Oral glucose ingestion attenuates exercise-induced activation of 5'-AMP-activated protein kinase in human skeletal muscle

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.

The role of MOTS-c-mediated antioxidant defense in aerobic exercise alleviating diabetic myocardial injury

Myocardial remodeling and dysfunction are commonly observed in type 2 diabetes mellitus (T2DM). Aerobic exercise can partly alleviate diabetes-induced myocardial dysfunction through its antioxidant actions. MOTS-c is a potential exercise mimic. This study aimed to investigate the effects of MOTS-c on improving diabetic heart function and its mechanism and to identify whether MOTS-c improved antioxidant defenses due to aerobic exercise. Herein, we established a rat model of T2DM induced by high-fat diet combined with a low-dose streptozotocin injection. Interventions were performed using intraperitoneal injections of MOTS-c (i.p. 0.5 mg/kg/day, 7 days/week) or aerobic exercise training (treadmill, 20 m/min, 60 min/day, 5 days/week) for 8 weeks. Myocardial ultrastructure was assessed using transmission electron microscopy (TEM), myocardial lipid peroxidation levels (MDA), superoxide dismutase (SOD), glutathione (GSH), and catalase (CAT) levels were assessed using colorimetric methods, and molecular analyses including MOTS-c, Kelch-like ECH-associated protein 1 (Keap1), Nuclear factor E2-related factor 2 (Nrf2), adenosine 5'-monophosphate (AMP)-activated protein kinase (AMPK)and phospho-AMPK (p-AMPK) were examined using Western blot. The results showed that MOTS-c, with or without exercise, reduced myocardial ultrastructural damage and improved glucolipid metabolism and cardiac function in T2DM. Furthermore, MOTS-c increased antioxidant markers such as SOD, CAT, and the protein expression of myocardial MOTS-c, Keap1, Nrf2, and p-AMPK. MOTS-c with exercise treatment reduced myocardial MDA and increased p-AMPK significantly comparing to only exercise or MOTS-c alone. Our findings suggest that MOTS-c may be a helpful supplement for overcoming exercise insufficiency and improving myocardial structure and function in diabetes.

Periodised Carbohydrate Intake Does Not Affect Substrate Oxidation but May Contribute to Endurance Capacity

The aim of this study was to investigate whether periodising carbohydrate intake around specific training sessions will enhance endurance training adaptations.Seventeen healthy recreationally endurance-trained males (n = 5) and females (n = 12) (27.5 ± 5.4 years) participated in a four-week training intervention. Participants were divided into two groups: FASTED (stayed fasted between evening high-intensity interval training session and low-intensity training session in the following morning) and FED (no restriction in food intake). Pre- and post-testing included peak oxygen uptake (VO_2peak), anaerobic capacity, and 60 min submaximal running tests. Fasted venous blood samples were drawn for the determination of triglyceride and glucose concentrations.VO_2peak increased in both FASTED (4.4 ± 3.0%, p = 0.001) and FED (4.6 ± 4.2%, p = 0.017), whereas maximal running velocity increased only in the FASTED (3.5 ± 2.7%, p = 0.002). Lactate concentrations in the anaerobic test after intervention were greater in FASTED than FED (p = 0.025-0.041). Running time in the anaerobic test was improved in FASTED (from 64.1 ± 15.6-86.3 ± 23.2 s, p < 0.001) but not in FED (from 56.4 ± 15.2-66.9 ± 21.3 s, p = 0.099). Substrate oxidation did not change after intervention in either of the groups (p = 0.052-0.597). Heart rate was lower in the submaximal running test in FASTED (p < 0.001) but not in FED (p = 0.097).Training with periodised carbohydrate availability does not have any effect on substrate oxidation. However, it seems to enhance the capacity to perform high-intensity exercise.

A potent physiological method to magnify and sustain soleus oxidative metabolism improves glucose and lipid regulation

Slow oxidative muscle, most notably the soleus, is inherently well equipped with the molecular machinery for regulating blood-borne substrates. However, the entire human musculature accounts for only ∼15% of the body’s oxidative metabolism of glucose at the resting energy expenditure, despite being the body’s largest lean tissue mass. We found the human soleus muscle could raise local oxidative metabolism to high levels for hours without fatigue, during a type of soleus-dominant activity while sitting, even in unfit volunteers. Muscle biopsies revealed there was minimal glycogen use. Magnifying the otherwise negligible local energy expenditure with isolated contractions improved systemic VLDL-triglyceride and glucose homeostasis by a large magnitude, e.g., 52% less postprandial glucose excursion (∼50 mg/dL less between ∼1 and 2 h) with 60% less hyperinsulinemia. Targeting a small oxidative muscle mass (∼1% body mass) with local contractile activity is a potent method for improving systemic metabolic regulation while prolonging the benefits of oxidative metabolism.

