Impact of training state on fasting-induced regulation of adipose tissue metabolism in humans

A detective story of intermittent fasting effect on immunity

Intermittent fasting (IF) refers to periodic fasting routines, that caloric intake is minimized not by meal portion size reduction but by intermittently eliminating ingestion of one or several consecutive meals. IF can instigate comprehensive and multifaceted alterations in energy metabolism, these metabolic channels may aboundingly function as primordial mechanisms that interface with the immune system, instigating intricate immune transformations. This review delivers a comprehensive understanding of IF, paying particular attention to its influence on the immune system, thus seeking to bridge these two research domains. We explore how IF effects lipid metabolism, hormonal levels, circadian rhythm, autophagy, oxidative stress, gut microbiota, and intestinal barrier integrity, and conjecture about the mechanisms orchestrating the intersect between these factors and the immune system. Moreover, the review includes research findings on the implications of IF on the immune system and patients burdened with autoimmune diseases.

Multitissue responses to exercise: a MoTrPAC feasibility study

We assessed the feasibility of the Molecular Transducers of Physical Activity Consortium (MoTrPAC) human adult clinical exercise protocols, while also documenting select cardiovascular, metabolic, and molecular responses to these protocols. After phenotyping and familiarization sessions, 20 subjects (25 ± 2 yr, 12 M, 8 W) completed an endurance exercise bout (n = 8, 40 min cycling at 70% V̇o2max), a resistance exercise bout (n = 6, ∼45 min, 3 sets of ∼10 repetition maximum, 8 exercises), or a resting control period (n = 6, 40 min rest). Blood samples were taken before, during, and after (10 min, 2 h, and 3.5 h) exercise or rest for levels of catecholamines, cortisol, glucagon, insulin, glucose, free fatty acids, and lactate. Heart rate was recorded throughout exercise (or rest). Skeletal muscle (vastus lateralis) and adipose (periumbilical) biopsies were taken before and ∼4 h following exercise or rest for mRNA levels of genes related to energy metabolism, growth, angiogenesis, and circadian processes. Coordination of the timing of procedural components (e.g., local anesthetic delivery, biopsy incisions, tumescent delivery, intravenous line flushes, sample collection and processing, exercise transitions, and team dynamics) was reasonable to orchestrate while considering subject burden and scientific objectives. The cardiovascular and metabolic alterations reflected a dynamic and unique response to endurance and resistance exercise, whereas skeletal muscle was transcriptionally more responsive than adipose 4 h postexercise. In summary, the current report provides the first evidence of protocol execution and feasibility of key components of the MoTrPAC human adult clinical exercise protocols. Scientists should consider designing exercise studies in various populations to interface with the MoTrPAC protocols and DataHub.NEW & NOTEWORTHY This study highlights the feasibility of key aspects of the MoTrPAC adult human clinical protocols. This initial preview of what can be expected from acute exercise trial data from MoTrPAC provides an impetus for scientists to design exercise studies to interlace with the rich phenotypic and -omics data that will populate the MoTrPAC DataHub at the completion of the parent protocol.

Exercise Equals the Mobilization of Visceral versus Subcutaneous Adipose Fatty Acid Molecules in Fasted Rats Associated with the Modulation of the AMPK/ATGL/HSL Axis

Combining exercise with fasting is known to boost fat mass-loss, but detailed analysis on the consequential mobilization of visceral and subcutaneous WAT-derived fatty acids has not been performed. In this study, a subset of fasted male rats (66 h) was submitted to daily bouts of mild exercise. Subsequently, by using gas chromatography-flame ionization detection, the content of 22 fatty acids (FA) in visceral (v) versus subcutaneous (sc) white adipose tissue (WAT) depots was compared to those found in response to the separate events. Findings were related to those obtained in serum and liver samples, the latter taking up FA to increase gluconeogenesis and ketogenesis. Each separate intervention reduced scWAT FA content, associated with increased levels of adipose triglyceride lipase (ATGL) protein despite unaltered AMP-activated protein kinase (AMPK) Thr172 phosphorylation, known to induce ATGL expression. The mobility of FAs from vWAT during fasting was absent with the exception of the MUFA 16:1 n-7 and only induced by combining fasting with exercise which was accompanied with reduced hormone sensitive lipase (HSL) Ser563 and increased Ser565 phosphorylation, whereas ATGL protein levels were elevated during fasting in association with the persistently increased phosphorylation of AMPK at Thr172 both during fasting and in response to the combined intervention. As expected, liver FA content increased during fasting, and was not further affected by exercise, despite additional FA release from vWAT in this condition, underlining increased hepatic FA metabolism. Both fasting and its combination with exercise showed preferential hepatic metabolism of the prominent saturated FAs C:16 and C:18 compared to the unsaturated FAs 18:1 n-9 and 18:2 n-6:1. In conclusion, depot-specific differences in WAT fatty acid molecule release during fasting, irrelevant to their degree of saturation or chain length, are mitigated when combined with exercise, to provide fuel to surrounding organs such as the liver which is correlated with increased ATGL/ HSL ratios, involving AMPK only in vWAT.

