Reduced glycogen availability is associated with increased AMPKalpha2 activity, nuclear AMPKalpha2 protein abundance, and GLUT4 mRNA expression in contracting human skeletal muscle

An IL-6 promoter variant (-174 G/C) augments IL-6 production and alters skeletal muscle transcription in response to exercise in mice

Interleukin-6 (IL-6) is produced and secreted by skeletal muscle cells during exercise and plays an important role in mediating metabolic responses to exercise. The promoter region of the IL-6 gene contains a common genetic variant (-174 G/C, rs1800795), which may alter responses to exercise training. To isolate the impact of this gene variant on exercise-induced IL-6 expression and skeletal muscle transcription responses following exercise, we generated knock-in mice with a GG or variant CC genotype for the murine homolog of rs1800795. The overall gross metabolic phenotype of resting mice was similar between genotypes; however, following acute treadmill running, the variant CC genotype was associated with a greater increase in skeletal muscle Il6 mRNA and circulating IL-6. Furthermore, we observed that mice with the variant CC genotype exhibited sex-specific differences in skeletal muscle master metabolism regulatory genes and had greater increases in genes controlling mitochondrial biogenesis in skeletal muscle post exercise. However, there was no effect of genotype on exercise-induced skeletal muscle glycogen depletion, circulating free fatty acids, blood glucose and lactate production, or exercise-responsive gene expression in subcutaneous fat. These findings suggest that the IL-6 promoter variant -174 G/C may result in enhanced skeletal muscle adaptations in response to exercise training and could mean that individuals with the "C" allele may more readily gain improvements in metabolic health in response to exercise training.NEW & NOTEWORTHY Interleukin-6 (IL-6) is produced and secreted by skeletal muscle during exercise and mediates metabolic responses to exercise. A common variant in the IL-6 promoter region (-174G/C) may alter responses to exercise training. Mice with the variant "CC" genotype exhibited higher skeletal muscle IL-6 mRNA and circulating IL-6 levels post exercise, as well as altered skeletal muscle gene transcription. This suggests that this variant might enhance muscle adaptations to exercise, potentially benefiting metabolic health.

Responses to Exercise with Low Carbohydrate Availability on Muscle Glycogen and Cell Signaling: A Systematic Review and Meta-analysis

BACKGROUND

The growing interest in how exercise and carbohydrate (CHO) restriction may modify molecular responses that promote endurance adaptations has led to many interesting controversies.

OBJECTIVE

We conducted a systematic review and a meta-analysis regarding the effect of low-carbohydrate availability (LOW) pre-, during, or post-exercise, on the mRNA content of commonly measured genes involved in mitochondrial biogenesis (PGC-1α, TFAM mRNA) and metabolism (PDK4, UCP3 and GLUT4 mRNA), and on muscle glycogen levels, compared with a high-CHO (CON) condition.

METHODS

MEDLINE, Scopus, and Web of Science databases were searched following the PRISMA 2020 guidelines (with an end date of November 2023). In total, 19 randomized-controlled studies were considered for inclusion. We evaluated the methodological quality of all studies using the Cochrane Risk of Bias tool for randomized clinical studies. A meta-analysis was performed using a random effects model to calculate the standardized mean difference (SMD), estimated by Hedges' g, and 95% confidence intervals (CIs).

RESULTS

The LOW condition was associated with an increased mRNA content of several genes during the early recovery period post-exercise, such as PDK4 (SMD 1.61; 95% CI 0.80-2.42), GLUT4 (SMD 1.38; 95% CI 0.46-2.30), and UCP3 (SMD 2.05; 95% CI 0.40-3.69). However, overall, there was no significant effect on the mRNA content of PGC-1α or TFAM. Finally, CHO restriction and exercise significantly reduced muscle glycogen levels (SMD 3.69; 95% CI 2.82-5.09). A meta-analysis of subgroups from studies with a difference in muscle glycogen concentration of > 200 mmol kg dw-1 between the LOW and CON conditions showed an increase in exercise-induced PGC-1α mRNA (SMD 2.08; 95% CI 0.64-3.52; p = 0.005; I2 = 75%) and a greater effect in PDK4 and GLUT4 mRNA.

CONCLUSION

The meta-analysis results show that CHO restriction was associated with an increase in the exercise-induced mRNA content of PDK4, UCP3, and GLUT4, but not the exercise-induced mRNA content of PGC-1ɑ and TFAM. However, when there were substantial differences in glycogen depletion between CON and LOW CHO conditions (> 200 mmol kg dw-1), there was a greater effect of CHO restriction on the exercise-induced mRNA content of metabolic genes, and an increase in exercise-induced PGC-1α mRNA.

Does AMPK bind glycogen in skeletal muscle or is the relationship correlative?

Since its discovery over five decades ago, an emphasis on better understanding the structure and functional role of AMPK has been prevalent. In that time, the role of AMPK as a heterotrimeric enzyme that senses the energy state of various cell types has been established. Skeletal muscle is a dynamic, plastic tissue that adapts to both functional and metabolic demands of the human body, such as muscle contraction or exercise. With a deliberate focus on AMPK in skeletal muscle, this review places a physiological lens to the association of AMPK and glycogen that has been established biochemically. It discusses that, to date, no in vivo association of AMPK with glycogen has been shown and this is not altered with interventions, either by physiological or biochemical utilisation of glycogen in skeletal muscle. The reason for this is likely due to the persistent phosphorylation of Thr148 in the β-subunit of AMPK which prevents AMPK from binding to carbohydrate domains. This review presents the correlative data that suggests AMPK senses glycogen utilisation through a direct interaction with glycogen, the biochemical data showing that AMPK can bind carbohydrate in vitro, and highlights that in a physiological setting of rodent skeletal muscle, AMPK does not directly bind to glycogen.

Delaying post-exercise carbohydrate intake impairs next-day exercise capacity but not muscle glycogen or molecular responses

AIM

To investigate how delayed post-exercise carbohydrate intake affects muscle glycogen, metabolic- and mitochondrial-related molecular responses, and subsequent high-intensity interval exercise (HIIE) capacity.

METHODS

In a double-blind cross-over design, nine recreationally active men performed HIIE (10 × 2-min cycling, ~94% W˙peak) in the fed state, on two occasions. During 0-3 h post-HIIE, participants drank either carbohydrates ("Immediate Carbohydrate" [IC], providing 2.4 g/kg) or water ("Delayed Carbohydrate" [DC]); total carbohydrate intake over 24 h post-HIIE was matched (~7 g/kg/d). Skeletal muscle (sampled pre-HIIE, post-HIIE, +3 h, +8 h, +24 h) was analyzed for whole-muscle glycogen and mRNA content, plus signaling proteins in cytoplasmic- and nuclear-enriched fractions. After 24 h, participants repeated the HIIE protocol until failure, to test subsequent HIIE capacity; blood lactate, heart rate, and ratings of perceived effort (RPE) were measured throughout.

RESULTS

Muscle glycogen concentrations, and relative changes, were similar between conditions throughout (p > 0.05). Muscle glycogen was reduced from baseline (mean ± SD mmol/kg dm; IC: 409 ± 166; DC: 352 ± 76) at post-HIIE (IC: 253 ± 96; DC: 214 ± 82), +3 h (IC: 276 ± 62; DC: 269 ± 116) and + 8 h (IC: 321 ± 56; DC: 269 ± 116), returning to near-baseline by +24 h. Several genes (PGC-1ɑ, p53) and proteins (p-ACCSer79, p-P38 MAPKThr180/Tyr182) elicited typical exercise-induced changes irrespective of condition. Delaying carbohydrate intake reduced next-day HIIE capacity (5 ± 3 intervals) and increased RPE (~2 ratings), despite similar physiological responses between conditions.

CONCLUSION

Molecular responses to HIIE (performed in the fed state) were not enhanced by delayed post-exercise carbohydrate intake. Our findings support immediate post-exercise refueling if the goal is to maximize next-day HIIE capacity and recovery time is ≤24 h.

Microbiota Mediate Enhanced Exercise Capacity Induced by Exercise Training

PURPOSE

We investigated the effects of gut microbes, and the mechanisms mediating the enhanced exercise performance induced by exercise training, i.e., skeletal muscle blood flow, and mitochondrial biogenesis and oxidative function in male mice.

METHODS

All mice received a graded exercise test before (PRE) and after exercise training via forced treadmill running at 60% to 70% of maximal running capacity 5 d·wk -1 for 5 wk (POST). To examine the role of the gut microbes, the graded exercise was repeated after 7 d of access to antibiotic (ABX)-treated water, used to eliminate gut microbes. Peripheral blood flow, mitochondrial oxidative capacity, and markers of mitochondrial biogenesis were collected at each time point.

RESULTS

Exercise training led to increases of 60% ± 13% in maximal running distance and 63% ± 11% work to exhaustion ( P < 0.001). These increases were abolished after ABX ( P < 0.001). Exercise training increased hindlimb blood flow and markers of mitochondrial biogenesis and oxidative function, including AMP-activated protein kinase, sirtuin-1, PGC-1α citrate synthase, complex IV, and nitric oxide, all of which were also abolished by ABX treatment.

