Effects of fasting on insulin action and glucose kinetics in lean and obese men and women

Intact ketogenesis predicted reduced risk of moderate-severe metabolic-associated fatty liver disease assessed by liver transient elastography in newly diagnosed type 2 diabetes

Aim

Hepatic ketogenesis is a key metabolic pathway that regulates energy homeostasis. Some related controversies exist regarding the pathogenesis of metabolic-associated fatty liver disease (MAFLD). We aimed to investigate whether intact ketogenic capacity could reduce the risk of MAFLD based on transient electrography (TE) in patients with newly diagnosed type 2 diabetes (T2D).

Methods

A total of 361 subjects with newly diagnosed T2D were recruited and classified into two groups based on the median serum β-hydroxybutyrate (βHB) level, referred to as the intact and impaired ketogenesis groups. The glucometabolic relevance of ketogenic capacity and associations of the baseline serum β-HB and MAFLD assessed with TE were investigated.

Results

Compared to the impaired ketogenesis group, the intact ketogenesis group showed better insulin sensitivity, lower serum triglyceride levels, and higher glycated hemoglobin levels. The controlled attenuation parameter (CAP) was lower in the intact ketogenesis group without statistical significance (289.7 ± 52.1 vs. 294.5 ± 43.6; p=0.342) but the prevalence of moderate-severe steatosis defined by CAP ≥260 dB/m was significantly lower in the intact group. Moreover, intact ketogenesis was significantly associated with a lower risk of moderate-severe MAFLD after adjusting for potential confounders (adjusted odds ratio 0.55, 95% confidence interval 0.30-0.98; p=0.044).

Conclusion

In drug-naïve, newly diagnosed T2D patients, intact ketogenesis predicted a lower risk of moderate-severe MAFLD assessed by TE.

Branched chain amino acids harbor distinct and often opposing effects on health and disease

BACKGROUND

The branched chain amino acids (BCAA) leucine, isoleucine, and valine are essential nutrients that have been associated with diabetes, cancers, and cardiovascular diseases. Observational studies suggest that BCAAs exert homogeneous phenotypic effects, but these findings are inconsistent with results from experimental human and animal studies.

METHODS

Hypothesizing that inconsistencies between observational and experimental BCAA studies reflect bias from shared lifestyle and genetic factors in observational studies, we used data from the UK Biobank and applied multivariable Mendelian randomization causal inference methods designed to address these biases.

RESULTS

In n = 97,469 participants of European ancestry (mean age = 56.7 years; 54.1% female), we estimate distinct and often opposing total causal effects for each BCAA. For example, of the 117 phenotypes with evidence of a statistically significant total causal effect for at least one BCAA, almost half (44%, n = 52) are associated with only one BCAA. These 52 associations include total causal effects of valine on diabetic eye disease [odds ratio = 1.51, 95% confidence interval (CI) = 1.31, 1.76], valine on albuminuria (odds ratio = 1.14, 95% CI = 1.08, 1.20), and isoleucine on angina (odds ratio = 1.17, 95% CI = 1.31, 1.76).

CONCLUSIONS

Our results suggest that the observational literature provides a flawed picture of BCAA phenotypic effects that is inconsistent with experimental studies and could mislead efforts developing novel therapeutics. More broadly, these findings motivate the development and application of causal inference approaches that enable 'omics studies conducted in observational settings to account for the biasing effects of shared genetic and lifestyle factors.

Effects of ketone bodies on energy expenditure, substrate utilization, and energy intake in humans

The potential of ketogenic approaches to regulate energy balance has recently gained attention since ketones may influence both energy expenditure and energy intake. In this narrative review, we summarized the most relevant evidence about the role of ketosis on energy expenditure, substrate utilization, and energy intake in humans. We considered different strategies to induce ketosis, such as fasting, dietary manipulation, and exogenous ketone sources. In general, ketosis does not have a major influence on energy expenditure but promotes a shift in substrate utilization towards ketone body oxidation. The strategies to induce ketosis by reduction of dietary carbohydrate availability (e.g., ketogenic diets) do not independently influence energy intake, being thus equally effective for weight loss as diets with higher carbohydrate content. In contrast, the intake of medium-chain triglycerides and ketone esters induces ketosis and appears to increase energy expenditure and reduce energy intake in the context of high carbohydrate availability. These latter strategies lead to slightly enhanced weight loss. Unfortunately, distinguishing the effects of the various ketogenic strategies per se from the effects of other physiological responses is not possible with the available human data. Highly controlled, inpatient studies using targeted strategies to isolate the independent effects of ketones are required to adequately address this knowledge gap.

The adipocyte supersystem of insulin and cAMP signaling

Adipose tissue signals to brain, liver, and muscles to control whole body metabolism through secreted lipid and protein factors as well as neurotransmission, but the mechanisms involved are incompletely understood. Adipocytes sequester triglyceride (TG) in fed conditions stimulated by insulin, while in fasting catecholamines trigger TG hydrolysis, releasing glycerol and fatty acids (FAs). These antagonistic hormone actions result in part from insulin's ability to inhibit cAMP levels generated through such G-protein-coupled receptors as catecholamine-activated β-adrenergic receptors. Consistent with these antagonistic signaling modes, acute actions of catecholamines cause insulin resistance. Yet, paradoxically, chronically activating adipocytes by catecholamines cause increased glucose tolerance, as does insulin. Recent results have helped to unravel this conundrum by revealing enhanced complexities of these hormones' signaling networks, including identification of unexpected common signaling nodes between these canonically antagonistic hormones.

Galectin-3 S-glutathionylation regulates its effect on adipocyte insulin signaling

Protein-S-glutathionylation promotes redox signaling in physiological and oxidative distress conditions. Galectin-3 (Gal-3) promotes insulin resistance by down-regulating adipocyte insulin signaling, however, its S-glutathionylation and significance is not known. In this context, we report reversible S-glutathionylation of Gal-3. Site-directed mutagenesis established Gal-3 Cys187 as the putative S-glutathionylation site. Glutathionylated Gal-3 prevents Gal-3(WT)-Insulin Receptor interaction and facilitates insulin-induced murine adipocyte p-IRS1(tyr895) and p-AKT(ser473) signaling and glucose uptake in a Gal-3 Cys187 glutathionylation dependent manner in murine adipocytes, as assessed by Western blotting and 2-NBDG uptake assay respectively. Pre-glutathionylated Gal-3 at Cys187 resisted irreversible oxidation by H₂O₂. M2 macrophages showed enhanced Gal-3 S-glutathionylation when compared to M1 phenotype. Serum and stromal vascular fraction (SVF) isolated from control mice showed increased Gal-3 S-glutathionylation as compared to db/db mice. A significant increase in Gal-3 S-glutathionylation was observed in metformin-treated db/db mice when compared to db/db mice alone. Similar to murine, enhanced Gal-3 S-glutathionylation is observed in primary human monocyte derived M2 macrophages when compared to the M1 macrophage phenotype and Gal-3 regulates primary human adipocyte insulin signaling in a glutathionylation dependent manner. Collectively, we identified Gal-3 S-glutathionylation as a protective phenomenon, which relieves its inhibitory effect on adipocyte insulin signaling.

Short-term fasting attenuates overall steroid hormone biosynthesis in healthy young women

Context

Fasting is stressful for the human body. It is managed by metabolic adaptations maintaining energy homeostasis and involves steroid hormone biosynthesis, but the exact interplay between energy and steroid metabolism remains elusive. Women with polycystic ovary syndrome (PCOS) suffer from disturbed metabolism and androgen excess, while in women with anorexia nervosa, cortisol and androgen production are decreased. By contrast, starvation of steroidogenic cells shifts adrenal steroid biosynthesis toward enhanced androgen production.

Aim

This study investigated the effect of fasting on steroid production in healthy women.

Methods

Twenty healthy young women fasted for 48 hours; steroid profiles from plasma and urine samples were assessed at baseline, after 24 hours, and 48 hours by liquid and gas chromatography–mass spectrometry.

Results

Fasting did not change overall steroidogenesis, although it increased progestogen production and lowered relative mineralocorticoid, glucocorticoid, and androgen production. The largest decrease in urine metabolites was seen for β-cortol, dehydroepiandrosterone, and androstenediol; higher levels were found for pregnanediol in urine and progesterone and aldosterone in serum. Activity of 17α-hydroxylase/17,20-lyase (CYP17A1), essential for androgen biosynthesis, was decreased after fasting in healthy women as were 21-hydroxylase (CYP21A2) and 5α-reductase activities. By contrast, hydroxysteroid 11-beta dehydrogenase 1 (HSD11B1) activity for cortisol inactivation seemed to increase with fasting.

Conclusion

Significant changes in steroid metabolism occurred after 48 hours of fasting in healthy women. In contrast to metabolic changes seen at baseline in PCOS women compared to healthy women, and after starving of steroidogenic cells, no androgen excess was observed after short-term fasting in healthy young women.

Physiological responses to acute fasting: implications for intermittent fasting programs

Intermittent fasting (IF) is a dietary strategy that involves alternating periods of abstention from calorie consumption with periods of ad libitum food intake. There is significant interest in the body of literature describing longitudinal adaptations to IF. Less attention has been given to the acute physiological responses that occur during the fasting durations that are commonly employed by IF practitioners. Thus, the purpose of this review was to examine the physiological responses - including alterations in substrate metabolism, systemic hormones, and autophagy - that occur throughout an acute fast. Literature searches were performed to locate relevant research describing physiological responses to acute fasting and short-term starvation. A single fast demonstrated the ability to alter glucose and lipid metabolism within the initial 24 hours, but variations in protein metabolism appeared to be minimal within this time frame. The ability of an acute fast to elicit significant increases in autophagy is still unknown. The information summarized in this review can be used to help contextualize existing research and better inform development of future IF interventions.