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We developed a method to capitalize upon the unique phenotype of the soleus

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“A high quality versus large quantity perspective” for muscle activation

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Singular movement targeting the 1 kg soleus easily sustains oxidative metabolism

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This method provides a distinct muscular activity stimulus for metabolic control

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.

Divergent serum metabolomic, skeletal muscle signaling, transcriptomic, and performance adaptations to fasted versus whey protein-fed sprint interval training

Sprint interval training (SIT) is a time-efficient alternative to endurance exercise, conferring beneficial skeletal muscle metabolic adaptations. Current literature has investigated the nutritional regulation of acute and chronic exercise-induced metabolic adaptations in muscle following endurance exercise, principally comparing the impact of training in fasted and carbohydrate-fed (CHO) conditions. Alternative strategies such as exercising in low CHO, protein-fed conditions remain poorly characterized, specifically pertaining to adaptations associated with SIT. Thus, this study aimed to compare the metabolic and performance adaptations to acute and short-term SIT in the fasted state with preexercise hydrolyzed (WPH) or concentrated (WPC) whey protein supplementation. In healthy males, preexercise protein ingestion did not alter exercise-induced increases in PGC-1α, PDK4, SIRT1, and PPAR-δ mRNA expression following acute SIT. However, supplementation of WPH beneficially altered acute exercise-induced CD36 mRNA expression. Preexercise protein ingestion attenuated acute exercise-induced increases in muscle pan-acetylation and PARP1 protein content compared with fasted SIT. Acute serum metabolomic differences confirmed greater preexercise amino acid delivery in protein-fed compared with fasted conditions. Following 3 wk of SIT, training-induced increases in mitochondrial enzymatic activity and exercise performance were similar across nutritional groups. Interestingly, resting muscle acetylation status was downregulated in WPH conditions following training. Such findings suggest preexercise WPC and WPH ingestion positively influences metabolic adaptations to SIT compared with fasted training, resulting in either similar or enhanced performance adaptations. Future studies investigating nutritional modulation of metabolic adaptations to exercise are warranted to build upon these novel findings.NEW & NOTEWORTHY These are the first data to show the influence of preexercise protein on serum and skeletal muscle metabolic adaptations to acute and short-term sprint interval training (SIT). Preexercise whey protein concentrate (WPC) or hydrolysate (WPH) feeding acutely affected the serum metabolome, which differentially influenced acute and chronic changes in mitochondrial gene expression, intracellular signaling (acetylation and PARylation) resulting in either similar or enhanced performance outcomes when compared with fasted training.

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.

Effects of energy supplements on the differentiation of skeletal muscle satellite cells

To investigate the effects of the activator of AMPK and high glucose on the differentiation of mouse SMSCs, primary SMSCs were isolated from mouse extensor digitorum longus muscle and grown to near confluence (80%). Postconfluent cells were cultured in a growth medium with different inductors: AICAR, glucose, and AICAR mixed with glucose. The specific protein expressions of SMSCs, myoblasts, adipocytes, and brown adipocytes were analyzed on days 0, 3, 5, 7, and 10. The results showed treatment with AICAR in SMSCs markedly activated AMPK phosphorylation (p < .05) and increased protein expression of Pax7 and MyoD (p < .05), high concentrations of intracellular glucose upregulated UCP-1 protein expression and enhanced lipid accumulation (p < .05), the cowork of AICAR and glucose affected a decrease on MyoD, PPARg, and UCP-1 expression (p < .05) and an increase on Pax7. The present study indicated that the certain energy supplements influence the direction of SMSC differentiation which may contribution on the structure of muscle and meat quality, sequentially.

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.

Supplement with whey protein hydrolysate in contrast to carbohydrate supports mitochondrial adaptations in trained runners

Background

Protein supplementation has been suggested to augment endurance training adaptations by increasing mixed muscle and myofibrillar protein synthesis and lean body mass. However, a potential beneficial effect on mitochondrial adaptations is yet to be clarified.

The aim of the present study was to investigate the effect of consuming whey protein hydrolysate before and whey protein hydrolysate plus carbohydrate (PRO-CHO) after each exercise session during a six-week training period compared to similarly timed intake of isocaloric CHO supplements on biomarkers of mitochondrial biogenesis, VO_2max and performance in trained runners.