CONCLUSIONS

Our results support the concept that gut microbiota mediate enhanced exercise capacity after exercise training and the mechanisms responsible, i.e., hindlimb blood flow, mitochondrial biogenesis, and metabolic profile. Finally, results of this study emphasize the need to fully examine the impact of prescribing ABX to athletes during their training regimens and how this may affect their performance.

Transient changes to metabolic homeostasis initiate mitochondrial adaptation to endurance exercise

Endurance exercise is well established to increase mitochondrial content and function in skeletal muscle, a process termed mitochondrial biogenesis. Current understanding is that exercise initiates skeletal muscle mitochondrial remodeling via modulation of cellular nutrient, energetic and contractile stress pathways. These subtle changes in the cellular milieu are sensed by numerous transduction pathways that serve to initiate and coordinate an increase in mitochondrial gene transcription and translation. The result of these acute signaling events is the promotion of growth and assembly of mitochondria, coupled to a greater capacity for aerobic ATP provision in skeletal muscle. The aim of this review is to highlight the acute metabolic events induced by endurance exercise and the subsequent molecular pathways that sense this transient change in cellular homeostasis to drive mitochondrial adaptation and remodeling.

Numerous Trigger-like Interactions of Kinases/Protein Phosphatases in Human Skeletal Muscles Can Underlie Transient Processes in Activation of Signaling Pathways during Exercise

Optimizing physical training regimens to increase muscle aerobic capacity requires an understanding of the internal processes that occur during exercise that initiate subsequent adaptation. During exercise, muscle cells undergo a series of metabolic events that trigger downstream signaling pathways and induce the expression of many genes in working muscle fibers. There are a number of studies that show the dependence of changes in the activity of AMP-activated protein kinase (AMPK), one of the mediators of cellular signaling pathways, on the duration and intensity of single exercises. The activity of various AMPK isoforms can change in different directions, increasing for some isoforms and decreasing for others, depending on the intensity and duration of the load. This review summarizes research data on changes in the activity of AMPK, Ca²⁺/calmodulin-dependent protein kinase II (CaMKII), and other components of the signaling pathways in skeletal muscles during exercise. Based on these data, we hypothesize that the observed changes in AMPK activity may be largely related to metabolic and signaling transients rather than exercise intensity per se. Probably, the main events associated with these transients occur at the beginning of the exercise in a time window of about 1-10 min. We hypothesize that these transients may be partly due to putative trigger-like kinase/protein phosphatase interactions regulated by feedback loops. In addition, numerous dynamically changing factors, such as [Ca²⁺], metabolite concentration, and reactive oxygen and nitrogen species (RONS), can shift the switching thresholds and change the states of these triggers, thereby affecting the activity of kinases (in particular, AMPK and CaMKII) and phosphatases. The review considers the putative molecular mechanisms underlying trigger-like interactions. The proposed hypothesis allows for a reinterpretation of the experimental data available in the literature as well as the generation of ideas to optimize future training regimens.

Exercise Is Medicine for Nonalcoholic Fatty Liver Disease: Exploration of Putative Mechanisms

Exercise remains a key component of nonalcoholic fatty liver disease (NAFLD) treatment. The mechanisms that underpin improvements in NAFLD remain the focus of much exploration in our attempt to better understand how exercise benefits patients with NAFLD. In this review, we summarize the available scientific literature in terms of mechanistic studies which explore the role of exercise training in modulating fatty acid metabolism, reducing hepatic inflammation, and improving liver fibrosis. This review highlights that beyond simple energy expenditure, the activation of key receptors and pathways may influence the degree of NAFLD-related improvements with some pathways being sensitive to exercise type, intensity, and volume. Importantly, each therapeutic target of exercise training in this review is also the focus of previous or ongoing drug development studies in patients with nonalcoholic steatohepatitis (NASH), and even when a regulatory-agency-approved drug comes to market, exercise will likely remain an integral component in the clinical management of patients with NAFLD and NASH.

New insights into activation and function of the AMPK

The classical role of AMP-activated protein kinase (AMPK) is as a cellular energy sensor activated by falling energy status, signalled by increases in AMP to ATP and ADP to ATP ratios. Once activated, AMPK acts to restore energy homeostasis by promoting ATP-producing catabolic pathways while inhibiting energy-consuming processes. In this Review, we provide an update on this canonical (AMP/ADP-dependent) activation mechanism, but focus mainly on recently described non-canonical pathways, including those by which AMPK senses the availability of glucose, glycogen or fatty acids and by which it senses damage to lysosomes and nuclear DNA. We also discuss new findings on the regulation of carbohydrate and lipid metabolism, mitochondrial and lysosomal homeostasis, and DNA repair. Finally, we discuss the role of AMPK in cancer, obesity, diabetes, nonalcoholic steatohepatitis (NASH) and other disorders where therapeutic targeting may exert beneficial effects.

IL-6 signaling in acute exercise and chronic training: Potential consequences for health and athletic performance

The cytokine interleukin-6 (IL-6) is involved in a diverse set of physiological processes. Traditionally, IL-6 has been thought of in terms of its inflammatory actions during the acute phase response and in chronic conditions such as rheumatoid arthritis and obesity. However, IL-6 is also an important signaling molecule during exercise, being acutely released from working muscle fibers with increased exercise duration, intensity, and muscle glycogen depletion. In this context, IL-6 enables muscle-organ crosstalk, facilitating a coordinated response to help maintain muscle energy homeostasis, while also having anti-inflammatory actions. The range of actions of IL-6 can be explained by its dichotomous signaling pathways. Classical signaling involves IL-6 binding to a cell-surface receptor (mbIL-6R; present on only a small number of cell types) and is the predominant signaling mechanism during exercise. Trans-signaling involves IL-6 binding to a soluble version of its receptor (sIL-6R), with the resulting complex having a much greater half-life and the ability to signal in all cell types. Trans-signaling drives the inflammatory actions of IL-6 and is the predominant pathway in disease. A single nucleotide polymorphism (rs2228145) on the IL-6R gene can modify the classical/trans-signaling balance through increasing the levels of sIL-6R. This SNP has clinical significance, having been linked to inflammatory conditions such as rheumatoid arthritis and type 1 diabetes, as well as to the severity of symptoms experienced with COVID-19. This review will describe how acute exercise, chronic training and the rs2228145 SNP can modify the IL-6 signaling pathway and the consequent implications for health and athletic performance.

Post-translational Modifications: The Signals at the Intersection of Exercise, Glucose Uptake, and Insulin Sensitivity

Diabetes is a global epidemic, of which type 2 diabetes makes up the majority of cases. Nonetheless, for some individuals, type 2 diabetes is eminently preventable and treatable via lifestyle interventions. Glucose uptake into skeletal muscle increases during and in recovery from exercise, with exercise effective at controlling glucose homeostasis in individuals with type 2 diabetes. Furthermore, acute and chronic exercise sensitizes skeletal muscle to insulin. A complex network of signals converge and interact to regulate glucose metabolism and insulin sensitivity in response to exercise. Numerous forms of post-translational modifications (eg, phosphorylation, ubiquitination, acetylation, ribosylation, and more) are regulated by exercise. Here we review the current state of the art of the role of post-translational modifications in transducing exercise-induced signals to modulate glucose uptake and insulin sensitivity within skeletal muscle. Furthermore, we consider emerging evidence for noncanonical signaling in the control of glucose homeostasis and the potential for regulation by exercise. While exercise is clearly an effective intervention to reduce glycemia and improve insulin sensitivity, the insulin- and exercise-sensitive signaling networks orchestrating this biology are not fully clarified. Elucidation of the complex proteome-wide interactions between post-translational modifications and the associated functional implications will identify mechanisms by which exercise regulates glucose homeostasis and insulin sensitivity. In doing so, this knowledge should illuminate novel therapeutic targets to enhance insulin sensitivity for the clinical management of type 2 diabetes.

Factors Influencing AMPK Activation During Cycling Exercise: A Pooled Analysis and Meta-Regression

BACKGROUND

The 5' adenosine monophosphate (AMP)-activated protein kinase (AMPK) is a cellular energy sensor that is activated by increases in the cellular AMP/adenosine diphosphate:adenosine triphosphate (ADP:ATP) ratios and plays a key role in metabolic adaptations to endurance training. The degree of AMPK activation during exercise can be influenced by many factors that impact on cellular energetics, including exercise intensity, exercise duration, muscle glycogen, fitness level, and nutrient availability. However, the relative importance of these factors for inducing AMPK activation remains unclear, and robust relationships between exercise-related variables and indices of AMPK activation have not been established.

OBJECTIVES

The purpose of this analysis was to (1) investigate correlations between factors influencing AMPK activation and the magnitude of change in AMPK activity during cycling exercise, (2) investigate correlations between commonly reported measures of AMPK activation (AMPK-α2 activity, phosphorylated (p)-AMPK, and p-acetyl coenzyme A carboxylase (p-ACC), and (3) formulate linear regression models to determine the most important factors for AMPK activation during exercise.