Role of ketone bodies in diabetes-induced dementia: sirtuins, insulin resistance, synaptic plasticity, mitochondrial dysfunction, and neurotransmitter

Patients with type 2 diabetes can have several neuropathologies, such as memory deficits. Recent studies have focused on the association between metabolic imbalance and neuropathological problems, and the associated molecular pathology. Diabetes triggers neuroinflammation, impaired synaptic plasticity, mitochondrial dysfunction, and insulin resistance in the brain. Glucose is a main energy substrate for neurons, but under certain conditions, such as fasting and starvation, ketone bodies can be used as an energy fuel for these cells. Recent evidence has shed new light on the role of ketone bodies in regulating several anti-inflammation cellular pathways and improving glucose metabolism, insulin action, and synaptic plasticity, thereby being neuroprotective. However, very high amount of ketone bodies can be toxic for the brain, such as in ketoacidosis, a dangerous complication that may occur in type 1 diabetes mellitus or alcoholism. Recent findings regarding the relationship between ketone bodies and neuropathogenesis in dementia are reviewed in this article. They suggest that the adequately low amount of ketone bodies can be a potential energy source for the treatment of diabetes-induced dementia neuropathology, considering the multifaceted effects of the ketone bodies in the central nervous system. This review can provide useful information for establishing the therapeutic guidelines of a ketogenic diet for diabetes-induced dementia.

Impact of Dietary Carbohydrate Restriction versus Energy Restriction on Exogenous Carbohydrate Oxidation during Aerobic Exercise

Individuals with high physical activity levels, such as athletes and military personnel, are likely to experience periods of low muscle glycogen content. Reductions in glycogen stores are associated with impaired physical performance. Lower glycogen stores in these populations are likely due to sustained aerobic exercise coupled with sub-optimal carbohydrate or energy intake. Consuming exogenous carbohydrate during aerobic exercise may be an effective intervention to sustain physical performance during periods of low glycogen. However, research is limited in the area of carbohydrate recommendations to fuel performance during periods of sub-optimal carbohydrate and energy intake. Additionally, the studies that have investigated the effects of low glycogen stores on exogenous carbohydrate oxidation have yielded conflicting results. Discrepancies between studies may be the result of glycogen stores being lowered by restricting carbohydrate or restricting energy intake. This narrative review discusses the influence of low glycogen status resulting from carbohydrate restriction versus energy restriction on exogenous carbohydrate oxidation and examines the potential mechanism resulting in divergent responses in exogenous carbohydrate oxidation. Results from this review indicate that rates of exogenous carbohydrate oxidation can be maintained when glycogen content is lower following carbohydrate restrictions, but may be reduced following energy restriction. Reductions in exogenous carbohydrate oxidation following energy restriction appear to result from lower insulin sensitivity and glucose uptake. Exogenous carbohydrate may thus be an effective intervention to sustain performance following short-term energy adequate carbohydrate restriction, but may not be an effective ergogenic aid when glycogen stores are low due to energy restriction.

Intermittent fasting for the prevention of cardiovascular disease

BACKGROUND

Cardiovascular disease (CVD) is the leading cause of death worldwide. Lifestyle changes are at the forefront of preventing the disease. This includes advice such as increasing physical activity and having a healthy balanced diet to reduce risk factors. Intermittent fasting (IF) is a popular dietary plan involving restricting caloric intake to certain days in the week such as alternate day fasting and periodic fasting, and restricting intake to a number of hours in a given day, otherwise known as time-restricted feeding. IF is being researched for its benefits and many randomised controlled trials have looked at its benefits in preventing CVD.

OBJECTIVES

To determine the role of IF in preventing and reducing the risk of CVD in people with or without prior documented CVD.

SEARCH METHODS

We conducted our search on 12 December 2019; we searched CENTRAL, MEDLINE and Embase. We also searched three trials registers and searched the reference lists of included papers. Systematic reviews were also viewed for additional studies. There was no language restriction applied.

SELECTION CRITERIA

We included randomised controlled trials comparing IF to ad libitum feeding (eating at any time with no specific caloric restriction) or continuous energy restriction (CER). Participants had to be over the age of 18 and included those with and without cardiometabolic risk factors. Intermittent fasting was categorised into alternate-day fasting, modified alternate-day fasting, periodic fasting and time-restricted feeding.

DATA COLLECTION AND ANALYSIS

Five review authors independently selected studies for inclusion and extraction. Primary outcomes included all-cause mortality, cardiovascular mortality, stroke, myocardial infarction, and heart failure. Secondary outcomes include the absolute change in body weight, and glucose. Furthermore, side effects such as headaches and changes to the quality of life were also noted. For continuous data, pooled mean differences (MD) (with 95% confidence intervals (CIs)) were calculated. We contacted trial authors to obtain missing data. We used GRADE to assess the certainty of the evidence.  MAIN RESULTS: Our search yielded 39,165 records after the removal of duplicates. From this, 26 studies met our criteria, and 18 were included in the pooled analysis. The 18 studies included 1125 participants and observed outcomes ranging from four weeks to six months. No studies included data on all-cause mortality, cardiovascular mortality, stroke, myocardial infarction, and heart failure at any point during follow-up. Of quantitatively analysed data, seven studies compared IF with ab libitum feeding, eight studies compared IF with CER, and three studies compared IF with both ad libitum feeding and CER. Outcomes were reported at short term (≤ 3 months) and medium term (> 3 months to 12 months) follow-up. Body weight was reduced with IF compared to ad libitum feeding in the short term (MD -2.88 kg, 95% CI -3.96 to -1.80; 224 participants; 7 studies; low-certainty evidence). We are uncertain of the effect of IF when compared to CER in the short term (MD -0.88 kg, 95% CI -1.76 to 0.00; 719 participants; 10 studies; very low-certainty evidence) and there may be no effect in the medium term (MD -0.56 kg, 95% CI -1.68 to 0.56; 279 participants; 4 studies; low-certainty evidence). We are uncertain about the effect of IF on glucose when compared to ad libitum feeding in the short term (MD -0.03 mmol/L, 95% CI -0.26 to 0.19; 95 participants; 3 studies; very-low-certainty of evidence) and when compared to CER  in the short term: MD -0.02 mmol/L, 95% CI -0.16 to 0.12; 582 participants; 9 studies; very low-certainty; medium term: MD 0.01, 95% CI -0.10 to 0.11; 279 participants; 4 studies; low-certainty evidence). The changes in body weight and glucose were not deemed to be clinically significant. Four studies reported data on side effects, with some participants complaining of mild headaches. One study reported on the quality of life using the RAND SF-36 score. There was a modest increase in the physical component summary score.

AUTHORS' CONCLUSIONS

Intermittent fasting was seen to be superior to ad libitum feeding in reducing weight. However, this was not clinically significant. There was no significant clinical difference between IF and CER in improving cardiometabolic risk factors to reduce the risk of CVD. Further research is needed to understand the safety and risk-benefit analysis of IF in specific patient groups (e.g. patients with diabetes or eating disorders) as well as the effect on longer-term outcomes such as all-cause mortality and myocardial infarction.

The Influence of Obesity on Treatment and Outcome of Severely Burned Patients

Obesity and the related medical, social, and economic impacts are relevant multifactorial and chronic conditions that also have a meaningful impact on outcomes following a severe injury, including burns. In addition to burn-specific difficulties, such as adequate hypermetabolic response, fluid resuscitation, and early wound coverage, obese patients also present with common comorbidities, such as arterial hypertension, diabetes mellitus, or nonalcoholic fatty liver disease. In addition, the pathophysiologic response to severe burns can be enhanced. Besides the increased morbidity and mortality compared to burn patients with normal weight, obese patients present a challenge in fluid resuscitation, perioperative management, and difficulties in wound healing. The present work is an in-depth review of the current understanding of the influence of obesity on the management and outcome of severe burns.

β-Hydroxybutyrate is reduced in humans with obesity-related NAFLD and displays a dose-dependent effect on skeletal muscle mitochondrial respiration in vitro

Nonalcoholic fatty liver disease (NAFLD) is characterized by hepatic fat accumulation and impaired insulin sensitivity. Reduced hepatic ketogenesis may promote these pathologies, but data are inconclusive in humans and the link between NAFLD and reduced insulin sensitivity remains obscure. We investigated individuals with obesity-related NAFLD and hypothesized that β-hydroxybutyrate (βOHB; the predominant ketone species) would be reduced and related to hepatic fat accumulation and insulin sensitivity. Furthermore, we hypothesized that ketones would impact skeletal muscle mitochondrial respiration in vitro. Hepatic fat was assessed by ¹H-MRS in 22 participants in a parallel design, case control study [Control: n = 7, age 50 ± 6 yr, body mass index (BMI) 30 ± 1 kg/m²; NAFLD: n = 15, age 57 ± 3 yr, BMI 35 ± 1 kg/m²]. Plasma assessments were conducted in the fasted state. Whole body insulin sensitivity was determined by the gold-standard hyperinsulinemic-euglycemic clamp. The effect of ketone dose (0.5-5.0 mM) on mitochondrial respiration was conducted in human skeletal muscle cell culture. Fasting βOHB, a surrogate measure of hepatic ketogenesis, was reduced in NAFLD (-15.6%, P < 0.01) and correlated negatively with liver fat (r² = 0.21, P = 0.03) and positively with insulin sensitivity (r² = 0.30, P = 0.01). Skeletal muscle mitochondrial oxygen consumption increased with low-dose ketones, attributable to increases in basal respiration (135%, P < 0.05) and ATP-linked oxygen consumption (136%, P < 0.05). NAFLD pathophysiology includes impaired hepatic ketogenesis, which is associated with hepatic fat accumulation and impaired insulin sensitivity. This reduced capacity to produce ketones may be a potential link between NAFLD and NAFLD-associated reductions in whole body insulin sensitivity, whereby ketone concentrations impact skeletal muscle mitochondrial respiration.