Methods

Twenty-four trained runners (VO_2max 60.7 ± 3.7 ml O₂ kg^− 1 min¹) completed a six-week block randomized controlled intervention period, consisting of progressive running training. Subjects were randomly assigned to either PRO-CHO or CHO and matched in pairs for gender, age, VO_2max, training and performance status. The PRO-CHO group ingested a protein beverage (0.3 g kg^− 1) before and protein-carbohydrate beverage (0.3 g protein kg^− 1 and 1 g carbohydrate kg^− 1) after each exercise session. The CHO group ingested an energy matched carbohydrate beverage. Resting muscle biopsies obtained pre and post intervention were analyzed for mitochondrial specific enzyme activity and mitochondrial protein content. Subjects completed a 6 K time trial (6 K TT) and a VO_2max test pre, midway (only 6 K TT) and post intervention.

Results

Following six weeks of endurance training Cytochrome C (Cyt C) protein content was significantly higher in the PRO-CHO group compared to the CHO group (p < 0.05), with several other mitochondrial proteins (Succinate dehydrogenase (SDHA), Cytochrome C oxidase (COX-IV), Voltage-dependent anion channel (VDAC), Heat shock protein 60 (HSP60), and Prohibitin (PHB1)) following a similar, but non-significant pattern (p = 0.07–0.14). β-hydroxyacyl-CoA dehydrogenase (HAD) activity was significantly lower after training in the CHO group (p < 0.01), but not in the PRO-CHO group (p = 0.24). VO_2max and 6 K TT was significantly improved after training with no significant difference between groups.

Conclusion

Intake of whey PRO hydrolysate before and whey PRO hydrolysate plus CHO after each exercise session during a six-week endurance training period may augment training effects on specific mitochondrial proteins compared to intake of iso-caloric CHO but does not alter VO_2max or 6 K TT performance.

Trial registration

clinicaltrials.gov, NCT03561337. Registered 6 June 2018 – Retrospectively registered.

The Impact of Physical Activity on Glycemic Variability Assessed by Continuous Glucose Monitoring in Patients With Type 2 Diabetes Mellitus: A Systematic Review

Aim: Patients with Type 2 Diabetes Mellitus (T2DM) have increased risk of developing vascular complications due to chronic hyperglycemia. Glycemic variability (GV) has been suggested to play an even more important role in the risk of developing diabetic complications than sustained hyperglycemia. Physical activity (PA) has shown reducing effects on mean plasma glucose; however, the effect on GV in T2DM needs further description. The objective of this review is to evaluate the effect of PA on GV, assessed by continuous glucose monitoring (CGM) in people with T2DM. Methods: A systematic literature search was conducted on MEDLINE and Embase to find randomized controlled trials (RCTs) covering the aspects T2DM, PA, and CGM. Following eligibility screening, variables of population characteristics, PA interventions, and GV outcomes were extracted and processed through qualitative synthesis. Risk of bias (ROB) was assessed using Cochrane ROB tool v2.0. Results: Of 1,825 identified articles, 40 full texts were screened. In the ten included RCTs matching the eligibility criteria, sample sizes ranged from nine to 63, mean age from 51 (SD 11) to 65 (SD 2) years and mean T2DM duration from four (SD 3) to ten (SD 6) years. Eight RCTs examined GV following single bouts of exercise, while two RCTs examined GV following training interventions. One RCT applied parallel group design, while nine RCTs applied crossover design. Numeric reductions in GV following acute exercise were seen, with four RCTs reaching statistical significance. Numeric reductions in GV were seen following training interventions, with one RCT reaching statistical significance. Numeric reductions of GV after PA appeared independently of intensity and T2DM progression but higher in participants with high baseline HbA1c and GV than with low. 80% of the trials were evaluated as uncertain/high ROB. Conclusion: The systematic literature search revealed limited and biased evidence showing that acute PA numerically reduced GV in patients with T2DM. PA reduced GV independently of PA intensity and T2DM progression. Prolonged RCTs with low ROB are needed to confirm reducing effects of PA on GV and to assess the influence of patient- and intervention characteristics on the effect of PA on GV.