METHODS

Data were pooled from 89 studies, including 982 participants (93.8% male, maximal oxygen consumption [[Formula: see text]] 51.9 ± 7.8 mL kg-1 min-1). Pearson's correlation analysis was performed to determine relationships between effect sizes for each of the primary outcome markers (AMPK-α2 activity, p-AMPK, p-ACC) and factors purported to influence AMPK signaling (muscle glycogen, carbohydrate ingestion, exercise duration and intensity, fitness level, and muscle metabolites). General linear mixed-effect models were used to examine which factors influenced AMPK activation.

RESULTS

Significant correlations (r = 0.19-0.55, p < .05) with AMPK activity were found between end-exercise muscle glycogen, exercise intensity, and muscle metabolites phosphocreatine, creatine, and free ADP. All markers of AMPK activation were significantly correlated, with the strongest relationship between AMPK-α2 activity and p-AMPK (r = 0.56, p < 0.001). The most important predictors of AMPK activation were the muscle metabolites and exercise intensity.

CONCLUSION

Muscle glycogen, fitness level, exercise intensity, and exercise duration each influence AMPK activity during exercise when all other factors are held constant. However, disrupting cellular energy charge is the most influential factor for AMPK activation during endurance exercise.

Three weeks of a home-based "sleep low-train low" intervention improves functional threshold power in trained cyclists: A feasibility study

Background

“Sleep Low-Train Low” is a training-nutrition strategy intended to purposefully reduce muscle glycogen availability around specific exercise sessions, potentially amplifying the training stimulus via augmented cell signalling. The aim of this study was to assess the feasibility of a 3-week home-based “sleep low-train low” programme and its effects on cycling performance in trained athletes.

Methods

Fifty-five trained athletes (Functional Threshold Power [FTP]: 258 ± 52W) completed a home-based cycling training program consisting of evening high-intensity training (6 × 5 min at 105% FTP), followed by low-intensity training (1 hr at 75% FTP) the next morning, three times weekly for three consecutive weeks. Participant’s daily carbohydrate (CHO) intake (6 g·kg^-1·d^-1) was matched but timed differently to manipulate CHO availability around exercise: no CHO consumption post- HIT until post-LIT sessions [Sleep Low (SL), n = 28] or CHO consumption evenly distributed throughout the day [Control (CON), n = 27]. Sessions were monitored remotely via power data uploaded to an online training platform, with performance tests conducted pre-, post-intervention.

Results

LIT exercise intensity reduced by 3% across week 1, 3 and 2% in week 2 (P < 0.01) with elevated RPE in SL vs. CON (P < 0.01). SL enhanced FTP by +5.5% vs. +1.2% in CON (P < 0.01). Comparable increases in 5-min peak power output (PPO) were observed between groups (P < 0.01) with +2.3% and +2.7% in SL and CON, respectively (P = 0.77). SL 1-min PPO was unchanged (+0.8%) whilst CON improved by +3.9% (P = 0.0144).

Conclusion

Despite reduced relative training intensity, our data demonstrate short-term “sleep low-train low” intervention improves FTP compared with typically “normal” CHO availability during exercise. Importantly, training was completed unsupervised at home (during the COVID-19 pandemic), thus demonstrating the feasibility of completing a “sleep low-train low” protocol under non-laboratory conditions.

Skeletal Muscle Ribosome and Mitochondrial Biogenesis in Response to Different Exercise Training Modalities

Skeletal muscle adaptations to resistance and endurance training include increased ribosome and mitochondrial biogenesis, respectively. Such adaptations are believed to contribute to the notable increases in hypertrophy and aerobic capacity observed with each exercise mode. Data from multiple studies suggest the existence of a competition between ribosome and mitochondrial biogenesis, in which the first adaptation is prioritized with resistance training while the latter is prioritized with endurance training. In addition, reports have shown an interference effect when both exercise modes are performed concurrently. This prioritization/interference may be due to the interplay between the 5’ AMP-activated protein kinase (AMPK) and mechanistic target of rapamycin complex 1 (mTORC1) signaling cascades and/or the high skeletal muscle energy requirements for the synthesis and maintenance of cellular organelles. Negative associations between ribosomal DNA and mitochondrial DNA copy number in human blood cells also provide evidence of potential competition in skeletal muscle. However, several lines of evidence suggest that ribosome and mitochondrial biogenesis can occur simultaneously in response to different types of exercise and that the AMPK-mTORC1 interaction is more complex than initially thought. The purpose of this review is to provide in-depth discussions of these topics. We discuss whether a curious competition between mitochondrial and ribosome biogenesis exists and show the available evidence both in favor and against it. Finally, we provide future research avenues in this area of exercise physiology.

AMPK: restoring metabolic homeostasis over space and time

The evolution of AMPK and its homologs enabled exquisite responsivity and control of cellular energetic homeostasis. Recent work has been critical in establishing the mechanisms that determine AMPK activity, novel targets of AMPK action, and the distribution of AMPK-mediated control networks across the cellular landscape. The role of AMPK as a hub of metabolic control has led to intense interest in pharmacologic activation as a therapeutic avenue for a number of disease states, including obesity, diabetes, and cancer. As such, critical work on the compartmentalization of AMPK, its downstream targets, and the systems it influences has progressed in recent years. The variegated distribution of AMPK-mediated control of metabolic homeostasis has revealed key insights into AMPK in normal biology and future directions for AMPK-based therapeutic strategies.

Mice with Whole-Body Disruption of AMPK-Glycogen Binding Have Increased Adiposity, Reduced Fat Oxidation and Altered Tissue Glycogen Dynamics

The AMP-activated protein kinase (AMPK), a central regulator of cellular energy balance and metabolism, binds glycogen via its β subunit. However, the physiological effects of disrupting AMPK-glycogen interactions remain incompletely understood. To chronically disrupt AMPK-glycogen binding, AMPK β double knock-in (DKI) mice were generated with mutations in residues critical for glycogen binding in both the β1 (W100A) and β2 (W98A) subunit isoforms. We examined the effects of this DKI mutation on whole-body substrate utilization, glucose homeostasis, and tissue glycogen dynamics. Body composition, metabolic caging, glucose and insulin tolerance, serum hormone and lipid profiles, and tissue glycogen and protein content were analyzed in chow-fed male DKI and age-matched wild-type (WT) mice. DKI mice displayed increased whole-body fat mass and glucose intolerance associated with reduced fat oxidation relative to WT. DKI mice had reduced liver glycogen content in the fed state concomitant with increased utilization and no repletion of skeletal muscle glycogen in response to fasting and refeeding, respectively, despite similar glycogen-associated protein content relative to WT. DKI liver and skeletal muscle displayed reductions in AMPK protein content versus WT. These findings identify phenotypic effects of the AMPK DKI mutation on whole-body metabolism and tissue AMPK content and glycogen dynamics.

Augmented muscle glycogen utilization following a single session of sprint training in hypoxia

PURPOSE

This study determined the effect of a single session of sprint interval training in hypoxia on muscle glycogen content among athletes.

METHODS

Ten male college track and field sprinters (mean ± standard error of the mean: age, 21.1 ± 0.2 years; height, 177 ± 2 cm; body weight, 67 ± 2 kg) performed two exercise trials under either hypoxia [HYPO; fraction of inspired oxygen (F_iO₂), 14.5%] or normoxia (NOR: F_iO₂, 20.9%). The exercise consisted of 3 × 30 s maximal cycle sprints with 8-min rest periods between sets. Before and immediately after the exercise, the muscle glycogen content was measured using carbon magnetic resonance spectroscopy in vastus lateralis and vastus intermedius muscles. Moreover, power output, blood lactate concentrations, metabolic responses (respiratory oxygen uptake and carbon dioxide output), and muscle oxygenation were evaluated.

RESULTS

Exercise significantly decreased muscle glycogen content in both trials (interaction, P = 0.03; main effect for time, P < 0.01). Relative changes in muscle glycogen content following exercise were significantly higher in the HYPO trial (- 43.5 ± 0.4%) than in the NOR trial (- 34.0 ± 0.3%; P < 0.01). The mean power output did not significantly differ between the two trials (P = 0.80). The blood lactate concentration after exercise was not significantly different between trials (P = 0.31).

CONCLUSION

A single session of sprint interval training (3 × 30 s sprints) in hypoxia caused a greater decrease in muscle glycogen content compared with the same exercise under normoxia without interfering with the power output.

Carbohydrate improves exercise capacity but does not affect subcellular lipid droplet morphology, AMPK and p53 signalling in human skeletal muscle

KEY POINTS

Muscle glycogen and intramuscular triglycerides (IMTG, stored in lipid droplets) are important energy substrates during prolonged exercise. Exercise-induced changes in lipid droplet (LD) morphology (i.e. LD size and number) have not yet been studied under nutritional conditions typically adopted by elite endurance athletes, that is, after carbohydrate (CHO) loading and CHO feeding during exercise. We report for the first time that exercise reduces IMTG content in both central and peripheral regions of type I and IIa fibres, reflective of decreased LD number in both fibre types whereas reductions in LD size were exclusive to type I fibres. Additionally, CHO feeding does not alter subcellular IMTG utilisation, LD morphology or muscle glycogen utilisation in type I or IIa/II fibres. In the absence of alterations to muscle fuel selection, CHO feeding does not attenuate cell signalling pathways with regulatory roles in mitochondrial biogenesis.