Acute and long-term exercise adaptation of adipose tissue and skeletal muscle in humans: a matched transcriptomics approach after 8-week training-intervention

BACKGROUND

Exercise exerts many health benefits by directly inducing molecular alterations in physically utilized skeletal muscle. Molecular adaptations of subcutaneous adipose tissue (SCAT) might also contribute to the prevention of metabolic diseases.

AIM

To characterize the response of human SCAT based on changes in transcripts and mitochondrial respiration to acute and repeated bouts of exercise in comparison to skeletal muscle.

METHODS

Sedentary participants (27 ± 4 yrs) with overweight or obesity underwent 8-week supervised endurance exercise 3×1h/week at 80% VO2peak. Before, 60 min after the first and last exercise bout and 5 days post intervention, biopsies were taken for transcriptomic analyses and high-resolution respirometry (n = 14, 8 female/6 male).

RESULTS

In SCAT, we found 37 acutely regulated transcripts (FC > 1.2, FDR < 10%) after the first exercise bout compared to 394, respectively, in skeletal muscle. Regulation of only 5 transcripts overlapped between tissues highlighting their differential response. Upstream and enrichment analyses revealed reduced transcripts of lipid uptake, storage and lipogenesis directly after exercise in SCAT and point to β-adrenergic regulation as potential major driver. The data also suggest an exercise-induced modulation of the circadian clock in SCAT. Neither term was associated with transcriptomic changes in skeletal muscle. No evidence for beigeing/browning was found in SCAT along with unchanged respiration.

CONCLUSIONS

Adipose tissue responds completely distinct from adaptations of skeletal muscle to exercise. The acute and repeated reduction in transcripts of lipid storage and lipogenesis, interconnected with a modulated circadian rhythm, can counteract metabolic syndrome progression toward diabetes.

The impact of fasting on adipose tissue metabolism

Fasting and starvation were common occurrences during human evolution and accordingly have been an important environmental factor shaping human energy metabolism. Humans can tolerate fasting reasonably well through adaptative and well-orchestrated time-dependent changes in energy metabolism. Key features of the adaptive response to fasting are the breakdown of liver glycogen and muscle protein to produce glucose for the brain, as well as the gradual depletion of the fat stores, resulting in the release of glycerol and fatty acids into the bloodstream and the production of ketone bodies in the liver. In this paper, an overview is presented of our current understanding of the effects of fasting on adipose tissue metabolism. Fasting leads to reduced uptake of circulating triacylglycerols by adipocytes through inhibition of the activity of the rate-limiting enzyme lipoprotein lipase. In addition, fasting stimulates the degradation of stored triacylglycerols by activating the key enzyme adipose triglyceride lipase. The mechanisms underlying these events are discussed, with a special interest in insights gained from studies on humans. Furthermore, an overview is presented of the effects of fasting on other metabolic pathways in the adipose tissue, including fatty acid synthesis, glucose uptake, glyceroneogenesis, autophagy, and the endocrine function of adipose tissue.

Metabolic impact of feeding prior to a 60-min bout of moderate-intensity exercise in females in a fasted state

Background

The metabolic impact of pre-exercise feeding of protein or carbohydrate on fat oxidation and energy expenditure rates, especially, in females, is poorly understood.