Lipids and ketones dominate metabolism at the expense of glucose control in pulmonary arterial hypertension: a hyperglycaemic clamp and metabolomics study

Individuals with idiopathic pulmonary arterial hypertension (PAH) display reduced oral glucose tolerance. This may involve defects in pancreatic function or insulin sensitivity but this hypothesis has not been tested; moreover, fasting nutrient metabolism remains poorly described in PAH. Thus, we aimed to characterise fasting nutrient metabolism and investigated the metabolic response to hyperglycaemia in PAH.12 participants (six PAH, six controls) were administered a hyperglycaemic clamp, while 52 (21 PAH, 31 controls) underwent plasma metabolomic analysis. Glucose, insulin, C-peptide, free fatty acids and acylcarnitines were assessed from the clamp. Plasma metabolomics was conducted on fasting plasma samples.The clamp verified a reduced insulin response to hyperglycaemia in PAH (-53% versus control), but with similar pancreatic insulin secretion. Skeletal muscle insulin sensitivity was unexpectedly greater in PAH. Hepatic insulin extraction was elevated in PAH (+11% versus control). Plasma metabolomics identified 862 metabolites: 213 elevated, 145 reduced in PAH (p<0.05). In both clamp and metabolomic cohorts, lipid oxidation and ketones were elevated in PAH. Insulin sensitivity, fatty acids, acylcarnitines and ketones correlated with PAH severity, while hepatic extraction and fatty acid:ketone ratio correlated with longer six-min walk distance.Poor glucose control in PAH could not be explained by pancreatic β-cell function or skeletal muscle insulin sensitivity. Instead, elevated hepatic insulin extraction emerged as an underlying factor. In agreement, nutrient metabolism in PAH favours lipid and ketone metabolism at the expense of glucose control. Future research should investigate the therapeutic potential of reinforcing lipid and ketone metabolism on clinical outcomes in PAH.

Intermuscular adipose tissue directly modulates skeletal muscle insulin sensitivity in humans

Intermuscular adipose tissue (IMAT) is negatively related to insulin sensitivity, but a causal role of IMAT in the development of insulin resistance is unknown. IMAT was sampled in humans to test for the ability to induce insulin resistance in vitro and characterize gene expression to uncover how IMAT may promote skeletal muscle insulin resistance. Human primary muscle cells were incubated with conditioned media from IMAT, visceral (VAT), or subcutaneous adipose tissue (SAT) to evaluate changes in insulin sensitivity. RNAseq analysis was performed on IMAT with gene expression compared with skeletal muscle and SAT, and relationships to insulin sensitivity were determined in men and women spanning a wide range of insulin sensitivity measured by hyperinsulinemic-euglycemic clamp. Conditioned media from IMAT and VAT decreased insulin sensitivity similarly compared with SAT. Multidimensional scaling analysis revealed distinct gene expression patterns in IMAT compared with SAT and muscle. Pathway analysis revealed that IMAT expression of genes in insulin signaling, oxidative phosphorylation, and peroxisomal metabolism related positively to donor insulin sensitivity, whereas expression of macrophage markers, inflammatory cytokines, and secreted extracellular matrix proteins were negatively related to insulin sensitivity. Perilipin 5 gene expression suggested greater IMAT lipolysis in insulin-resistant individuals. Combined, these data show that factors secreted from IMAT modulate muscle insulin sensitivity, possibly via secretion of inflammatory cytokines and extracellular matrix proteins, and by increasing local FFA concentration in humans. These data suggest IMAT may be an important regulator of skeletal muscle insulin sensitivity and could be a novel therapeutic target for skeletal muscle insulin resistance.

Developing a network view of type 2 diabetes risk pathways through integration of genetic, genomic and functional data

BACKGROUND

Genome-wide association studies (GWAS) have identified several hundred susceptibility loci for type 2 diabetes (T2D). One critical, but unresolved, issue concerns the extent to which the mechanisms through which these diverse signals influencing T2D predisposition converge on a limited set of biological processes. However, the causal variants identified by GWAS mostly fall into a non-coding sequence, complicating the task of defining the effector transcripts through which they operate.

METHODS

Here, we describe implementation of an analytical pipeline to address this question. First, we integrate multiple sources of genetic, genomic and biological data to assign positional candidacy scores to the genes that map to T2D GWAS signals. Second, we introduce genes with high scores as seeds within a network optimization algorithm (the asymmetric prize-collecting Steiner tree approach) which uses external, experimentally confirmed protein-protein interaction (PPI) data to generate high-confidence sub-networks. Third, we use GWAS data to test the T2D association enrichment of the "non-seed" proteins introduced into the network, as a measure of the overall functional connectivity of the network.

RESULTS

We find (a) non-seed proteins in the T2D protein-interaction network so generated (comprising 705 nodes) are enriched for association to T2D (p = 0.0014) but not control traits, (b) stronger T2D-enrichment for islets than other tissues when we use RNA expression data to generate tissue-specific PPI networks and (c) enhanced enrichment (p = 3.9 × 10^- 5) when we combine the analysis of the islet-specific PPI network with a focus on the subset of T2D GWAS loci which act through defective insulin secretion.

CONCLUSIONS

These analyses reveal a pattern of non-random functional connectivity between candidate causal genes at T2D GWAS loci and highlight the products of genes including YWHAG, SMAD4 or CDK2 as potential contributors to T2D-relevant islet dysfunction. The approach we describe can be applied to other complex genetic and genomic datasets, facilitating integration of diverse data types into disease-associated networks.

24-h severe energy restriction impairs postprandial glycaemic control in young, lean males

Intermittent energy restriction (IER) involves short periods of severe energy restriction interspersed with periods of adequate energy intake, and can induce weight loss. Insulin sensitivity is impaired by short-term, complete energy restriction, but the effects of IER are not well known. In randomised order, fourteen lean men (age: 25 (sd 4) years; BMI: 24 (sd 2) kg/m2; body fat: 17 (4) %) consumed 24-h diets providing 100 % (10 441 (sd 812) kJ; energy balance (EB)) or 25 % (2622 (sd 204) kJ; energy restriction (ER)) of estimated energy requirements, followed by an oral glucose tolerance test (OGTT; 75 g of glucose drink) after fasting overnight. Plasma/serum glucose, insulin, NEFA, glucagon-like peptide-1 (GLP-1), glucose-dependent insulinotropic peptide (GIP) and fibroblast growth factor 21 (FGF21) were assessed before and after (0 h) each 24-h dietary intervention, and throughout the 2-h OGTT. Homoeostatic model assessment of insulin resistance (HOMA2-IR) assessed the fasted response and incremental AUC (iAUC) or total AUC (tAUC) were calculated during the OGTT. At 0 h, HOMA2-IR was 23 % lower after ER compared with EB (P<0·05). During the OGTT, serum glucose iAUC (P<0·001), serum insulin iAUC (P<0·05) and plasma NEFA tAUC (P<0·01) were greater during ER, but GLP-1 (P=0·161), GIP (P=0·473) and FGF21 (P=0·497) tAUC were similar between trials. These results demonstrate that severe energy restriction acutely impairs postprandial glycaemic control in lean men, despite reducing HOMA2-IR. Chronic intervention studies are required to elucidate the long-term effects of IER on indices of insulin sensitivity, particularly in the absence of weight loss.

Prolonged fasting-induced metabolic signatures in human skeletal muscle of lean and obese men

Insulin resistance is a well-known physiological adaptation to prolonged fasting in healthy skeletal muscle. Obesity is associated with insulin resistance and metabolic inflexibility in skeletal muscle, and a pronounced increase in the risk of metabolic complications. Under the hypothesis that the metabolic traits of insulin resistance associated with prolonged fasting are different from insulin resistance associated with obesity, we examined nine obese and nine lean participants during 12 and 72h of fasting, respectively. Insulin resistance in obese participants was associated with impaired insulin signaling, and reduced levels of glucose-6-phosphate and TCA-cycle intermediates. 72h of fasting in lean participants reduced insulin-stimulated glucose uptake to levels similar to obese participants fasted for 12h. This was associated with increased lipid oxidation, but not accumulation of diacylglycerol or acylcarnitines and impairment of insulin signaling. Prolonged fasting was associated with pronounced increases in β-hydroxybutyrate and β- hydroxybutyrylcarnitine levels in skeletal muscle suggesting augmented ketone body metabolism. Fasting induced insulin resistance may be a consequence of substrate competition. The underlying mechanism behind insulin resistance in obesity is thus not comparable to the physiological adaptations in skeletal muscle induced by prolonged fasting in lean participants.

