumption, mitochondrial membrane potential, ATP content and ROS (reactive oxygen species) production. We also determined the expression of several mitochondrial genes. Chronic insulin treatment of myotubes caused insulin resistance with reduced PI3K (phosphoinositide 3-kinase) and ERK (extracellular-signal-regulated kinase) signalling. Insulin treatment increased oxygen consumption but reduced mitochondrial membrane potential and ROS production. ATP cellular levels were maintained through an increased glycolytic rate. The expression of mitochondrial OXPHOS (oxidative phosphorylation) subunits or Mfn-2 (mitofusin 2) were not significantly altered in comparison with untreated cells, whereas expression of PGC-1α (peroxisome-proliferator-activated receptor γ co-activator-1α) and UCPs (uncoupling proteins) were reduced. In contrast, saturated fatty acid exposure caused insulin resistance, reducing PI3K (phosphoinositide 3-kinase) and ERK (extracellular-signal-regulated kinase) activation while increasing activation of stress kinases JNK (c-Jun N-terminal kinase) and p38. Fatty acids reduced oxygen consumption and mitochondrial membrane potential while up-regulating the expression of mitochondrial ETC (electron chain complex) protein subunits and UCP proteins. Mfn-2 expression was not modified by palmitate. Palmitate-treated cells also showed a reduced glycolytic rate. Taken together, our findings indicate that chronic insulin and fatty acid-induced insulin resistance differentially affect mitochondrial function. In both conditions, cells were able to maintain ATP levels despite the loss of membrane potential; however, different protein expression suggests different adaptation mechanisms.

Acute effects of different diet compositions on skeletal muscle insulin signalling in obese individuals during caloric restriction

OBJECTIVE

The cellular effects of restricting fat versus carbohydrate during a low-calorie diet are unclear. The aim of this study was to examine acute effects of energy and macronutrient restriction on skeletal muscle insulin signalling in obesity.

MATERIALS/METHODS

Eighteen obese individuals without diabetes underwent euglycemic-hyperinsulinemic clamp and skeletal muscle biopsy after: (a) 5days of eucaloric diet (30% fat, 50% carbohydrate), and (b) 5days of a 30% calorie-restricted diet, either low fat/high carbohydrate (LF/HC: 20% fat, 60% carbohydrate) or high-fat/low carbohydrate (HF/LC: 50% fat, 30% carbohydrate).

RESULTS

Weight, body composition, and insulin sensitivity were similar between groups after eucaloric diet. Weight loss was similar between groups after hypocaloric diet, 1.3±1.3kg (p<0.0001 compared with eucaloric). Whole-body insulin sensitivity was unchanged after calorie restriction and similar between groups. However, ex vivo skeletal muscle insulin signalling differed depending on macronutrient composition of calorie-restricted diet. Skeletal muscle of the LF/HC group had increased insulin-stimulated tyrosine phosphorylation of IRS-1, decreased insulin-stimulated Ser307 phosphorylation of IRS-1, and increased IRS-1-associated phosphatidylinositol (PI)3-kinase activity. Conversely, insulin stimulation of tyrosine phosphorylated IRS-1 was absent and serine 307 phosphorylation of IRS-1 was increased on HF/LC, with blunting of IRS-1-associated PI3-kinase activity.

CONCLUSION

Acute caloric restriction with an LF/HC diet alters skeletal muscle insulin signalling in a way that improves insulin sensitivity, while acute caloric restriction with an HF/LC diet induces changes compatible with insulin resistance. In both cases, ex vivo changes in skeletal muscle insulin signalling appear prior to changes in whole body insulin sensitivity.

The defence of body weight: a physiological basis for weight regain after weight loss

Although weight loss can usually be achieved by restricting food intake, the majority of dieters regain weight over the long-term. In the hypothalamus, hormonal signals from the gastrointestinal tract, adipose tissue and other peripheral sites are integrated to influence appetite and energy expenditure. Diet-induced weight loss is accompanied by several physiological changes which encourage weight regain, including alterations in energy expenditure, substrate metabolism and hormone pathways involved in appetite regulation, many of which persist beyond the initial weight loss period. Safe effective long-term strategies to overcome these physiological changes are needed to help facilitate maintenance of weight loss. The present review, which focuses on data from human studies, begins with an outline of body weight regulation to provide the context for the subsequent discussion of short- and long-term physiological changes which accompany diet-induced weight loss.