Methods

Recreationally active females (n = 15, 32 ± 10 years, 164.8 ± 5.6 cm, 63.5 ± 9.3 kg, 23.4 ± 3.2 kg/m²) completed four testing sessions in a randomized, double-blind, crossover fashion after fasting overnight. Participants ingested isovolumetric and isoenergetic solutions containing either 25 g of whey protein, casein protein, carbohydrate (CHO), or a non-caloric placebo (PLA). Participants then completed 60 min of treadmill exercise at 15% below ventilatory threshold 30 min after ingestion. Respiratory exchange ratio (RER) was evaluated throughout exercise and resting energy expenditure (REE) was assessed pre-exercise, and 0-, 60-, and 120-min post-exercise.

Results

A significant condition x time interaction was observed for RER (p = 0.008) during exercise, with CHO exhibiting higher RER values (vs. PLA) at four time points. A significant main effect for condition was observed for carbohydrate (p = 0.001) and fat (p = 0.02) oxidation rates during exercise, with fat oxidation rates being higher in PLA vs. CHO (p = 0.01). When total fat oxidized was calculated across the entire exercise bout, a significant main effect for condition was observed (p = 0.01), with PLA being greater than CHO (p = 0.04). A significant condition x time interaction (p = 0.02) was found for both absolute and normalized REE, with casein and whey protein having significantly higher values than CHO (p < 0.05) immediately post-exercise.

Conclusion

When compared to a fasted control (PLA), consuming CHO, but not protein, decreased total fat oxidation prior to a 60-min bout of moderate-intensity exercise in females.

The mitochondrial profile in women with polycystic ovary syndrome: impact of exercise

Polycystic ovary syndrome (PCOS) is a common endocrine disorder affecting pre-menopausal women and involves metabolic dysregulation. Despite the high prevalence of insulin resistance, the existence of mitochondrial dysregulation and its role in the pathogenesis of PCOS is not clear. Exercise is recommended as the first-line therapy for women with PCOS. In particular, high-intensity interval training (HIIT) is known to improve metabolic health and enhance mitochondrial characteristics. In this narrative review, the existing knowledge of mitochondrial characteristics in skeletal muscle and adipose tissue of women with PCOS and the effect of exercise interventions in ameliorating metabolic and mitochondrial health in these women are discussed. Even though the evidence on mitochondrial dysfunction in PCOS is limited, some studies point to aberrant mitochondrial functions mostly in skeletal muscle, while there is very little research in adipose tissue. Although most exercise intervention studies in PCOS report improvements in metabolic health, they show diverse and inconclusive findings in relation to mitochondrial characteristics. A limitation of the current study is the lack of comprehensive mitochondrial analyses and the diversity in exercise modalities, with only one study investigating the impact of HIIT alone. Therefore, further comprehensive large-scale exercise intervention studies are required to understand the association between metabolic dysfunction and aberrant mitochondrial profile, and the molecular mechanisms underlying the exercise-induced metabolic adaptations in women with PCOS.

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.

GDF15 is an exercise-induced hepatokine regulated by glucagon and insulin in humans

OBJECTIVE

Growth differentiation factor (GDF)-15 is implicated in regulation of metabolism and circulating GDF15 increases in response to exercise. The source and regulation of the exercise-induced increase in GDF15 is, however not known.

METHOD

Plasma GDF15 was measured by ELISA under the following conditions: 1) Arterial-to-hepatic venous differences sampled before, during, and after exercise in healthy male subjects (n=10); 2) exogenous glucagon infusion compared to saline infusion in resting healthy subjects (n=10); 3) an acute exercise bout with and without a pancreatic clamp (n=6); 4) healthy subjects for 36 hours (n=17), and 5) patients with anorexia nervosa (n=25) were compared to healthy age-matched subjects (n=25). Tissue GDF15 mRNA content was determined in mice in response to exhaustive exercise (n=16).

RESULTS

The splanchnic bed released GDF15 to the circulation during exercise and increasing the glucagon-to-insulin ratio in resting humans led to a 2.7-fold (P<0.05) increase in circulating GDF15. Conversely, inhibiting the exercise-induced increase in the glucagon-to-insulin ratio blunted the exercise-induced increase in circulating GDF15. Fasting for 36 hours did not affect circulating GDF15, whereas resting patients with anorexia nervosa displayed elevated plasma concentrations (1.4-fold, P<0.05) compared to controls. In mice, exercise increased GDF15 mRNA contents in liver, muscle, and adipose tissue.

CONCLUSION

In humans, GDF15 is a "hepatokine" which increases during exercise and is at least in part regulated by the glucagon-to-insulin ratio. Moreover, chronic energy deprivation is associated with elevated plasma GDF15, which supports that GDF15 is implicated in metabolic signalling in humans.