Hepatic ketogenic insufficiency reprograms hepatic glycogen metabolism and the lipidome

While several molecular targets are under consideration, mechanistic underpinnings of the transition from uncomplicated nonalcoholic fatty liver disease (NAFLD) to nonalcoholic steatohepatitis (NASH) remain unresolved. Here we apply multiscale chemical profiling technologies to mouse models of deranged hepatic ketogenesis to uncover potential NAFLD driver signatures. Use of stable-isotope tracers, quantitatively tracked by nuclear magnetic resonance (NMR) spectroscopy, supported previous observations that livers of wild-type mice maintained long term on a high-fat diet (HFD) exhibit a marked increase in hepatic energy charge. Fed-state ketogenesis rates increased nearly 3-fold in livers of HFD-fed mice, a greater proportionate increase than that observed for tricarboxylic acid (TCA) cycle flux, but both of these contributors to overall hepatic energy homeostasis fueled markedly increased hepatic glucose production (HGP). Thus, to selectively determine the role of the ketogenic conduit on HGP and oxidative hepatic fluxes, we studied a ketogenesis-insufficient mouse model generated by knockdown of the mitochondrial isoform of 3-hydroxymethylglutaryl-CoA synthase (HMGCS2). In response to ketogenic insufficiency, TCA cycle flux in the fed state doubled and HGP increased more than 60%, sourced by a 3-fold increase in glycogenolysis. Finally, high-resolution untargeted metabolomics and shotgun lipidomics performed using ketogenesis-insufficient livers in the fed state revealed accumulation of bis(monoacylglycero)phosphates, which also accumulated in livers of other models commonly used to study NAFLD. In summary, natural and interventional variations in ketogenesis in the fed state strongly influence hepatic energy homeostasis, glucose metabolism, and the lipidome. Importantly, HGP remains tightly linked to overall hepatic energy charge, which includes both terminal fat oxidation through the TCA cycle and partial oxidation via ketogenesis.

Intramuscular triglyceride synthesis: importance in muscle lipid partitioning in humans

Intramuscular triglyceride (IMTG) concentration is elevated in insulin-resistant individuals and was once thought to promote insulin resistance. However, endurance-trained athletes have equivalent concentration of IMTG compared with individuals with type 2 diabetes, and have very low risk of diabetes, termed the "athlete's paradox." We now know that IMTG synthesis is positively related to insulin sensitivity, but the exact mechanisms for this are unclear. To understand the relationship between IMTG synthesis and insulin sensitivity, we measured IMTG synthesis in obese control subjects, endurance-trained athletes, and individuals with type 2 diabetes during rest, exercise, and recovery. IMTG synthesis rates were positively related to insulin sensitivity, cytosolic accumulation of DAG, and decreased accumulation of C18:0 ceramide and glucosylceramide. Greater rates of IMTG synthesis in athletes were not explained by alterations in FFA concentration, DGAT1 mRNA expression, or protein content. IMTG synthesis during exercise in Ob and T2D indicate utilization as a fuel despite unchanged content, whereas IMTG concentration decreased during exercise in athletes. mRNA expression for genes involved in lipid desaturation and IMTG synthesis were increased after exercise and recovery. Further, in a subset of individuals, exercise decreased cytosolic and membrane di-saturated DAG content, which may help explain insulin sensitization after acute exercise. These data suggest IMTG synthesis rates may influence insulin sensitivity by altering intracellular lipid localization, and decreasing specific ceramide species that promote insulin resistance.

Ganglioside GM3 content in skeletal muscles is increased in type 2 but decreased in type 1 diabetes rat models: Implications of glycosphingolipid metabolism in pathophysiology of diabetes

BACKGROUND

Ganglioside GM3 is found in the plasma membrane, where its accumulation attenuates insulin receptor signaling. Considering the role of skeletal muscles in insulin-stimulated glucose uptake, the aim of the present study was to determine the expression of GM3 and its precursors in skeletal muscles of rat models of type 1 and type 2 diabetes mellitus (T1DM and T2DM, respectively).

METHODS

Diabetes was induced in male Sprague-Dawley rats by streptozotocin injection (55 mg/kg, i.p., for T1DM induction; 35 mg/kg, i.p., for T2DM induction), followed by feeding of rats with either a normal pellet diet (T1DM) or a high-fat diet (T2DM). Rats were killed 2 weeks after diabetes induction and samples of skeletal muscle were collected. Frozen quadriceps muscle sections were stained with a primary antibody against GM3 (Neu5Ac) and visualized using a secondary antibody coupled with Texas Red. The muscle content of ganglioside GM3 and its precursors was analyzed by high-performance thin-layer chromatography (HPTLC) followed by GM3 immunostaining.

RESULTS

Muscle GM3 content was significantly higher in T2DM compared with control rats (P < 0.001). Furthermore, levels of the GM3 precursors ceramide, glucosylceramide, and lactosylceramide were significantly higher in T2DM compared with control rats (P < 0.05), whereas ceramide content was significantly lower in T1DM rats (P < 0.05). The intensity of the GM3 band on HPTLC was significantly higher in T2DM rats (P < 0.001) and significantly lower in T1DM rats (P < 0.05) compared with control.

CONCLUSIONS

The expression patterns of GM3 ganglioside and its precursors in diabetic rats suggest that the role of glycosphingolipid metabolism may differ between T2DM and T1DM.

Your mitochondria are what you eat: a high-fat or a high-sucrose diet eliminates metabolic flexibility in isolated mitochondria from rat skeletal muscle

Extreme diets consisting of either high fat (HF) or high sucrose (HS) may lead to insulin resistance in skeletal muscle, often associated with mitochondrial dysfunction. However, it is not known if these diets alter normal interactions of pyruvate and fatty acid oxidation at the level of the mitochondria. Here, we report that rat muscle mitochondria does show the normal Randle‐type fat‐carbohydrate interaction seen in vivo. The mechanism behind this metabolic flexibility at the level of the isolated mitochondria is a regulation of the flux‐ratio: pyruvate dehydrogenase (PDH)/β‐oxidation to suit the actual substrate availability, with the PDH flux as the major point of regulation. We further report that this regulatory mechanism of carbohydrate‐fat metabolic interaction surprisingly is lost in mitochondria obtained from animals exposed for 12 weeks to a HF‐ or a HS diet as compared to rats given a normal chow diet. The mechanism seems to be a loss of the PDH flux decrease seen in controls, when fatty acid is supplied as substrate in addition to pyruvate, and vice versa for the supply of pyruvate as substrate to mitochondria oxidizing fatty acid. Finally, we report that the calculated TCA flux in the isolated mitochondria under these circumstances shows a significant reduction (~50%) after the HF diet and an even larger reduction (~75%) after the HS diet, compared with the chow group. Thus, it appears that obesogenic diets as those applied here have major influence on key metabolic performance of skeletal muscle mitochondria.

Multi-dimensional Roles of Ketone Bodies in Fuel Metabolism, Signaling, and Therapeutics

Ketone body metabolism is a central node in physiological homeostasis. In this review, we discuss how ketones serve discrete fine-tuning metabolic roles that optimize organ and organism performance in varying nutrient states and protect from inflammation and injury in multiple organ systems. Traditionally viewed as metabolic substrates enlisted only in carbohydrate restriction, observations underscore the importance of ketone bodies as vital metabolic and signaling mediators when carbohydrates are abundant. Complementing a repertoire of known therapeutic options for diseases of the nervous system, prospective roles for ketone bodies in cancer have arisen, as have intriguing protective roles in heart and liver, opening therapeutic options in obesity-related and cardiovascular disease. Controversies in ketone metabolism and signaling are discussed to reconcile classical dogma with contemporary observations.

Effect of 24-h severe energy restriction on appetite regulation and ad libitum energy intake in lean men and women

BACKGROUND

Intermittent severe energy restriction (SER) can induce substantial weight loss, but the appetite regulatory responses to SER are unknown and may dictate long-term dietary adherence.

OBJECTIVE

We determined the effect of 24-h SER on appetite regulation, metabolism, and energy intake.

DESIGN

Eighteen lean men and women completed two 3-d trials in randomized, counterbalanced order. On day 1 subjects consumed standardized diets containing 100% (mean ± SD: 9.3 ± 1.3 MJ; energy balance) or 25% [2.3 ± 0.3 MJ; energy restriction (ER)] of energy requirements. On day 2, a standardized breakfast was consumed, with plasma concentrations of acylated ghrelin, glucagon-like peptide 1, insulin, glucose, and nonesterified fatty acids determined for 4 h. Ad libitum energy intake was assessed at lunch and dinner with subjective appetite and resting metabolism assessed throughout. On day 3, ad libitum energy intake was assessed at breakfast and by weighed food records.

RESULTS

Energy intake was 7% greater on day 2 (P < 0.05) during ER but not significantly different on day 3 (P = 0.557). Subjective appetite was greater during ER on the morning of day 2 (P < 0.05) but was not significantly different thereafter (P > 0.145). During ER, postprandial concentrations of acylated ghrelin were lower (P < 0.05), whereas glucose (P < 0.05) and nonesterified fatty acids (P < 0.0001) were higher. Postprandial glucagon-like peptide 1_7-36 (P = 0.784) and insulin (P = 0.06) concentrations were not significantly different between trials. Energy expenditure was lower during ER in the morning (P < 0.01).