ABSTRACT

We examined the effects of carbohydrate (CHO) feeding on lipid droplet (LD) morphology, muscle glycogen utilisation and exercise-induced skeletal muscle cell signalling. After a 36 h CHO loading protocol and pre-exercise meal (12 and 2 g kg^-1 , respectively), eight trained males ingested 0, 45 or 90 g CHO h^-1 during 180 min cycling at lactate threshold followed by an exercise capacity test (150% lactate threshold). Muscle biopsies were obtained pre- and post-completion of submaximal exercise. Exercise decreased (P < 0.01) glycogen concentration to comparable levels (∼700 to 250 mmol kg^-1 DW), though utilisation was greater in type I (∼40%) versus type II fibres (∼10%) (P < 0.01). LD content decreased in type I (∼50%) and type IIa fibres (∼30%) (P < 0.01), with greater utilisation in type I fibres (P < 0.01). CHO feeding did not affect glycogen or IMTG utilisation in type I or II fibres (all P > 0.05). Exercise decreased LD number within central and peripheral regions of both type I and IIa fibres, though reduced LD size was exclusive to type I fibres. Exercise induced (all P < 0.05) comparable AMPK^Thr172 (∼4-fold), p53^Ser15 (∼2-fold) and CaMKII^Thr268 phosphorylation (∼2-fold) with no effects of CHO feeding (all P > 0.05). CHO increased exercise capacity where 90 g h^-1 (233 ± 133 s) > 45 g h^-1 (156 ± 66 s; P = 0.06) > 0 g h^-1 (108 ± 54 s; P = 0.03). In conditions of high pre-exercise CHO availability, we conclude CHO feeding does not influence exercise-induced changes in LD morphology, glycogen utilisation or cell signalling pathways with regulatory roles in mitochondrial biogenesis.

Association of Serum Hepcidin Levels with Aerobic and Resistance Exercise: A Systematic Review

BACKGROUND

Prevalence of iron deficiency is commonly reported among athletic population groups. It impairs physical performance due to insufficient oxygen delivery to target organs and low energy production. This is due to the high demand of exercise on oxygen delivery for systemic metabolism by the erythrocytes in the blood. Hepcidin, the key regulator of iron homeostasis, decreases to facilitate iron efflux into the circulation during enhanced erythropoiesis. However, acute anaemia of exercise is caused by increased hepcidin expression that is induced by stress and inflammatory signal. The study aimed to systematically review changes in serum hepcidin levels during resistance and aerobic exercise programmes.

METHODS

A systemic literature search from 2010 to April 2020 across seven databases comprised of Cochrane library, PubMed, Web of Science, Scopus, Embase, MEDLINE, and OpenGrey. The primary outcome was increased or decreased serum hepcidin from baseline after the exercise activity. Risks of bias were evaluated by using the National Institutes of Health (NIH) for quality assessment of before and after different exercise programmes.

RESULTS

Overall, twenty-three studies met the inclusion criteria. Out of the 23 studies, 16 studies reported significantly exercise-induced serum hepcidin elevation. Of the 17 studies that evaluated serum interleukin (IL)-6 levels, 14 studies showed significant exercise-induced serum IL-6 elevation. Changes in exercise-induced serum hepcidin and IL-6 levels were similar in both resistance and endurance exercise. Significant correlations were observed between post-exercise hepcidin and baseline ferritin levels (r = 0.69, p < 0.05) and between post-exercise hepcidin and post-exercise IL-6 (r = 0.625, p < 0.05).

CONCLUSION

Resistance and endurance training showed significant increase in serum hepcidin and IL-6 levels in response to exercise. Baseline ferritin and post-exercise IL-6 elevation are key determining factors in the augmentation of hepcidin response to exercise.

A ketogenic diet combined with exercise alters mitochondrial function in human skeletal muscle while improving metabolic health

Animal data indicate that ketogenic diets are associated with improved mitochondrial function, but human data are lacking. We aimed to characterize skeletal muscle mitochondrial changes in response to a ketogenic diet combined with exercise training in healthy individuals. Twenty-nine physically active adults completed a 12-wk supervised exercise program after self-selection into a ketogenic diet (KD, n = 15) group or maintenance of their habitual mixed diet (MD, n = 14). Measures of metabolic health and muscle biopsies (vastus lateralis) were obtained before and after the intervention. Mitochondria were isolated from muscle and studied after exposure to carbohydrate (pyruvate), fat (palmitoyl-l-carnitine), and ketone (β-hydroxybutyrate+acetoacetate) substrates. Compared with MD, the KD resulted in increased whole body resting fat oxidation (P < 0.001) and decreased fasting insulin (P = 0.019), insulin resistance [homeostatic model assessment of insulin resistance (HOMA-IR), P = 0.022], and visceral fat (P < 0.001). The KD altered mitochondrial function as evidenced by increases in mitochondrial respiratory control ratio (19%, P = 0.009), ATP production (36%, P = 0.028), and ATP/H₂O₂ (36%, P = 0.033) with the fat-based substrate. ATP production with the ketone-based substrate was four to eight times lower than with other substrates, indicating minimal oxidation. The KD resulted in a small decrease in muscle glycogen (14%, P = 0.035) and an increase in muscle triglyceride (81%, P = 0.006). These results expand our understanding of human adaptation to a ketogenic diet combined with exercise. In conjunction with weight loss, we observed altered skeletal muscle mitochondrial function and efficiency, an effect that may contribute to the therapeutic use of ketogenic diets in various clinical conditions, especially those associated with insulin resistance.

What Should I Eat before Exercise? Pre-Exercise Nutrition and the Response to Endurance Exercise: Current Prospective and Future Directions

The primary variables influencing the adaptive response to a bout of endurance training are exercise duration and exercise intensity. However, altering the availability of nutrients before and during exercise can also impact the training response by modulating the exercise stimulus and/or the physiological and molecular responses to the exercise-induced perturbations. The purpose of this review is to highlight the current knowledge of the influence of pre-exercise nutrition ingestion on the metabolic, physiological, and performance responses to endurance training and suggest directions for future research. Acutely, carbohydrate ingestion reduces fat oxidation, but there is little evidence showing enhanced fat burning capacity following long-term fasted-state training. Performance is improved following pre-exercise carbohydrate ingestion for longer but not shorter duration exercise, while training-induced performance improvements following nutrition strategies that modulate carbohydrate availability vary based on the type of nutrition protocol used. Contrasting findings related to the influence of acute carbohydrate ingestion on mitochondrial signaling may be related to the amount of carbohydrate consumed and the intensity of exercise. This review can help to guide athletes, coaches, and nutritionists in personalizing pre-exercise nutrition strategies, and for designing research studies to further elucidate the role of nutrition in endurance training adaptations.

Long-chain fatty acyl-CoA esters regulate metabolism via allosteric control of AMPK β1 isoforms

Long-chain fatty acids (LCFAs) play important roles in cellular energy metabolism, acting as both an important energy source and signalling molecules¹. LCFA-CoA esters promote their own oxidation by acting as allosteric inhibitors of acetyl-CoA carboxylase, which reduces the production of malonyl-CoA and relieves inhibition of carnitine palmitoyl-transferase 1, thereby promoting LCFA-CoA transport into the mitochondria for β-oxidation^2-6. Here we report a new level of regulation wherein LCFA-CoA esters per se allosterically activate AMP-activated protein kinase (AMPK) β1-containing isoforms to increase fatty acid oxidation through phosphorylation of acetyl-CoA carboxylase. Activation of AMPK by LCFA-CoA esters requires the allosteric drug and metabolite site formed between the α-subunit kinase domain and the β-subunit. β1 subunit mutations that inhibit AMPK activation by the small-molecule activator A769662, which binds to the allosteric drug and metabolite site, also inhibit activation by LCFA-CoAs. Thus, LCFA-CoA metabolites act as direct endogenous AMPK β1-selective activators and promote LCFA oxidation.

Resistance exercise causes greater serum hepcidin elevation than endurance (cycling) exercise

BACKGROUND

Hepcidin is an iron regulating hormone, and exercise-induced hepcidin elevation is suggested to increase the risk of iron deficiency among athletes.

OBJECTIVE

We compared serum hepcidin responses to resistance exercise and endurance (cycling) exercise.

METHODS

Ten males [mean ± standard error: 172 ± 2 cm, body weight: 70 ± 2 kg] performed three trials: a resistance exercise trial (RE), an endurance exercise trial (END), and a rest trial (REST). The RE consisted of 60 min of resistance exercise (3-5 sets × 12 repetitions, 8 exercises) at 65% of one repetition maximum, while 60 min of cycling exercise at 65% of [Formula: see text] was performed in the END. Blood samples were collected before exercise and during a 6-h post-exercise (0h, 1h, 2h, 3h, 6h after exercise).