Ameliorating effects of lifelong physical activity on healthy aging and mitochondrial function in human white adipose tissue

Growing old is patently among the most prominent risk factors for lifestyle related diseases and deterioration in physical performance. Aging in particular affects mitochondrial homeostasis and maintaining a well-functioning mitochondrial pool is imperative in order to avoid age-associated metabolic decline. White adipose tissue (WAT) is a key organ in energy balance and impaired mitochondrial function in adipocytes has been associated with increased low-grade inflammation, altered metabolism, excessive ROS production and an accelerated aging phenotype. Exercise training improves mitochondrial health but whether lifelong exercise training can sufficiently maintain WAT mitochondrial function is currently unknown. Therefore, to dissect the role and dose-dependence of lifelong exercise training on aging WAT metabolic parameters and mitochondrial function, young and older untrained, as well as moderately and highly exercise trained older male subjects were recruited and abdominal subcutaneous (s)WAT biopsies and venous blood samples were obtained to measure mitochondrial function and key metabolic factors in WAT and plasma. Mitochondrial intrinsic respiratory capacity was lower in sWAT from older than in young subjects. In spite of this, maximal mitochondrial respiration per wet weight, markers of oxidative capacity, and mitophagic capacity were increased in sWAT from lifelong highly exercise trained than all other groups. Furthermore, ROS emission was generally lower in sWAT from lifelong highly exercise trained than older untrained subjects. Taken together, aging reduces intrinsic mitochondrial respiration in human sWAT, but lifelong high volume exercise training increases oxidative capacity by increasing mitochondrial volume likely contributing to healthy aging.

Metabolic and Molecular Subacute Effects of a Single Moderate-Intensity Exercise Bout, Performed in the Fasted State, in Obese Male Rats

Introduction and objectives: Obesity represents a major global public health problem. Its etiology is multifactorial and includes poor dietary habits, such as hypercaloric and hyperlipidic diets (HFDs), physical inactivity, and genetic factors. Regular exercise is, per se, a tool for the treatment and prevention of obesity, and recent studies suggest that the beneficial effects of exercise can be potentiated by the fasting state, thus potentially promoting additional effects. Despite the significant number of studies showing results that corroborate such hypothesis, very few have evaluated the effects of fasted-state exercise in overweight/obese populations. Therefore, the aim of this study was to evaluate the subacute effects (12 h after conclusion) of a single moderate-intensity exercise bout, performed in either a fed or an 8 h fasted state, on serum profile, substrate-content and heat shock pathway–related muscle protein immunocontent in obese male rats. Methods: Male Wistar rats received a modified high-fat diet for 12 weeks to induce obesity and insulin resistance. The animals were allocated to four groups: fed rest (FER), fed exercise (FEE), fasted rest (FAR) and fasted exercise (FAE). The exercise protocol was a 30 min session on a treadmill, with an intensity of 60% of VO₂max. The duration of the fasting period was 8 h prior to the exercise session. After a 12 h recovery, the animals were killed and metabolic parameters of blood, liver, heart, gastrocnemius and soleus muscles were evaluated, as well as SIRT1 and HSP70 immunocontent in the muscles. Results: HFD induced obesity and insulin resistance. Soleus glycogen concentration decreased in the fasted groups and hepatic glycogen decreased in the fed exercise group. The combination of exercise and fasting promoted a decreased concentration of serum total cholesterol and triglycerides. In the heart, combination fasting plus exercise was able to decrease triglycerides to control levels. In the soleus muscle, both fasting and fasting plus exercise were able to decrease triglyceride concentrations. In addition, heat shock protein 70 and sirtuin 1 immunocontent increased after exercise in the gastrocnemius and soleus muscles. Conclusions: An acute bout of moderate intensity aerobic exercise, when realized in fasting, may induce, in obese rats with metabolic dysfunctions, beneficial adaptations to their health, such as better biochemical and molecular adaptations that last for at least 12 h. Considering the fact that overweight/obese populations present an increased risk of cardiovascular events/diseases, significant reductions in such plasma markers of lipid metabolism are an important achievement for these populations.