Systematic Assessment of Blood-Borne MicroRNAs Highlights Molecular Profiles of Endurance Sport and Carbohydrate Uptake

Multiple studies endorsed the positive effect of regular exercise on mental and physical health. However, the molecular mechanisms underlying training-induced fitness in combination with personal life-style remain largely unexplored. Circulating biomarkers such as microRNAs (miRNAs) offer themselves for studying systemic and cellular changes since they can be collected from the bloodstream in a low-invasive manner. In Homo sapiens miRNAs are known to regulate a substantial number of protein-coding genes in a post-transcriptional manner and hence are of great interest to understand differential gene expression profiles, offering a cost-effective mechanism to study molecular training adaption, and connecting the dots from genomics to observed phenotypes. Here, we investigated molecular expression patterns of 2549 miRNAs in whole-blood samples from 23 healthy and untrained adult participants of a cross-over study, consisting of eight weeks of endurance training, with several sessions per week, followed by 8 weeks of washout and another 8 weeks of running, using microarrays. Participants were randomly assigned to one of the two study groups, one of which administered carbohydrates before each session in the first training period, and switching the treatment group for the second training period. During running sessions clinical parameters as heartbeat frequency were recorded. This information was extended with four measurements of maximum oxygen uptake (VO2 max) for each participant. We observed that multiple circulating miRNAs show expression changes after endurance training, leveraging the capability to separate the blood samples by training status. To this end, we demonstrate that most of the variance in miRNA expression can be explained by both common and known biological and technical factors. Our findings highlight six distinct clusters of miRNAs, each exhibiting an oscillating expression profile across the four study timepoints, that can effectively be utilized to predict phenotypic VO2 max levels. In addition, we identified miR-532-5p as a candidate marker to determine personal alterations in physical training performance on a case-by-case analysis taking the influence of a carbohydrate-rich nutrition into account. In literature, miR-532-5p is known as a common down-regulated miRNA in diabetes and obesity, possibly providing a molecular link between cellular homeostasis, personal fitness levels, and health in aging. We conclude that circulating miRNA expression can be altered due to regular endurance training, independent of the carbohydrate (CHO) availability in the training timeframe. Further validation studies are required to confirm the role of exercise-affected miRNAs and the extraordinary function of miR-532-5p in modulating the metabolic response to a high availability of glucose.

Carbohydrate Restriction in Type 1 Diabetes: A Realistic Therapy for Improved Glycaemic Control and Athletic Performance?

Around 80% of individuals with Type 1 diabetes (T1D) in the United States do not achieve glycaemic targets and the prevalence of comorbidities suggests that novel therapeutic strategies, including lifestyle modification, are needed. Current nutrition guidelines suggest a flexible approach to carbohydrate intake matched with intensive insulin therapy. These guidelines are designed to facilitate greater freedom around nutritional choices but they may lead to higher caloric intakes and potentially unhealthy eating patterns that are contributing to the high prevalence of obesity and metabolic syndrome in people with T1D. Low carbohydrate diets (LCD; <130 g/day) may represent a means to improve glycaemic control and metabolic health in people with T1D. Regular recreational exercise or achieving a high level of athletic performance is important for many living with T1D. Research conducted on people without T1D suggests that training with reduced carbohydrate availability (often termed "train low") enhances metabolic adaptation compared to training with normal or high carbohydrate availability. However, these "train low" practices have not been tested in athletes with T1D. This review aims to investigate the known pros and cons of LCDs as a potentially effective, achievable, and safe therapy to improve glycaemic control and metabolic health in people with T1D. Secondly, we discuss the potential for low, restricted, or periodised carbohydrate diets in athletes with T1D.

A Framework for Periodized Nutrition for Athletics

Over the last decade, in support of training periodization, there has been an emergence around the concept of nutritional periodization. Within athletics (track and field), the science and art of periodization is a cornerstone concept with recent commentaries emphasizing the underappreciated complexity associated with predictable performance on demand. Nevertheless, with varying levels of evidence, sport and event specific sequencing of various training units and sessions (long [macrocycle; months], medium [mesocycle; weeks], and short [microcycle; days and within-day duration]) is a routine approach to training periodization. Indeed, implementation of strategic temporal nutrition interventions (macro, meso, and micro) can support and enhance training prescription and adaptation, as well as acute event specific performance. However, a general framework on how, why, and when nutritional periodization could be implemented has not yet been established. It is beyond the scope of this review to highlight every potential nutritional periodization application. Instead, this review will focus on a generalized framework, with specific examples of macro-, meso-, and microperiodization for the macronutrients of carbohydrates, and, by extension, fat. More specifically, the authors establish the evidence and rationale for situations of acute high carbohydrate availability, as well as the evidence for more chronic manipulation of carbohydrates coupled with training. The topic of periodized nutrition has made considerable gains over the last decade but is ripe for further scientific progress and field application.

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.

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.