RESULTS

Both RE and END significantly increased blood lactate levels, with significantly higher in the RE (P < 0.001). Serum iron levels were significantly elevated immediately after exercise (P < 0.001), with no significant difference between RE and END. Both the RE and END significantly increased serum growth hormone (GH), cortisol, and myoglobin levels (P < 0.01). However, exercise-induced elevations of GH and cortisol were significantly greater in the RE (trial × time: P < 0.001). Plasma interleukin-6 (IL-6) levels were significantly elevated after exercise (P = 0.003), with no significant difference between the trials. Plasma hepcidin levels were elevated after exercise (P < 0.001), with significantly greater in the RE (463 ± 125%) than in the END (137 ± 27%, P = 0.03). During the REST, serum hepcidin and plasma IL-6 levels did not change significantly.

CONCLUSION

Resistance exercise caused a greater exercise-induced elevation in hepcidin than did endurance (cycling) exercise. The present findings indicate that caution will be required to avoid iron deficiency even among athletes in strength (power) types of events who are regularly involved in resistance exercise.

Exercise twice-a-day potentiates markers of mitochondrial biogenesis in men

Endurance exercise begun with reduced muscle glycogen stores seems to potentiate skeletal muscle protein abundance and gene expression. However, it is unknown whether this greater signaling responses is due to performing two exercise sessions in close proximity-as a first exercise session is necessary to reduce the muscle glycogen stores. In the present study, we manipulated the recovery duration between a first muscle glycogen-depleting exercise and a second exercise session, such that the second exercise session started with reduced muscle glycogen in both approaches but was performed either 2 or 15 hours after the first exercise session (so-called "twice-a-day" and "once-daily" approaches, respectively). We found that exercise twice-a-day increased the nuclear abundance of transcription factor EB (TFEB) and nuclear factor of activated T cells (NFAT) and potentiated the transcription of peroxisome proliferator-activated receptor-ɣ coactivator 1-alpha (PGC-1α), peroxisome proliferator-activated receptor-alpha (PPARα), and peroxisome proliferator-activated receptor beta/delta (PPARβ/δ) genes, in comparison with the once-daily exercise. These results suggest that part of the elevated molecular signaling reported with previous "train-low" approaches might be attributed to performing two exercise sessions in close proximity. The twice-a-day approach might be an effective strategy to induce adaptations related to mitochondrial biogenesis and fat oxidation.

The role of skeletal muscle Akt in the regulation of muscle mass and glucose homeostasis

OBJECTIVE

Skeletal muscle insulin signaling is a major determinant of muscle growth and glucose homeostasis. Protein kinase B/Akt plays a prominent role in mediating many of the metabolic effects of insulin. Mice and humans harboring systemic loss-of-function mutations in Akt2, the most abundant Akt isoform in metabolic tissues, are glucose intolerant and insulin resistant. Since the skeletal muscle accounts for a significant amount of postprandial glucose disposal, a popular hypothesis in the diabetes field suggests that a reduction in Akt, specifically in skeletal muscle, leads to systemic glucose intolerance and insulin resistance. Despite this common belief, the specific role of skeletal muscle Akt in muscle growth and insulin sensitivity remains undefined.

METHODS

We generated multiple mouse models of skeletal muscle Akt deficiency to evaluate the role of muscle Akt signaling in vivo. The effects of these genetic perturbations on muscle mass, glucose homeostasis and insulin sensitivity were assessed using both in vivo and ex vivo assays.

RESULTS

Surprisingly, mice lacking Akt2 alone in skeletal muscle displayed normal skeletal muscle insulin signaling, glucose tolerance, and insulin sensitivity despite a dramatic reduction in phosphorylated Akt. In contrast, deletion of both Akt isoforms (M-AktDKO) prevented downstream signaling and resulted in muscle atrophy. Despite the absence of Akt signaling, in vivo and ex vivo insulin-stimulated glucose uptake were normal in M-AktDKO mice. Similar effects on insulin sensitivity were observed in mice with prolonged deletion (4 weeks) of both skeletal muscle Akt isoforms selectively in adulthood. Conversely, short term deletion (2 weeks) of skeletal muscle specific Akt in adult muscles impaired insulin tolerance paralleling the effect observed by acute pharmacological inhibition of Akt in vitro. Mechanistically, chronic ablation of Akt induced mitochondrial dysfunction and activation of AMPK, which was required for insulin-stimulated glucose uptake in the absence of Akt.

CONCLUSIONS

Together, these data indicate that chronic reduction in Akt activity alone in skeletal muscle is not sufficient to induce insulin resistance or prevent glucose uptake in all conditions. Therefore, since insulin-stimulated glucose disposal in skeletal muscle is markedly impaired in insulin-resistant states, we hypothesize that alterations in signaling molecules in addition to skeletal muscle Akt are necessary to perturb glucose tolerance and insulin sensitivity in vivo.

Graded reductions in preexercise muscle glycogen impair exercise capacity but do not augment skeletal muscle cell signaling: implications for CHO periodization

We examined the effects of graded muscle glycogen on exercise capacity and modulation of skeletal muscle signaling pathways associated with the regulation of mitochondrial biogenesis. In a repeated-measures design, eight men completed a sleep-low, train-low model comprising an evening glycogen-depleting cycling protocol followed by an exhaustive exercise capacity test [8 × 3 min at 80% peak power output (PPO), followed by 1-min efforts at 80% PPO until exhaustion] the subsequent morning. After glycogen-depleting exercise, subjects ingested a total of 0 g/kg (L-CHO), 3.6 g/kg (M-CHO), or 7.6 g/kg (H-CHO) of carbohydrate (CHO) during a 6-h period before sleeping, such that exercise was commenced the next morning with graded ( P < 0.05) muscle glycogen concentrations (means ± SD: L-CHO: 88 ± 43, M-CHO: 185 ± 62, H-CHO: 278 ± 47 mmol/kg dry wt). Despite differences ( P < 0.05) in exercise capacity at 80% PPO between trials (L-CHO: 18 ± 7, M-CHO: 36 ± 3, H-CHO: 44 ± 9 min), exercise induced comparable AMPKThr172 phosphorylation (~4-fold) and PGC-1α mRNA expression (~5-fold) after exercise and 3 h after exercise, respectively. In contrast, neither exercise nor CHO availability affected the phosphorylation of p38MAPKThr180/Tyr182 or CaMKIIThr268 or mRNA expression of p53, Tfam, CPT-1, CD36, or PDK4. Data demonstrate that when exercise is commenced with muscle glycogen < 300 mmol/kg dry wt, further graded reductions of 100 mmol/kg dry weight impair exercise capacity but do not augment skeletal muscle cell signaling.

NEW & NOTEWORTHY We provide novel data demonstrating that when exercise is commenced with muscle glycogen below 300 mmol/kg dry wt (as achieved with the sleep-low, train-low model) further graded reductions in preexercise muscle glycogen of 100 mmol/kg dry wt reduce exercise capacity at 80% peak power output by 20–50% but do not augment skeletal muscle cell signaling.

Dietary Fuels in Athletic Performance

Focusing on daily nutrition is important for athletes to perform and adapt optimally to exercise training. The major roles of an athlete's daily diet are to supply the substrates needed to cover the energy demands for exercise, to ensure quick recovery between exercise bouts, to optimize adaptations to exercise training, and to stay healthy. The major energy substrates for exercising skeletal muscles are carbohydrate and fat stores. Optimizing the timing and type of energy intake and the amount of dietary macronutrients is essential to ensure peak training and competition performance, and these strategies play important roles in modulating skeletal muscle adaptations to endurance and resistance training. In this review, recent advances in nutritional strategies designed to optimize exercise-induced adaptations in skeletal muscle are discussed, with an emphasis on mechanistic approaches, by describing the physiological mechanisms that provide the basis for different nutrition regimens.

Postexercise skeletal muscle signaling responses to moderate- to high-intensity steady-state exercise in the fed or fasted state

Exercise performed in the fasted state acutely increases fatty acid availability and utilization. Furthermore, activation of energy-sensing pathways and fatty acid metabolic genes can be augmented by fasting and fasted exercise. However, whether a similar effect occurs at higher exercise intensities remains poorly understood. This study aimed to assess the effect of fed and fasted exercise upon postexercise signaling and mRNA responses during moderate- to high-intensity steady-state exercise. Eight male participants [age: 25 (SD 2) yr, V̇o_2peak: 47.9 (SD 3.8) ml·kg^-1·min^-1] performed 1 h of cycling at 70% W_max in the fasted (FAST) state or 2 h following ingestion of a carbohydrate-rich mixed-macronutrient breakfast (FED). Muscle biopsies were collected pre-, immediately, and 3 h postexercise from the medial vastus lateralis, while venous blood samples were collected throughout the trial. Plasma, nonesterified fatty acid, and glycerol concentrations were elevated during FAST compared with FED, although substrate utilization during exercise was similar. AMPK^Thr172 phosphorylation was ~2.5-fold elevated postexercise in both trials and was significantly augmented by ~30% during FAST. CREB^Ser133 phosphorylation was elevated approximately twofold during FAST, although CREB^Ser133 phosphorylation acutely decreased by ~50% immediately postexercise. mRNA expression of PDK4 was approximately three- to fourfold augmented by exercise and approximately twofold elevated throughout FAST, while expression of PPARGC1A mRNA was similarly activated (~10-fold) by exercise in both FED and FAST. In summary, performing moderate- to high-intensity steady-state exercise in the fasted state increases systemic lipid availability, elevates phosphorylation of AMPK^Thr172 and CREB^Ser133, and augments PDK4 mRNA expression without corresponding increases in whole body fat oxidation and the mRNA expression of PPARGC1A.