CONCLUSIONS

In lean young adults, 24-h SER transiently elevated subjective appetite and marginally increased energy intake, but hormonal appetite markers did not respond in a manner indicative of hyperphagia. These results suggest that intermittent SER might be useful to attenuate energy intake and control body weight in this population. This trial was registered at www.clinicaltrials.gov.uk as NCT02696772.

Effects of acute dietary weight loss on postprandial plasma bile acid responses in obese insulin resistant subjects

BACKGROUND & AIMS

Bile acids (BA) are pleiotropic hormones affecting glucose and lipid metabolism. The physiochemical properties of different BA species affect their enterohepatic dynamics and their affinity for bile acid receptors. The BA pool composition is altered in patients with type 2 diabetes and obesity. In this study we used a 2-week very-low-calorie diet (VLCD) to investigate the effects of weight loss on BA pool composition and postprandial dynamics.

METHODS

We performed mixed meal tests in obese, insulin resistant subjects before and after the VLCD. We measured postprandial plasma levels of glucose, insulin, BA and the BA-induced enterokine fibroblast growth factor 19 (FGF19).

RESULTS

The VLCD decreased weight by 4.5 ± 2.3 kg (p < 0.0001) within 14 days. Weight loss increased peak postprandial deoxycholate (DCA) levels (median [IQR]: 0.90 [0.90] vs. 1.25 [1.35] μmol/L; p = 0.045*). Other BA species, glucose, insulin and FGF19 levels and prandial excursions were not significantly affected. The VLCD decreased resting and postprandial energy expenditure by 7 and 11% respectively.

CONCLUSIONS

VLCD induced weight loss increased postprandial DCA peak levels and decreased resting energy expenditure in obese insulin resistant subjects.

No effect of 24 h severe energy restriction on appetite regulation and ad libitum energy intake in overweight and obese males

BACKGROUND/OBJECTIVES

Long-term success of weight loss diets might depend on how the appetite regulatory system responds to energy restriction (ER). This study determined the effect of 24 h severe ER on subjective and hormonal appetite regulation, subsequent ad libitum energy intake and metabolism.

SUBJECTS/METHODS

In randomised order, eight overweight or obese males consumed a 24 h diet containing either 100% (12105 (1174 kJ; energy balance; EB) or 25% (3039 (295) kJ; ER) of estimated daily energy requirements (EER). An individualised standard breakfast containing 25% of EER (3216 (341) kJ) was consumed the following morning and resting energy expenditure, substrate utilisation and plasma concentrations of acylated ghrelin, glucagon-like peptide-1 (GLP-1_7-36), glucose-dependant insulinotropic peptide (GIP_1-42), glucose, insulin and non-esterified fatty acid (NEFA) were determined for 4 h after breakfast. Ad libitum energy intake was assessed in the laboratory on day 2 and via food records on day 3. Subjective appetite was assessed throughout.

RESULTS

Energy intake was not different between trials for day 2 (EB: 14946 (1272) kJ; ER: 15251 (2114) kJ; P=0.623), day 3 (EB: 10580 (2457) kJ; 10812 (4357) kJ; P=0.832) or day 2 and 3 combined (P=0.693). Subjective appetite was increased during ER on day 1 (P<0.01), but was not different between trials on day 2 (P>0.381). Acylated ghrelin, GLP-1_7-36 and insulin were not different between trials (P>0.104). Post-breakfast area under the curve (AUC) for NEFA (P<0.05) and GIP_1-42 (P<0.01) were greater during ER compared with EB. Fat oxidation was greater (P<0.01) and carbohydrate oxidation was lower (P<0.01) during ER, but energy expenditure was not different between trials (P=0.158).

CONCLUSIONS

These results suggest that 24 h severe ER does not affect appetite regulation or energy intake in the subsequent 48 h. This style of dieting may be conducive to maintenance of a negative EB by limiting compensatory eating behaviour, and therefore may assist with weight loss.

Dietary Fatty Acids Differentially Associate with Fasting Versus 2-Hour Glucose Homeostasis: Implications for The Management of Subtypes of Prediabetes

Over-nutrition has fuelled the global epidemic of type 2 diabetes, but the role of individual macronutrients to the diabetogenic process is not well delineated. We aimed to examine the impact of dietary fatty acid intake on fasting and 2-hour plasma glucose concentrations, as well as tissue-specific insulin action governing each. Normoglycemic controls (n = 15), athletes (n = 14), and obese (n = 23), as well as people with prediabetes (n = 10) and type 2 diabetes (n = 11), were queried about their habitual diet using a Food Frequency Questionnaire. All subjects were screened by an oral glucose tolerance test (OGTT) and studied using the hyperinsulinemic/euglycemic clamp with infusion of 6,62H2-glucose. Multiple regression was performed to examine relationships between dietary fat intake and 1) fasting plasma glucose, 2) % suppression of endogenous glucose production, 3) 2-hour post-OGTT plasma glucose, and 4) skeletal muscle insulin sensitivity (glucose rate of disappearance (Rd) and non-oxidative glucose disposal (NOGD)). The %kcal from saturated fat (SFA) was positively associated with fasting (β = 0.303, P = 0.018) and 2-hour plasma glucose (β = 0.415, P<0.001), and negatively related to % suppression of hepatic glucose production (β = -0.245, P = 0.049), clamp Rd (β = -0.256, P = 0.001) and NOGD (β = -0.257, P = 0.001). The %kcal from trans fat was also negatively related to clamp Rd (β = -0.209, P = 0.008) and NOGD (β = -0.210, P = 0.008). In contrast, the %kcal from polyunsaturated fat (PUFA) was negatively associated with 2-hour glucose levels (β = -0.383, P = 0.001), and positively related to Rd (β = 0.253, P = 0.007) and NOGD (β = 0.246, P = 0.008). Dietary advice to prevent diabetes should consider the underlying pathophysiology of the prediabetic state.

Investigation into the acute effects of total and partial energy restriction on postprandial metabolism among overweight/obese participants

The intermittent energy restriction (IER) approach to weight loss involves short periods of substantial (75-100 %) energy restriction (ER) interspersed with normal eating. This study aimed to characterise the early metabolic response to these varying degrees of ER, which occurs acutely and prior to weight loss. Ten (three female) healthy, overweight/obese participants (36 (SEM 5) years; 29·0 (sem 1·1) kg/m2) took part in this acute three-way cross-over study. Participants completed three 1-d dietary interventions in a randomised order with a 1-week washout period: isoenergetic intake, partial 75 % ER and total 100 % ER. Fasting and postprandial (6-h) metabolic responses to a liquid test meal were assessed the following morning via serial blood sampling and indirect calorimetry. Food intake was also recorded for two subsequent days of ad libitum intake. Relative to the isoenergetic control, postprandial glucose responses were increased following total ER (+142 %; P=0·015) and to a lesser extent after partial ER (+76 %; P=0·051). There was also a delay in the glucose time to peak after total ER only (P=0·024). Both total and partial ER interventions produced comparable reductions in postprandial TAG responses (-75 and -59 %, respectively; both P<0·05) and 3-d energy intake deficits of approximately 30 % (both P=0·015). Resting and meal-induced thermogenesis were not significantly affected by either ER intervention. In conclusion, our data demonstrate the ability of substantial ER to acutely alter postprandial glucose-lipid metabolism (with partial ER producing the more favourable overall response), as well as incomplete energy-intake compensation amongst overweight/obese participants. Further investigations are required to establish how metabolism adapts over time to the repeated perturbations experienced during IER, as well as the implications for long-term health.

The effects of short-term fasting on tolerance to (neo) adjuvant chemotherapy in HER2-negative breast cancer patients: a randomized pilot study

BACKGROUND

Preclinical evidence shows that short-term fasting (STF) protects healthy cells against side effects of chemotherapy and makes cancer cells more vulnerable to it. This pilot study examines the feasibility of STF and its effects on tolerance of chemotherapy in a homogeneous patient group with early breast cancer (BC).

METHODS

Eligible patients had HER2-negative, stage II/III BC. Women receiving (neo)-adjuvant TAC (docetaxel/doxorubicin/cyclophosphamide) were randomized to fast 24 h before and after commencing chemotherapy, or to eat according to the guidelines for healthy nutrition. Toxicity in the two groups was compared. Chemotherapy-induced DNA damage in peripheral blood mononuclear cells (PBMCs) was quantified by the level of γ-H2AX analyzed by flow cytometry.

RESULTS

Thirteen patients were included of whom seven were randomized to the STF arm. STF was well tolerated. Mean erythrocyte- and thrombocyte counts 7 days post-chemotherapy were significantly higher (P = 0.007, 95 % CI 0.106-0.638 and P = 0.00007, 95 % CI 38.7-104, respectively) in the STF group compared to the non-STF group. Non-hematological toxicity did not differ between the groups. Levels of γ-H2AX were significantly increased 30 min post-chemotherapy in CD45 + CD3- cells in non-STF, but not in STF patients.

CONCLUSIONS

STF during chemotherapy was well tolerated and reduced hematological toxicity of TAC in HER2-negative BC patients. Moreover, STF may reduce a transient increase in, and/or induce a faster recovery of DNA damage in PBMCs after chemotherapy. Larger studies, investigating a longer fasting period, are required to generate more insight into the possible benefits of STF during chemotherapy.

TRIAL REGISTRATION

ClinicalTrials.gov: NCT01304251 , March 2011.