Preexercise breakfast ingestion versus extended overnight fasting increases postprandial glucose flux after exercise in healthy men

The aim of this study was to characterize postprandial glucose flux after exercise in the fed versus overnight fasted state and to investigate the potential underlying mechanisms. In a randomized order, twelve men underwent breakfast-rest [(BR) 3 h semirecumbent], breakfast-exercise [(BE) 2 h semirecumbent before 60 min of cycling (50% peak power output)], and overnight fasted exercise [(FE) as per BE omitting breakfast] trials. An oral glucose tolerance test (OGTT) was completed after exercise (after rest on BR). Dual stable isotope tracers ([U-¹³C] glucose ingestion and [6,6-²H₂] glucose infusion) and muscle biopsies were combined to assess postprandial plasma glucose kinetics and intramuscular signaling, respectively. Plasma intestinal fatty acid binding (I-FABP) concentrations were determined as a marker of intestinal damage. Breakfast before exercise increased postexercise plasma glucose disposal rates during the OGTT, from 44 g/120 min in FE {35 to 53 g/120 min [mean (normalized 95% confidence interval)] to 73 g/120 min in BE [55 to 90 g/120 min; P = 0.01]}. This higher plasma glucose disposal rate was, however, offset by increased plasma glucose appearance rates (principally OGTT-derived), resulting in a glycemic response that did not differ between BE and FE ( P = 0.11). Plasma I-FABP concentrations during exercise were 264 pg/ml (196 to 332 pg/ml) lower in BE versus FE ( P = 0.01). Breakfast before exercise increases postexercise postprandial plasma glucose disposal, which is offset (primarily) by increased appearance rates of orally ingested glucose. Therefore, metabolic responses to fed-state exercise cannot be readily inferred from studies conducted in a fasted state.

Overnight fasting compromises exercise intensity and volume during sprint interval training but improves high-intensity aerobic endurance

BACKGROUND

The combined effects of sprint interval training (SIT) and exercising in the fasted state are unknown. We compared the effects of SIT with exogenous carbohydrate supplementation (SIT-CHO) and SIT following overnight fast (SIT-Fast) on aerobic capacity (peak oxygen consumption: V̇O2peak) and high-intensity aerobic endurance (time-to-exhaustion at 85% V̇O2peak [T85%]).

METHODS

Twenty male cyclists were randomized to SIT-CHO and SIT-Fast. Both groups performed 30-second all-out cycling followed by 4-minute active recovery 3 times per week for 4 weeks, with the number of sprint bouts progressing from 4 to 7. Peak power output (PPO) and total mechanical work were measured for each sprint interval bout. The SIT-CHO group performed exercise sessions following breakfast and consumed carbohydrate drink during exercise, whereas the SIT-Fast group performed exercise sessions following overnight fast and consumed water during exercise. Before and after training, V̇O2peak and T85% were assessed. Blood glucose, non-esterified fatty acids, insulin and glucagon concentrations were measured during T85%.

RESULTS

Overall PPO and mechanical work were lower in SIT-Fast than SIT-CHO (3664.9 vs. 3871.7 J/kg; P=0.021 and 10.6 vs. 9.9 W/kg; P=0.010, respectively). Post-training V̇O2peak did not differ between groups. Baseline-adjusted post-training T85% was longer in SIT-Fast compared to SIT-CHO (19.7±3.0 vs. 16.6±3.0 minutes, ANCOVA P=0.038) despite no changes in circulating energy substrates or hormones.

CONCLUSIONS

Our results suggest that SIT-Fast compromises exercise intensity and volume but still can have a greater impact on the ability to sustain high-intensity aerobic endurance exercise compared to SIT-CHO.

Regulation of Muscle Glycogen Metabolism during Exercise: Implications for Endurance Performance and Training Adaptations

Since the introduction of the muscle biopsy technique in the late 1960s, our understanding of the regulation of muscle glycogen storage and metabolism has advanced considerably. Muscle glycogenolysis and rates of carbohydrate (CHO) oxidation are affected by factors such as exercise intensity, duration, training status and substrate availability. Such changes to the global exercise stimulus exert regulatory effects on key enzymes and transport proteins via both hormonal control and local allosteric regulation. Given the well-documented effects of high CHO availability on promoting exercise performance, elite endurance athletes are typically advised to ensure high CHO availability before, during and after high-intensity training sessions or competition. Nonetheless, in recognition that the glycogen granule is more than a simple fuel store, it is now also accepted that glycogen is a potent regulator of the molecular cell signaling pathways that regulate the oxidative phenotype. Accordingly, the concept of deliberately training with low CHO availability has now gained increased popularity amongst athletic circles. In this review, we present an overview of the regulatory control of CHO metabolism during exercise (with a specific emphasis on muscle glycogen utilization) in order to discuss the effects of both high and low CHO availability on modulating exercise performance and training adaptations, respectively.