No evidence of direct association between GLUT4 and glycogen in human skeletal muscle

Previous studies have demonstrated that exercise increases whole body and skeletal muscle insulin sensitivity that is linked with increased GLUT4 at the plasma membrane following insulin stimulation and associated with muscle glycogen depletion. To assess the potential direct association between muscle glycogen and GLUT4, seven untrained, male subjects exercised for 60 min at ~75% VO₂ peak, with muscle samples obtained by percutaneous needle biopsy immediately before and after exercise. Exercise reduced muscle glycogen content by ~43%. An ultracentrifugation protocol resulted in a ~2-3-fold enriched glycogen fraction from muscle samples for analysis. Total GLUT4 content was unaltered by exercise and we were unable to detect any GLUT4 in glycogen fractions, either with or without amylase treatment. In skinned muscle fiber segments, there was very little, if any, GLUT4 detected in wash solutions, except following exposure to 1% Triton X-100. Amylase treatment of single fibers did not increase GLUT4 in the wash solution and there were no differences in GLUT4 content between fibers obtained before or after exercise for any of the wash treatments. Our results indicate no direct association between GLUT4 and glycogen in human skeletal muscle, before or after exercise, and suggest that alterations in GLUT4 translocation associated with exercise-induced muscle glycogen depletion are mediated via other mechanisms.

Fuel for the Work Required: A Theoretical Framework for Carbohydrate Periodization and the Glycogen Threshold Hypothesis

Deliberately training with reduced carbohydrate (CHO) availability to enhance endurance-training-induced metabolic adaptations of skeletal muscle (i.e. the 'train low, compete high' paradigm) is a hot topic within sport nutrition. Train-low studies involve periodically training (e.g., 30-50% of training sessions) with reduced CHO availability, where train-low models include twice per day training, fasted training, post-exercise CHO restriction and 'sleep low, train low'. When compared with high CHO availability, data suggest that augmented cell signalling (73% of 11 studies), gene expression (75% of 12 studies) and training-induced increases in oxidative enzyme activity/protein content (78% of 9 studies) associated with 'train low' are especially apparent when training sessions are commenced within a specific range of muscle glycogen concentrations. Nonetheless, such muscle adaptations do not always translate to improved exercise performance (e.g. 37 and 63% of 11 studies show improvements or no change, respectively). Herein, we present our rationale for the glycogen threshold hypothesis, a window of muscle glycogen concentrations that simultaneously permits completion of required training workloads and activation of the molecular machinery regulating training adaptations. We also present the 'fuel for the work required' paradigm (representative of an amalgamation of train-low models) whereby CHO availability is adjusted in accordance with the demands of the upcoming training session(s). In order to strategically implement train-low sessions, our challenge now is to quantify the glycogen cost of habitual training sessions (so as to inform the attainment of any potential threshold) and ensure absolute training intensity is not compromised, while also creating a metabolic milieu conducive to facilitating the endurance phenotype.

Interactive Roles for AMPK and Glycogen from Cellular Energy Sensing to Exercise Metabolism

The AMP-activated protein kinase (AMPK) is a heterotrimeric complex with central roles in cellular energy sensing and the regulation of metabolism and exercise adaptations. AMPK regulatory β subunits contain a conserved carbohydrate-binding module (CBM) that binds glycogen, the major tissue storage form of glucose. Research over the past two decades has revealed that the regulation of AMPK is impacted by glycogen availability, and glycogen storage dynamics are concurrently regulated by AMPK activity. This growing body of research has uncovered new evidence of physical and functional interactive roles for AMPK and glycogen ranging from cellular energy sensing to the regulation of whole-body metabolism and exercise-induced adaptations. In this review, we discuss recent advancements in the understanding of molecular, cellular, and physiological processes impacted by AMPK-glycogen interactions. In addition, we appraise how novel research technologies and experimental models will continue to expand the repertoire of biological processes known to be regulated by AMPK and glycogen. These multidisciplinary research advances will aid the discovery of novel pathways and regulatory mechanisms that are central to the AMPK signaling network, beneficial effects of exercise and maintenance of metabolic homeostasis in health and disease.

Nutritional Ketosis and Mitohormesis: Potential Implications for Mitochondrial Function and Human Health

Impaired mitochondrial function often results in excessive production of reactive oxygen species (ROS) and is involved in the etiology of many chronic diseases, including cardiovascular disease, diabetes, neurodegenerative disorders, and cancer. Moderate levels of mitochondrial ROS, however, can protect against chronic disease by inducing upregulation of mitochondrial capacity and endogenous antioxidant defense. This phenomenon, referred to as mitohormesis, is induced through increased reliance on mitochondrial respiration, which can occur through diet or exercise. Nutritional ketosis is a safe and physiological metabolic state induced through a ketogenic diet low in carbohydrate and moderate in protein. Such a diet increases reliance on mitochondrial respiration and may, therefore, induce mitohormesis. Furthermore, the ketone β-hydroxybutyrate (BHB), which is elevated during nutritional ketosis to levels no greater than those resulting from fasting, acts as a signaling molecule in addition to its traditionally known role as an energy substrate. BHB signaling induces adaptations similar to mitohormesis, thereby expanding the potential benefit of nutritional ketosis beyond carbohydrate restriction. This review describes the evidence supporting enhancement of mitochondrial function and endogenous antioxidant defense in response to nutritional ketosis, as well as the potential mechanisms leading to these adaptations.

Postexercise serum hepcidin response to repeated sprint exercise under normoxic and hypoxic conditions

We determined the effects of repeated sprint exercise under normoxic and hypoxic conditions on serum hepcidin levels. Ten male athletes (age: 20.9 ± 0.3 years; height: 175.7 ± 6.0 cm; weight: 67.3 ± 6.3 kg) performed 2 exercise trials under normoxic (NOR; fraction of inspiratory oxygen (FiO₂): 20.9%) or hypoxic conditions (HYPO; FiO₂: 14.5%). The exercise consisted of 3 sets of 5 × 6 s of maximal pedaling (30-s rest periods between sprints, 10-min rest periods between sets). Blood samples were collected before exercise, immediately after exercise, and 1 and 3 h after exercise. Serum hepcidin levels were significantly elevated after exercise in both trials (both P < 0.01), with no significant difference between the trials. The postexercise blood lactate levels were significantly higher in the HYPO than the NOR (P < 0.05). Both trials caused similar increases in plasma interleukin-6 and serum iron levels (P < 0.001), with no significant difference between the trials. A significant interaction (trial × time) was apparent in terms of serum erythropoietin (EPO) levels (P = 0.003). The EPO level was significantly higher in the HYPO than the NOR at 3 h after exercise (P < 0.05). In conclusion, repeated sprint exercise significantly increased serum hepcidin levels to similar extent in 2 trials, despite differences in the inspired oxygen concentrations during both the exercise and the 3-h postexercise period.

Activation of γ2-AMPK Suppresses Ribosome Biogenesis and Protects Against Myocardial Ischemia/Reperfusion Injury

RATIONALE

AMPK (AMP-activated protein kinase) is a heterotrimeric protein that plays an important role in energy homeostasis and cardioprotection. Two isoforms of each subunit are expressed in the heart, but the isoform-specific function of AMPK remains unclear.

OBJECTIVE

We sought to determine the role of γ2-AMPK in cardiac stress response using bioengineered cell lines and mouse models containing either isoform of the γ-subunit in the heart.

METHODS AND RESULTS

We found that γ2 but not γ1 or γ3 subunit translocated into nucleus on AMPK activation. Nuclear accumulation of AMPK complexes containing γ2-subunit phosphorylated and inactivated RNA Pol I (polymerase I)-associated transcription factor TIF-IA at Ser-635, precluding the assembly of transcription initiation complexes for rDNA. The subsequent downregulation of pre-rRNA level led to attenuated endoplasmic reticulum (ER) stress and cell death. Deleting γ2-AMPK led to increases in pre-rRNA level, ER stress markers, and cell death during glucose deprivation, which could be rescued by inhibition of rRNA processing or ER stress. To study the function of γ2-AMPK in the heart, we generated a mouse model with cardiac-specific deletion of γ2-AMPK (cardiac knockout [cKO]). Although the total AMPK activity was unaltered in cKO hearts because of upregulation of γ1-AMPK, the lack of γ2-AMPK sensitizes the heart to myocardial ischemia/reperfusion injury. The cKO failed to suppress pre-rRNA level during ischemia/reperfusion and showed a greater infarct size. Conversely, cardiac-specific overexpression of γ2-AMPK decreased ribosome biosynthesis and ER stress during ischemia/reperfusion insult, and the infarct size was reduced.