Glucose uptake in human brown adipose tissue is impaired upon fasting-induced insulin resistance

AIMS/HYPOTHESIS

Human brown adipose tissue (BAT) has recently emerged as a potential target in the treatment of type 2 diabetes, owing to its capacity to actively clear glucose from the circulation—at least upon cold exposure. The effects of insulin resistance on the capacity of human BAT to take up glucose are unknown. Prolonged fasting is known to induce insulin resistance in peripheral tissues in order to spare glucose for the brain.

METHODS

We studied the effect of fasting-induced insulin resistance on the capacity of BAT to take up glucose during cold exposure as well as on cold-stimulated thermogenesis. BAT glucose uptake was assessed by means of cold-stimulated dynamic 2-deoxy-2-[(18)F]fluoro-D-glucose positron emission tomography/computed tomography ([(18)F]FDG-PET/CT) imaging.

RESULTS

We show that a 54 h fasting period markedly decreases both cold-induced BAT glucose uptake and nonshivering thermogenesis (NST) during cold stimulation. In vivo molecular imaging and modelling revealed that the reduction of glucose uptake in BAT was due to impaired cellular glucose uptake and not due to decreased supply. Interestingly, decreased BAT glucose uptake upon fasting was related to a decrease in core temperature during cold exposure, pointing towards a role for BAT in maintaining normothermia in humans.

CONCLUSIONS/INTERPRETATION

Cold-stimulated glucose uptake in BAT is strongly reduced upon prolonged fasting. When cold-stimulated glucose uptake in BAT is also reduced under other insulin-resistant states, such as diabetes, cold-induced activation of BAT may not be a valid way to improve glucose clearance by BAT under such conditions.

TRIAL REGISTRATION

www.trialregister.nl NTR3523 FUNDING: This work was supported by the EU FP7 project DIABAT (HEALTH-F2-2011-278373 to WDvML) and by the Netherlands Organization for Scientific Research (TOP 91209037 to WDvML).

Ketogenesis prevents diet-induced fatty liver injury and hyperglycemia

Nonalcoholic fatty liver disease (NAFLD) spectrum disorders affect approximately 1 billion individuals worldwide. However, the drivers of progressive steatohepatitis remain incompletely defined. Ketogenesis can dispose of much of the fat that enters the liver, and dysfunction in this pathway could promote the development of NAFLD. Here, we evaluated mice lacking mitochondrial 3-hydroxymethylglutaryl CoA synthase (HMGCS2) to determine the role of ketogenesis in preventing diet-induced steatohepatitis. Antisense oligonucleotide-induced loss of HMGCS2 in chow-fed adult mice caused mild hyperglycemia, increased hepatic gluconeogenesis from pyruvate, and augmented production of hundreds of hepatic metabolites, a suite of which indicated activation of the de novo lipogenesis pathway. High-fat diet feeding of mice with insufficient ketogenesis resulted in extensive hepatocyte injury and inflammation, decreased glycemia, deranged hepatic TCA cycle intermediate concentrations, and impaired hepatic gluconeogenesis due to sequestration of free coenzyme A (CoASH). Supplementation of the CoASH precursors pantothenic acid and cysteine normalized TCA intermediates and gluconeogenesis in the livers of ketogenesis-insufficient animals. Together, these findings indicate that ketogenesis is a critical regulator of hepatic acyl-CoA metabolism, glucose metabolism, and TCA cycle function in the absorptive state and suggest that ketogenesis may modulate fatty liver disease.

Variability in HOMA-IR, lipoprotein profile and selected hormones in young active men

Resistance to insulin actions is contributing to many metabolic disturbances. Such factors as age, sex, nutrition, body fat, and physical activity determine body insulin resistance. Present study attempted to asses insulin resistance and its metabolic effects with respect to energy intake in young, lean, and active men. A total of 87 men aged 18-23 participated in the study. Plasma levels of glucose, insulin, lipoproteins, cortisol, and TSH were determined. Insulin resistance was expressed as Homeostasis Model Assessment for Insulin Resistance (HOMA-IR) and calculated using homeostatic model. The median value of HOMA-IR (1.344) was used to divide subjects into two groups. Men did not differ in anthropometric parameters, daily physical activity, and plasma TSH and cortisol levels. However, in men with higher HOMA-IR significantly lower daily energy intake was observed concomitantly with higher TG, TC, and HDL-C concentrations in plasma versus their counterparts with lower HOMA-IR. Exclusively in subjects with higher HOMA-IR significant and positive correlation was noted between HOMA-IR and TC and LDL-C. We concluded that despite a normal body weight and physical activity, a subset of young men displayed unfavorable changes in insulin sensitivity and lipid profile, probably due to insufficient energy intake.

Adiposity and insulin resistance in humans: the role of the different tissue and cellular lipid depots

Human adiposity has long been associated with insulin resistance and increased cardiovascular risk, and abdominal adiposity is considered particularly adverse. Intra-abdominal fat is associated with insulin resistance, possibly mediated by greater lipolytic activity, lower adiponectin levels, resistance to leptin, and increased inflammatory cytokines, although the latter contribution is less clear. Liver lipid is also closely associated with, and likely to be an important contributor to, insulin resistance, but it may also be in part the consequence of the lipogenic pathway of insulin action being up-regulated by hyperinsulinemia and unimpaired signaling. Again, intramyocellular triglyceride is associated with muscle insulin resistance, but anomalies include higher intramyocellular triglyceride in insulin-sensitive athletes and women (vs men). Such issues could be explained if the "culprits" were active lipid moieties such as diacylglycerol and ceramide species, dependent more on lipid metabolism and partitioning than triglyceride amount. Subcutaneous fat, especially gluteofemoral, appears metabolically protective, illustrated by insulin resistance and dyslipidemia in patients with lipodystrophy. However, some studies suggest that deep sc abdominal fat may have adverse properties. Pericardial and perivascular fat relate to atheromatous disease, but not clearly to insulin resistance. There has been recent interest in recognizable brown adipose tissue in adult humans and its possible augmentation by a hormone, irisin, from exercising muscle. Brown adipose tissue is metabolically active, oxidizes fatty acids, and generates heat but, because of its small and variable quantities, its metabolic importance in humans under usual living conditions is still unclear. Further understanding of specific roles of different lipid depots may help new approaches to control obesity and its metabolic sequelae.

Effects of prolonged fasting on AMPK signaling, gene expression, and mitochondrial respiratory chain content in skeletal muscle from lean and obese individuals

Obesity in humans is often associated with metabolic inflexibility, but the underlying molecular mechanisms remain incompletely understood. The aim of the present study was to investigate how adaptation to prolonged fasting affects energy/nutrient-sensing pathways and metabolic gene expression in skeletal muscle from lean and obese individuals. Twelve lean and 14 nondiabetic obese subjects were fasted for 48 h. Whole body glucose/lipid oxidation rates were determined by indirect calorimetry, and blood and skeletal muscle biopsies were collected and analyzed. In response to fasting, body weight loss was similar in both groups, but the decrease in plasma insulin and leptin and the concomitant increase in growth hormone were significantly attenuated in obese subjects. The fasting-induced shift from glucose toward lipid oxidation was also severely blunted. At the molecular level, the expression of insulin receptor-β (IRβ) was lower in skeletal muscle from obese subjects at baseline, whereas the fasting-induced reductions in insulin signaling were similar in both groups. The protein expression of mitochondrial respiratory chain components, although not modified by fasting, was significantly reduced in obese subjects. Some minor differences in metabolic gene expression were observed at baseline and in response to fasting. Surprisingly, fasting reduced AMPK activity in lean but not in obese subjects, whereas the expression of AMPK subunits was not affected. We conclude that whole body metabolic inflexibility in response to prolonged fasting in obese humans is associated with lower skeletal muscle IRβ and mitochondrial respiratory chain content as well as a blunted decline of AMPK activity.

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.

Ketone body metabolism and cardiovascular disease

Ketone bodies are metabolized through evolutionarily conserved pathways that support bioenergetic homeostasis, particularly in brain, heart, and skeletal muscle when carbohydrates are in short supply. The metabolism of ketone bodies interfaces with the tricarboxylic acid cycle, β-oxidation of fatty acids, de novo lipogenesis, sterol biosynthesis, glucose metabolism, the mitochondrial electron transport chain, hormonal signaling, intracellular signal transduction pathways, and the microbiome. Here we review the mechanisms through which ketone bodies are metabolized and how their signals are transmitted. We focus on the roles this metabolic pathway may play in cardiovascular disease states, the bioenergetic benefits of myocardial ketone body oxidation, and prospective interactions among ketone body metabolism, obesity, metabolic syndrome, and atherosclerosis. Ketone body metabolism is noninvasively quantifiable in humans and is responsive to nutritional interventions. Therefore, further investigation of this pathway in disease models and in humans may ultimately yield tailored diagnostic strategies and therapies for specific pathological states.

Adaptive reciprocity of lipid and glucose metabolism in human short-term starvation

The human organism has tools to cope with metabolic challenges like starvation that are crucial for survival. Lipolysis, lipid oxidation, ketone body synthesis, tailored endogenous glucose production and uptake, and decreased glucose oxidation serve to protect against excessive erosion of protein mass, which is the predominant supplier of carbon chains for synthesis of newly formed glucose. The starvation response shows that the adaptation to energy deficit is very effective and coordinated with different adaptations in different organs. From an evolutionary perspective, this lipid-induced effect on glucose oxidation and uptake is very strong and may therefore help to understand why insulin resistance in obesity and type 2 diabetes mellitus is difficult to treat. The importance of reciprocity in lipid and glucose metabolism during human starvation should be taken into account when studying lipid and glucose metabolism in general and in pathophysiological conditions in particular.