Carbohydrate intake and training efficacy - a randomized cross-over study

Carbohydrate (CHO) availability during endurance exercise seems to attenuate exercise-induced perturbations of cellular homeostasis and might consequently diminish the stimulus for training adaptation. Therefore, a negative effect of CHO intake on endurance training efficacy seems plausible. This study aimed to test the influence of carbohydrate intake on the efficacy of an endurance training program on previously untrained healthy adults. A randomized cross-over trial (8-week wash-out period) was conducted in 23 men and women with two 8-week training periods (with vs. without intake of 50g glucose before each training bout). Training intervention consisted of 4x45 min running/walking sessions/week at 70% of heart rate reserve. Exhaustive, ramp-shaped exercise tests with gas exchange measurements were conducted before and after each training period. Outcome measures were maximum oxygen uptake (VO_2max) and ventilatory anaerobic threshold (VT). VO_2max and VT increased after training regardless of CHO intake (VO_2max: Non-CHO 2.6 ± 3.0 ml*min^-1*kg^-1 p = 0.004; CHO 1.4 ± 2.5 ml*min^-1*kg^-1 p = 0.049; VT: Non-CHO 4.2 ± 4.2 ml*min^-1*kg^-1 p < 0.001; CHO 3.0 ± 4.2 ml*min^-1*kg^-1 p = 0.003). The 95% confidence interval (CI) for the difference between conditions was between +0.1 and +2.1 ml*min^-1*kg^-1 for VO_2max and between -1.2 and +3.1 for VT. It is concluded that carbohydrate intake could potentially impair the efficacy of an endurance training program.

Periodized Nutrition for Athletes

It is becoming increasingly clear that adaptations, initiated by exercise, can be amplified or reduced by nutrition. Various methods have been discussed to optimize training adaptations and some of these methods have been subject to extensive study. To date, most methods have focused on skeletal muscle, but it is important to note that training effects also include adaptations in other tissues (e.g., brain, vasculature), improvements in the absorptive capacity of the intestine, increases in tolerance to dehydration, and other effects that have received less attention in the literature. The purpose of this review is to define the concept of periodized nutrition (also referred to as nutritional training) and summarize the wide variety of methods available to athletes. The reader is referred to several other recent review articles that have discussed aspects of periodized nutrition in much more detail with primarily a focus on adaptations in the muscle. The purpose of this review is not to discuss the literature in great detail but to clearly define the concept and to give a complete overview of the methods available, with an emphasis on adaptations that are not in the muscle. Whilst there is good evidence for some methods, other proposed methods are mere theories that remain to be tested. 'Periodized nutrition' refers to the strategic combined use of exercise training and nutrition, or nutrition only, with the overall aim to obtain adaptations that support exercise performance. The term nutritional training is sometimes used to describe the same methods and these terms can be used interchangeably. In this review, an overview is given of some of the most common methods of periodized nutrition including 'training low' and 'training high', and training with low- and high-carbohydrate availability, respectively. 'Training low' in particular has received considerable attention and several variations of 'train low' have been proposed. 'Training-low' studies have generally shown beneficial effects in terms of signaling and transcription, but to date, few studies have been able to show any effects on performance. In addition to 'train low' and 'train high', methods have been developed to 'train the gut', train hypohydrated (to reduce the negative effects of dehydration), and train with various supplements that may increase the training adaptations longer term. Which of these methods should be used depends on the specific goals of the individual and there is no method (or diet) that will address all needs of an individual in all situations. Therefore, appropriate practical application lies in the optimal combination of different nutritional training methods. Some of these methods have already found their way into training practices of athletes, even though evidence for their efficacy is sometimes scarce at best. Many pragmatic questions remain unanswered and another goal of this review is to identify some of the remaining questions that may have great practical relevance and should be the focus of future research.

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.

Targeting specific interstitial glycemic parameters with high-intensity interval exercise and fasted-state exercise in type 2 diabetes

AIMS

To compare the acute glycemic responses to a bout of high-intensity interval exercise (HIIE) and energy-matched moderate-intensity continuous exercise (MICE) performed under fasted and postprandial conditions.