Visceral Adipose Tissue Displays Unique Metabolomic Fingerprints in Obesity, Pre-Diabetes and Type 2 Diabetes

Visceral adipose tissue (VAT) metabolic profiling harbors the potential to disentangle molecular changes underlying obesity-related dysglycemia. In this study, the VAT exometabolome of subjects with obesity and different glycemic statuses are analyzed. The subjects (n = 19) are divided into groups according to body mass index and glycemic status: subjects with obesity and euglycemia (Ob+NGT, n = 5), subjects with obesity and pre-diabetes (Ob+Pre-T2D, n = 5), subjects with obesity and type 2 diabetes under metformin treatment (Ob+T2D, n = 5) and subjects without obesity and with euglycemia (Non-Ob, n = 4), used as controls. VATs are incubated in culture media and extracellular metabolite content is determined by proton nuclear magnetic resonance (1H-NMR). Glucose consumption is not different between the groups. Pyruvate and pyroglutamate consumption are significantly lower in all groups of subjects with obesity compared to Non-Ob, and significantly lower in Ob+Pre-T2D as compared to Ob+NGT. In contrast, isoleucine consumption is significantly higher in all groups of subjects with obesity, particularly in Ob+Pre-T2D, compared to Non-Ob. Acetate production is also significantly lower in Ob+Pre-T2D compared to Non-Ob. In sum, the VAT metabolic fingerprint is associated with pre-diabetes and characterized by higher isoleucine consumption, accompanied by lower acetate production and pyruvate and pyroglutamate consumption. We propose that glucose metabolism follows different fates within the VAT, depending on the individuals' health status.

Post-exercise Effects and Long-Term Training Adaptations of Hormone Sensitive Lipase Lipolysis Induced by High-Intensity Interval Training in Adipose Tissue of Mice

Although studies have proven that high-intensity interval training (HIIT) shows a comparable effect to moderate-intensity continuous training (MICT) on reducing body fat, especially visceral fat, the mechanism is still unclear. Since MICT consumes more fat during exercise, the mechanism of HIIT weight loss may be related to post-exercise effects, long-term adaptive changes, and hormone sensitive lipase (HSL). The objective of this study was to compare the post-effects of acute exercise, long-term adaptive changes on HSL activity, and catecholamine-induced lipolysis between HIIT and MICT. Following a 14-week high-fat diet (HFD), obese female C57Bl/6 mice were divided into acute exercise groups (one time training, sacrificed at rest and 0, 1, and 12 h after exercise, n = 49), -L groups (12-week long-term training, 12-h fasting, n = 21), and -C groups (12-week training, primary adipocytes were isolated and stimulated by catecholamine in vitro, n = 18). MICT or HIIT treadmill protocols (running distance matched) were carried out during training. Comparison of acute exercise effects by two-way ANOVA showed no time × group interaction effect, however, a significant increase in HSL-Ser563 (at 0 and 1 h) and Ser660 phosphorylation (at 0, 1, and 12 h) in inguinal (subcutaneous) fat was only observed in HIIT mice (p < 0.05 vs. rest), but not in MICT mice. The periuterine (visceral) fat HSL expression and phosphorylation of HIIT mice was similar to or lower than MICT mice. After long-term training, 12-h fasting significantly increased periuterine fat Ser563 phosphorylation in HIIT mice (p < 0.05), but there was no change in MICT mice. Under stimulation of catecholamine in vitro, isolated primary adipocytes from periuterine fat of long-term HIIT mice showed a higher Ser563 increase than that found in MICT mice (p < 0.05). The quantity of triglyceride (TG) lipid bonds (representing lipolysis level) was significantly lower after HIIT than MICT (p < 0.05). The results indicate that (1) acute HIIT can induce an increase of HSL phosphorylation in subcutaneous fat lasting at least 12 h, implying longer post-exercise lipolysis than MICT and (2) long-time HIIT has a better effect on improving catecholamine resistance of visceral adipocytes caused by a HFD, which allows fat to be mobilized more easily when stimulated.

Response of Mitochondrial Respiration in Adipose Tissue and Muscle to 8 Weeks of Endurance Exercise in Obese Subjects

CONTEXT

Exercise training improves glycemic control and increases mitochondrial content and respiration capacity in skeletal muscle. Rodent studies suggest that training increases mitochondrial respiration in adipose tissue.

OBJECTIVE

To assess the effects of endurance training on respiratory capacities of human skeletal muscle and abdominal subcutaneous adipose tissue and to study the correlation with improvement in insulin sensitivity.