Novel and reversible mechanisms of smoking-induced insulin resistance in humans

Smoking is the most common cause of preventable morbidity and mortality in the United States, in part because it is an independent risk factor for the development of insulin resistance and type 2 diabetes. However, mechanisms responsible for smoking-induced insulin resistance are unclear. In this study, we found smokers were less insulin sensitive compared with controls, which increased after either 1 or 2 weeks of smoking cessation. Improvements in insulin sensitivity after smoking cessation occurred with normalization of IRS-1(ser636) phosphorylation. In muscle cell culture, nicotine exposure significantly increased IRS-1(ser636) phosphorylation and decreased insulin sensitivity, recapitulating the phenotype of smoking-induced insulin resistance in humans. The two pathways known to stimulate IRS-1(ser636) phosphorylation (p44/42 mitogen-activated protein kinase [MAPK] and mammalian target of rapamycin [mTOR]) were both stimulated by nicotine in culture. Inhibition of mTOR, but not p44/42 MAPK, during nicotine exposure prevented IRS-1(ser636) phosphorylation and normalized insulin sensitivity. These data indicate nicotine induces insulin resistance in skeletal muscle by activating mTOR. Therapeutic agents designed to oppose skeletal muscle mTOR activation may prevent insulin resistance in humans who are unable to stop smoking or are chronically exposed to secondhand smoke.

Features of hepatic and skeletal muscle insulin resistance unique to type 1 diabetes

CONTEXT

Type 1 diabetes is known to be a state of insulin resistance; however, the tissues involved in whole-body insulin resistance are less well known. It is unclear whether insulin resistance is due to glucose toxicity in the post-Diabetes Control and Complications Trial era of tighter glucose control.

OBJECTIVE

We performed this study to determine muscle and liver insulin sensitivity individuals with type 1 diabetes after overnight insulin infusion to lower fasting glucose concentration.

DESIGN, PATIENTS, AND METHODS

Fifty subjects [25 controls without and 25 individuals with type 1 diabetes (diabetes duration 22.9 ± 1.7 yr, without known end organ damage] were frequency matched on age and body mass index by group and studied. After 3 d of dietary control and overnight insulin infusion to normalize glucose, we performed a three-stage hyperinsulinemic/euglycemic clamp infusing insulin at 4, 8, and 40 mU/m(2) · min. Glucose metabolism was quantified using an infusion of [6,6-(2)H(2)]glucose. Hepatic insulin sensitivity was measured using the insulin IC(50) for glucose rate of appearance (Ra), whereas muscle insulin sensitivity was measured using the glucose rate of disappearance during the highest insulin dose.

RESULTS

Throughout the study, glucose Ra was significantly greater in individuals compared with those without type 1 diabetes. The concentration of insulin required for 50% suppression of glucose Ra was 2-fold higher in subjects with type 1 diabetes. Glucose rate of disappearance was significantly lower in individuals with type 1 diabetes during the 8- and 40-mU/m(2) · min stages.

CONCLUSION

Insulin resistance in liver and skeletal muscle was a significant feature in type 1 diabetes. Nevertheless, the etiology of insulin resistance was not explained by body mass index, percentage fat, plasma lipids, visceral fat, and physical activity and was also not fully explained by hyperglycemia.

Insulin resistance after a 72-h fast is associated with impaired AS160 phosphorylation and accumulation of lipid and glycogen in human skeletal muscle

During fasting, human skeletal muscle depends on lipid oxidation for its energy substrate metabolism. This is associated with the development of insulin resistance and a subsequent reduction of insulin-stimulated glucose uptake. The underlying mechanisms controlling insulin action on skeletal muscle under these conditions are unresolved. In a randomized design, we investigated eight healthy subjects after a 72-h fast compared with a 10-h overnight fast. Insulin action on skeletal muscle was assessed by a hyperinsulinemic euglycemic clamp and by determining insulin signaling to glucose transport. In addition, substrate oxidation, skeletal muscle lipid content, regulation of glycogen synthesis, and AMPK signaling were assessed. Skeletal muscle insulin sensitivity was reduced profoundly in response to a 72-h fast and substrate oxidation shifted to predominantly lipid oxidation. This was associated with accumulation of both lipid and glycogen in skeletal muscle. Intracellular insulin signaling to glucose transport was impaired by regulation of phosphorylation at specific sites on AS160 but not TBC1D1, both key regulators of glucose uptake. In contrast, fasting did not impact phosphorylation of AMPK or insulin regulation of Akt, both of which are established upstream kinases of AS160. These findings show that insulin resistance in muscles from healthy individuals is associated with suppression of site-specific phosphorylation of AS160, without Akt or AMPK being affected. This impairment of AS160 phosphorylation, in combination with glycogen accumulation and increased intramuscular lipid content, may provide the underlying mechanisms for resistance to insulin in skeletal muscle after a prolonged fast.

Prolonged fasting identifies skeletal muscle mitochondrial dysfunction as consequence rather than cause of human insulin resistance

OBJECTIVE

Type 2 diabetes and insulin resistance have been associated with mitochondrial dysfunction, but it is debated whether this is a primary factor in the pathogenesis of the disease. To test the concept that mitochondrial dysfunction is secondary to the development of insulin resistance, we employed the unique model of prolonged fasting in humans. Prolonged fasting is a physiologic condition in which muscular insulin resistance develops in the presence of increased free fatty acid (FFA) levels, increased fat oxidation and low glucose and insulin levels. It is therefore anticipated that skeletal muscle mitochondrial function is maintained to accommodate increased fat oxidation unless factors secondary to insulin resistance exert negative effects on mitochondrial function.

RESEARCH DESIGN AND METHODS

While in a respiration chamber, twelve healthy males were subjected to a 60 h fast and a 60 h normal fed condition in a randomized crossover design. Afterward, insulin sensitivity was assessed using a hyperinsulinemic-euglycemic clamp, and mitochondrial function was quantified ex vivo in permeabilized muscle fibers using high-resolution respirometry.

RESULTS

Indeed, FFA levels were increased approximately ninefold after 60 h of fasting in healthy male subjects, leading to elevated intramuscular lipid levels and decreased muscular insulin sensitivity. Despite an increase in whole-body fat oxidation, we observed an overall reduction in both coupled state 3 respiration and maximally uncoupled respiration in permeabilized skeletal muscle fibers, which could not be explained by changes in mitochondrial density.

CONCLUSIONS

These findings confirm that the insulin-resistant state has secondary negative effects on mitochondrial function. Given the low insulin and glucose levels after prolonged fasting, hyperglycemia and insulin action per se can be excluded as underlying mechanisms, pointing toward elevated plasma FFA and/or intramuscular fat accumulation as possible causes for the observed reduction in mitochondrial capacity.

Inflexibility in intramuscular triglyceride fractional synthesis distinguishes prediabetes from obesity in humans

Whether intramuscular triglyceride (IMTG) concentration or flux is more important in the progression to type 2 diabetes is controversial. Therefore, this study examined IMTG concentration, as well as its fractional synthesis rate (FSR), in obese people with normal glucose tolerance (NGT; n = 20) vs. obese people with prediabetes (PD; n = 19), at rest and during exercise. Insulin action and secretion were assessed using an intravenous glucose tolerance test. [U-(13)C]palmitate was infused for 4 h before and throughout 1.5 h of treadmill walking at 50% VO(2(max)). IMTG concentration was measured by gas chromatograph/mass spectrometer, and FSR by gas chromatography-combustion isotope ratio mass spectrometer, from muscle biopsies taken immediately before and after exercise. Basal IMTG concentration was higher (43 +/- 5.7 vs. 27 +/- 3.9 mg/mg dry weight, P = 0.03) and FSR trended lower (0.23 +/- 0.04 vs. 0.32 +/- 0.05/h, P = 0.075), as did insulin action (S(i); 2.9 +/- 0.43 vs. 3.3 +/- 0.35 x 10(-4)/mU/ml, P = 0.07), in PD vs. NGT. IMTG concentration did not change significantly during exercise, but was no longer different in PD vs. NGT (45 +/- 7.7 vs. 37 +/- 5.8 mg/mg dry weight, P = 0.41). IMTG FSR suppressed during exercise in NGT (-81% to 0.06 +/- 0.13/h, P = 0.02), but not PD (+4% to 0.24 +/- 0.13%/h, P = 0.95). Palmitate oxidation was similar during rest (P = 0.92) and exercise (P = 0.94) between groups, but its source appeared different with more coming from muscle at rest and plasma during exercise in NGT, whereas the converse was true in PD. Altogether, higher basal IMTG concentration that is metabolically inflexible distinguishes obese people with PD from those with NGT.