METHODS

A randomized, controlled, crossover design was used. Ten individuals with type 2 diabetes were each tested in five experimental conditions after an overnight fast: 1) fasted-state HIIE (HIIEfast); 2) post-breakfast HIIE (HIIEfed); 3) fasted-state MICE (MICEfast); 4) post-breakfast MICE (MICEfed); and 5) no exercise (control). MICE was performed at workload corresponding to 55% of V.V̇O2peak, whereas HIIE was composed of repetitions of three minutes at workload corresponding to 40% followed by one minute at workload corresponding to 100% V.V̇̇O2peak. Interstitial glucose was monitored by continuous glucose monitoring over 24h under standardized diet and medication.

RESULTS

Fasted-state exercise attenuated postprandial glycemic increments (p<0.05) to a greater extent than post-breakfast exercise did. HIIE reduced nocturnal and fasting glycemia on the day following exercise more than MICE did (main effect: both p<0.05). Compared to the control condition, HIIEfast lowered most interstitial glycemic parameters, i.e., 24-h mean glucose (-1.5mmol·l(-1); p<0.05), fasting glucose (-1.0mmol·l(-1); p<0.05), overall postprandial glycemic increment (-257mmol·360min·l(-1); p<0.05), glycemic variability (-1.79mmol·l(-1); p<0.05), and time spent in hyperglycemia (-283min; p<0.05).

CONCLUSION

This study showed that HIIE is more effective than MICE in lowering nocturnal/fasting glycemia. Exercise performed in the fasted state reduces postprandial glycemic increments to a greater extent than post-breakfast exercise does. Performing HIIE under fasted condition may be most advantageous as it lowered most aspects of glycemia.

Transient energy deficit induced by exercise increases 24-h fat oxidation in young trained men

Whole body fat oxidation increases during exercise. However, 24-h fat oxidation on a day with exercise often remains similar to that of sedentary day, when energy intake is increased to achieve an energy-balanced condition. The present study aimed to examine a possibility that time of the day when exercise is performed makes differences in 24-h fat oxidation. As a potential mechanism of exercise affecting 24-h fat oxidation, its relation to exercise-induced transient energy deficit was examined. Nine young male endurance athletes underwent three trials of indirect calorimetry using a metabolic chamber, in which they performed a session of 100 min of exercise before breakfast (AM), after lunch (PM), or two sessions of 50 min of exercise before breakfast and after lunch (AM/PM) at 65% of maximal oxygen uptake. Experimental meals were designed to achieve individual energy balance. Twenty-four-hour energy expenditure was similar among the trials, but 24-h fat oxidation was 1,142 ± 97, 809 ± 88, and 608 ± 46 kcal/24 h in descending order of its magnitude for AM, AM/PM, and PM, respectively (P < 0.05). Twenty-four-hour carbohydrate oxidation was 2,558 ± 110, 2,374 ± 114, and 2,062 ± 96 kcal/24 h for PM, AM/PM, and AM, respectively. In spite of energy-balanced condition over 24 h, exercise induced a transient energy deficit, the magnitude of which was negatively correlated with 24-h fat oxidation (r = -0.72, P < 0.01). Similarly, transient carbohydrate deficit after exercise was negatively correlated with 24-h fat oxidation (r = -0.40, P < 0.05). The time of the day when exercise is performed affects 24-h fat oxidation, and the transient energy/carbohydrate deficit after exercise is implied as a factor affecting 24-h fat oxidation.

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.

Interaction of physical trainings and coffee intakes in fuel utilization during exercise in rats

This study investigates the impact of exercises, coffee intakes, and physical trainings on fuel utilization in rats. Ninety-six rats were fed a control diet with either water (C) or coffee (CF; 0.12 g freeze-dried instant coffee/100 g body weight/d). Additionally, the animals go through physical training (TC and TCF) or no training (NTC and NTCF) for 4 weeks. For physical training, animals have to exercise on treadmills for 30 minutes (5 d per week, 15° incline, 0.5-0.8 km/h). At the end of week 4, the animals in each group were subdivided into three exercise groups: before exercise (BE), during exercise (DE), and after exercise (AE). The DE rats exercised on treadmills for 1 hour immediately before being sacrificed. Hemoglobin, hematocrit, glucose, glycogen, protein, triglyceride (TG), and free fatty acid (FFA) levels in the plasma, liver, and skeletal muscle of the rats were compared accordingly. Organ weights were also measured. Coffee-training interaction had a significant impact on heart weight, visceral fat, hemoglobin, hematocrit, liver glycogen in DE and AE, and liver triglyceride in DE and AE. Exercise (meaning exercised on a treadmill for 1 hour immediately before being sacrificed) training interaction was significant in liver glycogen, muscle glycogen in control diet and control diet with coffee, FFA and muscle TG levels at control diet with coffee group. Exercise-coffee interactions significantly influenced the FFA with no training groups. Exercise-coffee-training interaction significantly effects on FFA, Liver TG and Muscle TG. Coffee intakes can increase lipolysis during exercising but coffee consumptions delay the recovery of liver glycogen levels in trained rats after exercising. Coffee intakes can increase lipolysis during exercising but coffee consumptions delay the recovery of liver glycogen levels in trained rats after exercising. Coffee can be an effective ergogenic aid during exercise for physically trained rats.