CONCLUSIONS

The γ2-AMPK translocates into the nucleus to suppress pre-rRNA transcription and ribosome biosynthesis during stress, thus ameliorating ER stress and cell death. Increased γ2-AMPK activity is required to protect against ischemia/reperfusion injury. Our study reveals an isoform-specific function of γ2-AMPK in modulating ribosome biosynthesis, cell survival, and cardioprotection.

Carbohydrate intake and resistance-based exercise: are current recommendations reflective of actual need?

Substantial research has been completed examining the impact of carbohydrate (CHO) intake on endurance exercise, whereas its role in resistance-based exercise performance, adaptation and cell signalling has yet to be fully characterised. This empirical shortcoming has precluded the ability to establish specific CHO recommendations for resistance exercise. This results in recommendations largely stemming from findings based on endurance exercise and/or anecdotal evidence despite the distinct energetic demands and molecular responses mediating adaptation from endurance- and resistance-based exercise. Moreover, the topic of CHO and exercise has become one of polarising nature with divergent views - some substantiated, others lacking evidence. Current literature suggests a moderately high daily CHO intake (3-7 g/kg per d) for resistance training, which is thought to prevent glycogen depletion and facilitate performance and adaptation. However, contemporary investigation, along with an emerging understanding of the molecular underpinnings of resistance exercise adaptation, may suggest that such an intake may not be necessary. In addition to the low likelihood of true glycogen depletion occurring in response to resistance exercise, a diet restrictive in CHO may not be detrimental to acute resistance exercise performance or the cellular signalling activity responsible for adaptation, even when muscle glycogen stores are reduced. Current evidence suggests that signalling of the mammalian target of rapamycin complex 1, the key regulatory kinase for gene translation (protein synthesis), is unaffected by CHO restriction or low muscular glycogen concentrations. Such findings may call into question the current view and subsequent recommendations of CHO intake with regard to resistance-based exercise.

Fuel for the work required: a practical approach to amalgamating train-low paradigms for endurance athletes

Using an amalgamation of previously studied "train-low" paradigms, we tested the effects of reduced carbohydrate (CHO) but high leucine availability on cell-signaling responses associated with exercise-induced regulation of mitochondrial biogenesis and muscle protein synthesis (MPS). In a repeated-measures crossover design, 11 males completed an exhaustive cycling protocol with high CHO availability before, during, and after exercise (HIGH) or alternatively, low CHO but high protein (leucine enriched) availability (LOW + LEU). Muscle glycogen was different (P < 0.05) pre-exercise (HIGH: 583 ± 158, LOW + LEU: 271 ± 85 mmol kg(-1) dw) but decreased (P < 0.05) to comparable levels at exhaustion (≈100 mmol kg(-1) dw). Despite differences (P < 0.05) in exercise capacity (HIGH: 158 ± 29, LOW + LEU: 100 ± 17 min), exercise induced (P < 0.05) comparable AMPKα2 (3-4-fold) activity, PGC-1α (13-fold), p53 (2-fold), Tfam (1.5-fold), SIRT1 (1.5-fold), Atrogin 1 (2-fold), and MuRF1 (5-fold) gene expression at 3 h post-exercise. Exhaustive exercise suppressed p70S6K activity to comparable levels immediately post-exercise (≈20 fmol min(-1) mg(-1)). Despite elevated leucine availability post-exercise, p70S6K activity remained suppressed (P < 0.05) 3 h post-exercise in LOW + LEU (28 ± 14 fmol min(-1) mg(-1)), whereas muscle glycogen resynthesis (40 mmol kg(-1) dw h(-1)) was associated with elevated (P < 0.05) p70S6K activity in HIGH (53 ± 30 fmol min(-1) mg(-1)). We conclude: (1) CHO restriction before and during exercise induces "work-efficient" mitochondrial-related cell signaling but; (2) post-exercise CHO and energy restriction maintains p70S6K activity at basal levels despite feeding leucine-enriched protein. Our data support the practical concept of "fuelling for the work required" as a potential strategy for which to amalgamate train-low paradigms into periodized training programs.

AMPK-dependent regulation of GLP1 expression in L-like cells

This study examined whether AMPK, an evolutionarily conserved sensor of cellular energy status, determines the production of glucagon-like peptide-1 (GLP1). A negative relation existed between phosphorylation of AMPKα and the expression and secretion of GLP1 during changes in energy status in STC-1 cells, an L-like cell line. High concentration of glucose (25 mmol/L) decreased AMPKα phosphorylation, whereas it stimulated the expression and secretion of GLP1 relative to 5.6 mmol/L glucose. Serum starvation upregulated AMPKα phosphorylation, whereas it reduced GLP1 production significantly. Stimulation of AMPK phosphorylation by AICAR and overexpression of wild-type AMPKα1, constitutively active AMPKα1 plasmids, or AMPKα1 lentivirus particles suppressed proglucagon mRNA and protein contents in STC-1 cells. Inactivation of AMPK by Compound C, AMPKα1 siRNA or kinase-inactive AMPKα1 mutant increased the expression and secretion of GLP1. Our results suggest that AMPKα1 may link energy supply with the production of GLP1 in L-like cells.

When phosphorylated at Thr148, the β2-subunit of AMP-activated kinase does not associate with glycogen in skeletal muscle

The 5′-AMP-activated protein kinase (AMPK), a heterotrimeric complex that functions as an intracellular fuel sensor that affects metabolism, is activated in skeletal muscle in response to exercise and utilization of stored energy. The diffusibility properties of α- and β-AMPK were examined in isolated skeletal muscle fiber segments dissected from rat fast-twitch extensor digitorum longus and oxidative soleus muscles from which the surface membranes were removed by mechanical dissection. After the muscle segments were washed for 1 and 10 min, ∼60% and 75%, respectively, of the total AMPK pools were found in the diffusible fraction. After in vitro stimulation of the muscle, which resulted in an ∼80% decline in maximal force, 20% of the diffusible pool became bound in the fiber. This bound pool was not associated with glycogen, as determined by addition of a wash step containing amylase. Stimulation of extensor digitorum longus muscles resulted in 28% glycogen utilization and a 40% increase in phosphorylation of the downstream AMPK target acetyl carboxylase-CoA. This, however, had no effect on the proportion of total β2-AMPK that was phosphorylated in whole muscle homogenates measured by immunoprecipitation. These findings suggest that, in rat skeletal muscle, β2-AMPK is not associated with glycogen and that activation of AMPK by muscle contraction does not dephosphorylate β2-AMPK. These findings question the physiological relevance of the carbohydrate-binding function of β2-AMPK in skeletal muscle.

Creatine ingestion augments dietary carbohydrate mediated muscle glycogen supercompensation during the initial 24 h of recovery following prolonged exhaustive exercise in humans

Muscle glycogen availability can limit endurance exercise performance. We previously demonstrated 5 days of creatine (Cr) and carbohydrate (CHO) ingestion augmented post-exercise muscle glycogen storage compared to CHO feeding alone in healthy volunteers. Here, we aimed to characterise the time-course of this Cr-induced response under more stringent and controlled experimental conditions and identify potential mechanisms underpinning this phenomenon. Fourteen healthy, male volunteers cycled to exhaustion at 70 % VO_2peak. Muscle biopsies were obtained at rest immediately post-exercise and after 1, 3 and 6 days of recovery, during which Cr or placebo supplements (20 g day⁻¹) were ingested along with a prescribed high CHO diet (37.5 kcal kg body mass⁻¹ day⁻¹, >80 % calories CHO). Oral-glucose tolerance tests (oral-GTT) were performed pre-exercise and after 1, 3 and 6 days of Cr and placebo supplementation. Exercise depleted muscle glycogen content to the same extent in both treatment groups. Creatine supplementation increased muscle total-Cr, free-Cr and phosphocreatine (PCr) content above placebo following 1, 3 and 6 days of supplementation (all P < 0.05). Creatine supplementation also increased muscle glycogen content noticeably above placebo after 1 day of supplementation (P < 0.05), which was sustained thereafter. This study confirmed dietary Cr augments post-exercise muscle glycogen super-compensation, and demonstrates this occurred during the initial 24 h of post-exercise recovery (when muscle total-Cr had increased by <10 %). This marked response ensued without apparent treatment differences in muscle insulin sensitivity (oral-GTT, muscle GLUT4 mRNA), osmotic stress (muscle c-fos and HSP72 mRNA) or muscle cell volume (muscle water content) responses, such that another mechanism must be causative.