DESIGN

Using high-resolution respirometry, we analyzed biopsies from 25 sedentary (VO2 peak 25.1 ± 4.0 VO2 mL/[kg*min]) subjects (16 female, 9 male; 29.8 ± 8.4 years) with obesity (body mass index [BMI] 31.5 ± 4.3 kg/m2), who did not have diabetes. They performed a supervised endurance training over 8 weeks (3 × 1 hour/week at 80% VO2 peak).

RESULTS

Based on change in insulin sensitivity after intervention (using the Matsuda insulin sensitivity index [ISIMats]), subjects were grouped in subgroups as responders (>15% increase in ISIMats) and low-responders. The response in ISIMats was correlated to a reduction of subcutaneous and visceral adipose tissue volume. Both groups exhibited similar increases in fitness, respiratory capacity, and abundance of mitochondrial enzymes in skeletal muscle fibers. Respiratory capacities in subcutaneous adipose tissue were not altered by the intervention. Compared with muscle fibers, adipose tissue respiration showed a preference for β-oxidation and complex II substrates. Respiratory capacities were higher in adipose tissue from female participants.

CONCLUSION

Our data show that the improvement of peripheral insulin sensitivity after endurance training is not directly related to an increase in mitochondrial respiratory capacities in skeletal muscle and occurs without an increase in the respiratory capacity of subcutaneous adipose tissue.

Mobilising vitamin D from adipose tissue: The potential impact of exercise

Vitamin D is lipophilic and accumulates substantially in adipose tissue. Even without supplementation, the amount of vitamin D in the adipose of a typical adult is equivalent to several months of the daily reference nutrient intake (RNI). Paradoxically, despite the large amounts of vitamin D located in adipose tissue, individuals with obesity are often vitamin D deficient according to consensus measures of vitamin D status (serum 25-hydroxyvitamin D concentrations). Thus, it appears that vitamin D can become 'trapped' in adipose tissue, potentially due to insufficient lipolytic stimulation and/or due to tissue dysfunction/adaptation resulting from adipose expansion. Emerging evidence suggests that exercise may mobilise vitamin D from adipose (even in the absence of weight loss). If exercise helps to mobilise vitamin D from adipose tissue, then this could have important ramifications for practitioners and policymakers regarding the management of low circulating levels of vitamin D, as well as chronically low levels of physical activity, obesity and associated health conditions. This perspective led us to design a study to examine the impact of exercise on vitamin D status, vitamin D turnover and adipose tissue vitamin D content (the VitaDEx project). The VitaDEx project will determine whether increasing physical activity (via exercise) represents a potentially useful strategy to mobilise vitamin D from adipose tissue.

Molecular adaptations in human subcutaneous adipose tissue after ten weeks of endurance exercise training in healthy males

Endurance exercise training induces adaptations in metabolically active organs, but adaptations in human subcutaneous adipose tissue (scAT) remains incompletely understood. On the basis of animal studies, we hypothesized that endurance exercise training would increase the expression of proteins involved in lipolysis and glucose uptake in scAT. To test these hypotheses, 19 young and healthy males were randomized to either endurance exercise training (TR; age 18-24 yr; BMI 19.0-25.4 kg/m²) or a nonexercising control group (CON; age 21-35 yr; BMI 20.5-28.8 kg/m²). Abdominal subcutaneous fat biopsies and blood were obtained at rest before and after intervention. By using Western blotting and PCR, we determined expression of lipid droplet-associated proteins, various proteins involved in substrate metabolism, and mRNA abundance of cell surface G protein-coupled receptors (GPCRs). Adipose tissue insulin sensitivity was determined from fasting plasma insulin and nonesterified fatty acids (adipose tissue insulin resistance index; Adipo-IR). Adipo-IR improved in TR compared with CON ( P = 0.03). This was accompanied by increased insulin receptor (IR) protein expression in scAT with a 1.54-fold (SD 0.79) change from baseline in TR vs. 0.85 (SD 0.30) in CON ( P = 0.007). Additionally, hexokinase II (HKII) and succinate dehydrogenase complex subunit A (SDHA) protein increased in TR compared with CON ( P = 0.006 and P = 0.04, respectively). We did not observe changes in lipid droplet-associated proteins or mRNA abundance of GPCRs. Collectively, 10 weeks of endurance exercise training improved adipose tissue insulin sensitivity, which was accompanied by increased IR, HKII, and SDHA protein expression in scAT. We suggest that these adaptations contribute to an improved metabolic flexibility. NEW & NOTEWORTHY This study is the first to investigate the molecular adaptations in human subcutaneous adipose tissue (scAT) after endurance exercise training compared with a nonexercising control group. We show that endurance exercise training improves insulin sensitivity in human scAT, and this is accompanied by increased expression of insulin receptor, hexokinase II, and succinate dehydrogenase complex subunit A. Collectively, our data suggest that endurance exercise training induces molecular adaptations in human scAT, which may contribute to an improved metabolic flexibility.