Enhanced glycogen metabolism in adipose tissue decreases triglyceride mobilization

Adipose tissue is a primary site for lipid storage containing trace amounts of glycogen. However, refeeding after a prolonged partial fast produces a marked transient spike in adipose glycogen, which dissipates in coordination with the initiation of lipid resynthesis. To further study the potential interplay between glycogen and lipid metabolism in adipose tissue, the aP2-PTG transgenic mouse line was utilized since it contains a 100- to 400-fold elevation of adipocyte glycogen levels that are mobilized upon fasting. To determine the fate of the released glucose 1-phosphate, a series of metabolic measurements were made. Basal and isoproterenol-stimulated lactate production in vitro was significantly increased in adipose tissue from transgenic animals. In parallel, basal and isoproterenol-induced release of nonesterified fatty acids (NEFAs) was significantly reduced in transgenic adipose tissue vs. control. Interestingly, glycerol release was unchanged between the genotypes, suggesting that enhanced triglyceride resynthesis was occurring in the transgenic tissue. Qualitatively similar results for NEFA and glycerol levels between wild-type and transgenic animals were obtained in vivo during fasting. Additionally, the physiological upregulation of the phosphoenolpyruvate carboxykinase cytosolic isoform (PEPCK-C) expression in adipose upon fasting was significantly blunted in transgenic mice. No changes in whole body metabolism were detected through indirect calorimetry. Yet weight loss following a weight gain/loss protocol was significantly impeded in the transgenic animals, indicating a further impairment in triglyceride mobilization. Cumulatively, these results support the notion that the adipocyte possesses a set point for glycogen, which is altered in response to nutritional cues, enabling the coordination of adipose glycogen turnover with lipid metabolism.

Short-term sprint interval training increases insulin sensitivity in healthy adults but does not affect the thermogenic response to beta-adrenergic stimulation

Sprint interval training (SIT) and traditional endurance training elicit similar physiological adaptations. From the perspective of metabolic function, superior glucose regulation is a common characteristic of endurance-trained adults. Accordingly, we have investigated the hypothesis that short-term SIT will increase insulin sensitivity in sedentary/recreationally active humans. Thirty one healthy adults were randomly assigned to one of three conditions: (1) SIT (n = 12): six sessions of repeated (4-7) 30 s bouts of very high-intensity cycle ergometer exercise over 14 days; (2) sedentary control (n = 10); (3) single-bout SIT (n = 9): one session of 4 x 30 s cycle ergometer sprints. Insulin sensitivity was determined (hyperinsulinaemic euglycaemic clamp) prior to and 72 h following each intervention. Compared with baseline, and sedentary and single-bout controls, SIT increased insulin sensitivity (glucose infusion rate: 6.3 +/- 0.6 vs. 8.0 +/- 0.8 mg kg(1) min(1); mean +/- s.e.m.; P = 0.04). In a separate study, we investigated the effect of SIT on the thermogenic response to beta-adrenergic receptor (beta-AR) stimulation, an important determinant of energy balance. Compared with baseline, and sedentary and single-bout control groups, SIT did not affect resting energy expenditure (EE: ventilated hood technique; 6274 +/- 226 vs. 6079 +/- 297 kJ day(1); P = 0.51) or the thermogenic response to isoproterenol (6, 12 and 24 ng (kg fat-free mass)(1) min(1): %EE 11 +/- 2, 14 +/- 3, 23 +/- 2 vs. 11 +/- 1, 16 +/- 2, 25 +/- 3; P = 0.79). Combined data from both studies revealed no effect of SIT on fasted circulating concentrations of glucose, insulin, adiponectin, pigment epithelial-derived factor, non-esterified fatty acids or noradrenaline (all P > 0.05). Sixteen minutes of high-intensity exercise over 14 days augments insulin sensitivity but does not affect the thermogenic response to beta-AR stimulation.

Short-term sprint interval training increases insulin sensitivity in healthy adults but does not affect the thermogenic response to beta-adrenergic stimulation

Sprint interval training (SIT) and traditional endurance training elicit similar physiological adaptations. From the perspective of metabolic function, superior glucose regulation is a common characteristic of endurance-trained adults. Accordingly, we have investigated the hypothesis that short-term SIT will increase insulin sensitivity in sedentary/recreationally active humans. Thirty one healthy adults were randomly assigned to one of three conditions: (1) SIT (n = 12): six sessions of repeated (4-7) 30 s bouts of very high-intensity cycle ergometer exercise over 14 days; (2) sedentary control (n = 10); (3) single-bout SIT (n = 9): one session of 4 x 30 s cycle ergometer sprints. Insulin sensitivity was determined (hyperinsulinaemic euglycaemic clamp) prior to and 72 h following each intervention. Compared with baseline, and sedentary and single-bout controls, SIT increased insulin sensitivity (glucose infusion rate: 6.3 +/- 0.6 vs. 8.0 +/- 0.8 mg kg(1) min(1); mean +/- s.e.m.; P = 0.04). In a separate study, we investigated the effect of SIT on the thermogenic response to beta-adrenergic receptor (beta-AR) stimulation, an important determinant of energy balance. Compared with baseline, and sedentary and single-bout control groups, SIT did not affect resting energy expenditure (EE: ventilated hood technique; 6274 +/- 226 vs. 6079 +/- 297 kJ day(1); P = 0.51) or the thermogenic response to isoproterenol (6, 12 and 24 ng (kg fat-free mass)(1) min(1): %EE 11 +/- 2, 14 +/- 3, 23 +/- 2 vs. 11 +/- 1, 16 +/- 2, 25 +/- 3; P = 0.79). Combined data from both studies revealed no effect of SIT on fasted circulating concentrations of glucose, insulin, adiponectin, pigment epithelial-derived factor, non-esterified fatty acids or noradrenaline (all P > 0.05). Sixteen minutes of high-intensity exercise over 14 days augments insulin sensitivity but does not affect the thermogenic response to beta-AR stimulation.

Adaptation of myocardial substrate metabolism to a ketogenic nutrient environment

Heart muscle is metabolically versatile, converting energy stored in fatty acids, glucose, lactate, amino acids, and ketone bodies. Here, we use mouse models in ketotic nutritional states (24 h of fasting and a very low carbohydrate ketogenic diet) to demonstrate that heart muscle engages a metabolic response that limits ketone body utilization. Pathway reconstruction from microarray data sets, gene expression analysis, protein immunoblotting, and immunohistochemical analysis of myocardial tissue from nutritionally modified mouse models reveal that ketotic states promote transcriptional suppression of the key ketolytic enzyme, succinyl-CoA:3-oxoacid CoA transferase (SCOT; encoded by Oxct1), as well as peroxisome proliferator-activated receptor alpha-dependent induction of the key ketogenic enzyme HMGCS2. Consistent with reduction of SCOT, NMR profiling demonstrates that maintenance on a ketogenic diet causes a 25% reduction of myocardial (13)C enrichment of glutamate when (13)C-labeled ketone bodies are delivered in vivo or ex vivo, indicating reduced procession of ketones through oxidative metabolism. Accordingly, unmetabolized substrate concentrations are higher within the hearts of ketogenic diet-fed mice challenged with ketones compared with those of chow-fed controls. Furthermore, reduced ketone body oxidation correlates with failure of ketone bodies to inhibit fatty acid oxidation. These results indicate that ketotic nutrient environments engage mechanisms that curtail ketolytic capacity, controlling the utilization of ketone bodies in ketotic states.

Metabolic actions of insulin in men and women

Insulin is an important regulator of glucose, lipid, and protein metabolism. It suppresses hepatic glucose and triglyceride production, inhibits adipose tissue lipolysis and whole-body and muscle proteolysis, and stimulates glucose uptake in muscle. In this review we discuss what is currently known about the control of substrate metabolism by insulin in men and women. The data available so far indicate that women are more sensitive to insulin with regards to glucose metabolism (both in the liver and in muscle), whereas there are no differences between men and women in insulin action on lipolysis. Potential differences exist in the regulation of plasma triglyceride concentration and protein metabolism by insulin and in changes in insulin action in response to stimuli (e.g., weight loss and exercise) that are known to alter insulin sensitivity. However, these areas have not been studied comprehensively enough to draw firm conclusions.

Intermittent fasting does not affect whole-body glucose, lipid, or protein metabolism

BACKGROUND

Intermittent fasting (IF) was shown to increase whole-body insulin sensitivity, but it is uncertain whether IF selectively influences intermediary metabolism. Such selectivity might be advantageous when adapting to periods of food abundance and food shortage.

OBJECTIVE

The objective was to assess effects of IF on intermediary metabolism and energy expenditure.

DESIGN

Glucose, glycerol, and valine fluxes were measured after 2 wk of IF and a standard diet (SD) in 8 lean healthy volunteers in a crossover design, in the basal state and during a 2-step hyperinsulinemic euglycemic clamp, with assessment of energy expenditure and phosphorylation of muscle protein kinase B (AKT), glycogen synthase kinase (GSK), and mammalian target of rapamycine (mTOR). We hypothesized that IF selectively increases peripheral glucose uptake and lowers proteolysis, thereby protecting protein stores.

RESULTS

No differences in body weight were observed between the IF and SD groups. Peripheral glucose uptake and hepatic insulin sensitivity during the clamp did not significantly differ between the IF and SD groups. Likewise, lipolysis and proteolysis were not different between the IF and SD groups. IF decreased resting energy expenditure. IF had no effect on the phosphorylation of AKT but significantly increased the phosphorylation of glycogen synthase kinase. Phosphorylation of mTOR was significantly lower after IF than after the SD.

CONCLUSIONS

IF does not affect whole-body glucose, lipid, or protein metabolism in healthy lean men despite changes in muscle phosphorylation of GSK and mTOR. The decrease in resting energy expenditure after IF indicates the possibility of an increase in weight during IF when caloric intake is not adjusted. This study was registered at www.trialregister.nl as NTR1841.