AMPK/Snf1 signaling regulates histone acetylation: Impact on gene expression and epigenetic functions

AMP-activated protein kinase (AMPK) and its yeast homolog, Snf1, are critical regulators in the maintenance of energy metabolic balance not only stimulating energy production but also inhibiting energy-consuming processes. The AMPK/Snf1 signaling controls energy metabolism by specific phosphorylation of many metabolic enzymes and transcription factors, enhancing or suppressing their functions. The AMPK/Snf1 complexes can be translocated from cytoplasm into nuclei where they are involved in the regulation of transcription. Recent studies have indicated that AMPK/Snf1 activation can control histone acetylation through different mechanisms affecting not only gene transcription but also many other epigenetic functions. For instance, AMPK/Snf1 enzymes can phosphorylate the histone H3S10 (yeast) and H2BS36 (mammalian) sites which activate specific histone acetyltransferases (HAT), consequently enhancing histone acetylation. Moreover, nuclear AMPK can phosphorylate type 2A histone deacetylases (HDAC), e.g. HDAC4 and HDAC5, triggering their export from nuclei thus promoting histone acetylation reactions. AMPK activation can also increase the level of acetyl CoA, e.g. by inhibiting fatty acid and cholesterol syntheses. Acetyl CoA is a substrate for HATs, thus increasing their capacity for histone acetylation. On the other hand, AMPK can stimulate the activity of nicotinamide phosphoribosyltransferase (NAMPT) which increases the level of NAD(+). NAD(+) is a substrate for nuclear sirtuins, especially for SIRT1 and SIRT6, which deacetylate histones and transcription factors, e.g. those regulating ribosome synthesis and circadian clocks. Histone acetylation is an important epigenetic modification which subsequently can affect chromatin remodeling, e.g. via bromodomain proteins. We will review the signaling mechanisms of AMPK/Snf1 in the control of histone acetylation and subsequently clarify their role in the epigenetic regulation of ribosome synthesis and circadian clocks.

Physiological and molecular responses to an acute bout of reduced-exertion high-intensity interval training (REHIT)

PURPOSE

We have previously shown that 6 weeks of reduced-exertion high-intensity interval training (REHIT) improves VO2max in sedentary men and women and insulin sensitivity in men. Here, we present two studies examining the acute physiological and molecular responses to REHIT.

METHODS

In Study 1, five men and six women (age: 26 ± 7 year, BMI: 23 ± 3 kg m(-2), VO2max: 51 ± 11 ml kg(-1) min(-1)) performed a single 10-min REHIT cycling session (60 W and two 20-s 'all-out' sprints), with vastus lateralis biopsies taken before and 0, 30, and 180 min post-exercise for analysis of glycogen content, phosphorylation of AMPK, p38 MAPK and ACC, and gene expression of PGC1α and GLUT4. In Study 2, eight men (21 ± 2 year; 25 ± 4 kg·m(-2); 39 ± 10 ml kg(-1) min(-1)) performed three trials (REHIT, 30-min cycling at 50 % of VO2max, and a resting control condition) in a randomised cross-over design. Expired air, venous blood samples, and subjective measures of appetite and fatigue were collected before and 0, 15, 30, and 90 min post-exercise.

RESULTS

Acutely, REHIT was associated with a decrease in muscle glycogen, increased ACC phosphorylation, and activation of PGC1α. When compared to aerobic exercise, changes in VO2, RER, plasma volume, and plasma lactate and ghrelin were significantly more pronounced with REHIT, whereas plasma glucose, NEFAs, PYY, and measures of appetite were unaffected.

CONCLUSIONS

Collectively, these data demonstrate that REHIT is associated with a pronounced disturbance of physiological homeostasis and associated activation of signalling pathways, which together may help explain previously observed adaptations once considered exclusive to aerobic exercise.

Ramping up the signal: promoting endurance training adaptation in skeletal muscle by nutritional manipulation

Mitochondrial biogenesis in skeletal muscle results from the cumulative effect of transient increases in mRNA transcripts encoding mitochondrial proteins in response to repeated exercise sessions. This process requires the coordinated expression of both nuclear and mitochondrial (mt) DNA genomes and is regulated, for the most part, by the peroxisome proliferator-activated receptor γ coactivator 1α. Several other exercise-inducible proteins also play important roles in promoting an endurance phenotype, including AMP-activated protein kinase, p38 mitogen-activated protein kinase and tumour suppressor protein p53. Commencing endurance-based exercise with low muscle glycogen availability results in greater activation of many of these signalling proteins compared with when the same exercise is undertaken with normal glycogen concentration, suggesting that nutrient availability is a potent signal that can modulate the acute cellular responses to a single bout of exercise. When exercise sessions are repeated in the face of low glycogen availability (i.e. chronic training), the phenotypic adaptations resulting from such interventions are also augmented.

Influence of resistance exercise intensity and metabolic stress on anabolic signaling and expression of myogenic genes in skeletal muscle

INTRODUCTION

We investigated the effect of resistance exercise intensity and exercise-induced metabolic stress on the activation of anabolic signaling and expression of myogenic genes in skeletal muscle.

METHODS

Ten strength-trained athletes performed high-intensity [HI, 74% of 1-repetition maximum (RM)], middle-intensity (MI, 54% 1RM), or middle-intensity (54% 1RM) no-relaxation exercise (MIR). Kinase phosphorylation level and myogenic gene expression in muscle samples were evaluated before, 45 min, 5 h, and 20 h after exercise.

RESULTS

The lactate concentration in MI was approximately 2-fold lower than in the 2 other sessions, and was highest in MIR. The phosphorylation level of extracellular kinase 1/2Thr202/Tyr204 after exercise was related to metabolic stress. Metabolic stress induced a decrease in myostatin mRNA expression, whereas mechano-growth factor mRNA level depended on exercise intensity.

CONCLUSIONS

This study demonstrates that both intensity and exercise-induced metabolic stress can be manipulated to affect muscle anabolic signaling.

Carbohydrate availability and exercise training adaptation: too much of a good thing?

Traditional nutritional approaches to endurance training have typically promoted high carbohydrate (CHO) availability before, during and after training sessions to ensure adequate muscle substrate to meet the demands of high daily training intensities and volumes. However, during the past decade, data from our laboratories and others have demonstrated that deliberately training in conditions of reduced CHO availability can promote training-induced adaptations of human skeletal muscle (i.e. increased maximal mitochondrial enzyme activities and/or mitochondrial content, increased rates of lipid oxidation and, in some instances, improved exercise capacity). Such data have led to the concept of 'training low, but competing high' whereby selected training sessions are completed in conditions of reduced CHO availability (so as to promote training adaptation), but CHO reserves are restored immediately prior to an important competition. The augmented training response observed with training-low strategies is likely regulated by enhanced activation of key cell signalling kinases (e.g. AMPK, p38MAPK), transcription factors (e.g. p53, PPARδ) and transcriptional co-activators (e.g. PGC-1α), such that a co-ordinated up-regulation of both the nuclear and mitochondrial genomes occurs. Although the optimal practical strategies to train low are not currently known, consuming additional caffeine, protein, and practising CHO mouth-rinsing before and/or during training may help to rescue the reduced training intensities that typically occur when 'training low', in addition to preventing protein breakdown and maintaining optimal immune function. Finally, athletes should practise 'train-low' workouts in conjunction with sessions undertaken with normal or high CHO availability so that their capacity to oxidise CHO is not blunted on race day.

Effects of AMPK activation on insulin sensitivity and metabolism in leptin-deficient ob/ob mice

AMP-activated protein kinase (AMPK) is a heterotrimeric complex, composed of a catalytic subunit (α) and two regulatory subunits (β and γ), which act as a metabolic sensor to regulate glucose and lipid metabolism. A mutation in the γ3 subunit (AMPKγ3(R225Q)) increases basal AMPK phosphorylation, while concomitantly reducing sensitivity to AMP. AMPKγ3(R225Q) (γ3(R225Q)) transgenic mice are protected against dietary-induced triglyceride accumulation and insulin resistance. We determined whether skeletal muscle-specific expression of AMPKγ3(R225Q) prevents metabolic abnormalities in leptin-deficient ob/ob (ob/ob-γ3(R225Q)) mice. Glycogen content was increased, triglyceride content was decreased, and diacylglycerol and ceramide content were unaltered in gastrocnemius muscle from ob/ob-γ3(R225Q) mice, whereas glucose tolerance was unaltered. Insulin-stimulated glucose uptake in extensor digitorum longus muscle during the euglycemic-hyperinsulinemic clamp was increased in lean γ3(R225Q) mice, but not in ob/ob-γ3(R225Q) mice. Acetyl-CoA carboxylase phosphorylation was increased in gastrocnemius muscle from γ3(R225Q) mutant mice independent of adiposity. Glycogen and triglyceride content were decreased after leptin treatment (5 days) in ob/ob mice, but not in ob/ob-γ3(R225Q) mice. In conclusion, metabolic improvements arising from muscle-specific expression of AMPKγ3(R225Q) are insufficient to ameliorate insulin resistance and obesity in leptin-deficient mice. Central defects due to leptin deficiency may override any metabolic benefit conferred by peripheral overexpression of the AMPKγ3(R225Q) mutation.