Training state and skeletal muscle autophagy in response to 36 h of fasting

The present study aimed at investigating fasting-induced responses in regulators and markers of autophagy in vastus lateralis muscle from untrained and trained human subjects. Untrained and trained subjects (based on maximum oxygen uptake, muscle citrate synthase activity, and oxidative phosphorylation protein level) fasted for 36 h with vastus lateralis muscle biopsies obtained at 2, 12, 24, and 36 h after a standardized meal. Fasting reduced ( P < 0.05) skeletal muscle microtubule-associated protein-1A/1B light chain 3 (LC3)I, LC3II, and adaptor protein sequestosome 1/p62 protein content in untrained subjects only. Moreover, skeletal muscle RAC-alpha serine/threonine-protein kinase (AKT)Thr308, AMP-activated protein kinase (AMPK)Thr172, and Unc-51-like autophagy-activating kinase-1 (ULK1)Ser555 phosphorylation state, as well as Bcl-2-interacting coiled-coil protein-1 (Beclin1) and ULK1Ser757 phosphorylation, was lower ( P < 0.05) in trained than untrained subjects during fasting. In addition, the plasma concentrations of several amino acids were higher ( P < 0.05) in trained than untrained subjects, and the plasma concentration profile of several amino acids was different in untrained and trained subjects during fasting. Taken together, these findings suggest that 36-h fasting has effects on some mediators of autophagy in untrained human skeletal muscle and that training state influences the effect of fasting on autophagy signaling and on mediators of autophagy in skeletal muscle.

NEW & NOTEWORTHY This study showed that skeletal muscle autophagy was only modestly affected in humans by 36 h of fasting. Hence, 36-h fasting has effects on some mediators of autophagy in untrained human skeletal muscle, and training state influences the effect of fasting on autophagy signaling and on mediators of autophagy in skeletal muscle.

Impact of skeletal muscle IL-6 on regulation of liver and adipose tissue metabolism during fasting

The liver and adipose tissue are important tissues in whole-body metabolic regulation during fasting. Interleukin 6 (IL-6) is a cytokine shown to be secreted from contracting muscle in humans and suggested to signal to the liver and adipose tissue. Furthermore, skeletal muscle IL-6 has been proposed to play a role during fasting. Therefore the aim of the present study was to investigate the role of skeletal muscle IL-6 in the regulation of substrate production in the liver and adipose tissue during fasting. Male skeletal muscle-specific IL-6 knockout (IL-6 MKO) mice and littermate floxed (control) mice fasted for 6 or 18 h (6 h fasting or 18 h fasting) with corresponding fed control groups (6 h fed or 18 h fed) and liver and adipose tissue were quickly obtained. Plasma β-hydroxybutyrate increased and hepatic glucose, lactate and glycogen decreased with fasting. In addition, fasting increased phosphoenolpyruvate carboxykinase protein and phosphorylation of pyruvate dehydrogenase (PDH) in the liver as well as hormone-sensitive lipase (HSL)^Ser660 and HSL^Ser563 phosphorylation, PDH phosphorylation, adipose triglyceride lipase phosphorylation and perilipin phosphorylation and protein content in adipose tissue without any effect of lack of skeletal muscle IL-6. In conclusion, fasting induced regulation of enzymes in adipose tissue lipolysis and glyceroneogenesis as well as regulation of hepatic gluconeogenic capacity and hepatic substrate utilization in mice. However, skeletal muscle IL-6 was not required for these fasting-induced effects, but had minor effects on markers of lipolysis and glyceroneogenesis in adipose tissue as well as markers of hepatic gluconeogenesis in the fed state.