Reduced carbohydrate availability enhances exercise-induced p53 signaling in human skeletal muscle: implications for mitochondrial biogenesis

The mechanisms that regulate the enhanced skeletal muscle oxidative capacity observed when training with reduced carbohydrate (CHO) availability are currently unknown. The aim of the present study was to test the hypothesis that reduced CHO availability enhances p53 signaling and expression of genes associated with regulation of mitochondrial biogenesis and substrate utilization in human skeletal muscle. In a repeated-measures design, muscle biopsies (vastus lateralis) were obtained from eight active males before and after performing an acute bout of high-intensity interval running with either high (HIGH) or low CHO availability (LOW). Resting muscle glycogen (HIGH, 467 ± 19; LOW, 103 ± 9 mmol/kg dry wt) was greater in HIGH compared with LOW (P < 0.05). Phosphorylation (P-) of ACC(Ser79) (HIGH, 1.4 ± 0.4; LOW, 2.9 ± 0.9) and p53(Ser15) (HIGH, 0.9 ± 0.4; LOW, 2.6 ± 0.8) was higher in LOW immediately postexercise and 3 h postexercise, respectively (P < 0.05). Before and 3 h postexercise, mRNA content of pyruvate dehydrogenase kinase 4, mitochondrial transcription factor A, cytochrome-c oxidase IV, and PGC-1α were greater in LOW compared with HIGH (P < 0.05), whereas carnitine palmitoyltransferase-1 showed a trend toward significance (P = 0.09). However, only PGC-1α expression was increased by exercise (P < 0.05), where three-fold increases occurred independently of CHO availability. We conclude that the exercise-induced increase in p53 phosphorylation is enhanced in conditions of reduced CHO availability, which may be related to upstream signaling through AMPK. Given the emergence of p53 as a molecular regulator of mitochondrial biogenesis, such nutritional modulation of contraction-induced p53 activation has implications for both athletic and clinical populations.

More than a store: regulatory roles for glycogen in skeletal muscle adaptation to exercise

The glycogen content of muscle determines not only our capacity for exercise but also the signaling events that occur in response to exercise. The result of the shift in signaling is that frequent training in a low-glycogen state results in improved fat oxidation during steady-state submaximal exercise. This review will discuss how the amount or localization of glycogen particles can directly or indirectly result in this differential response to training. The key direct effect discussed is carbohydrate binding, whereas the indirect effects include the metabolic shift toward fat oxidation, the increase in catecholamines, and osmotic stress. Although our understanding of the role of glycogen in response to training has expanded exponentially over the past 5 years, there are still many questions remaining as to how stored carbohydrate affects the muscular adaptation to exercise.

The effect of hypoxia and work intensity on insulin resistance in type 2 diabetes

CONTEXT

Hypoxia and muscle contraction stimulate glucose transport in vitro. We have previously demonstrated that exercise and hypoxia have an additive effect on insulin sensitivity in type 2 diabetics.

OBJECTIVES

Our objective was to examine the effects of three different hypoxic/exercise (Hy Ex) trials on glucose metabolism and insulin resistance in the 48 h after acute hypoxia in type 2 diabetics.

DESIGN, PARTICIPANTS, AND INTERVENTIONS

Eight male type 2 diabetics completed 60 min of hypoxic [mean (sem) O(2) = ∼14.7 (0.2)%] exercise at 90% of lactate threshold [Hy Ex(60); 49 (1) W]. Patients completed an additional two hypoxic trials of equal work, lasting 40 min [Hy Ex(40); 70 (1) W] and 20 min [Hy Ex(20); 140 (12) W].

MAIN OUTCOME MEASURES

Glucose rate of appearance and rate of disappearance were determined using the one-compartment minimal model. Homeostasis models of insulin resistance (HOMA(IR)), fasting insulin resistance index and β-cell function (HOMA(β-cell)) were calculated at 24 and 48 h after trials.

RESULTS

Peak glucose rate of appearance was highest during Hy Ex(20) [8.89 (0.56) mg/kg · min, P < 0.05]. HOMA(IR) and fasting insulin resistance index were improved in the 24 and 48 h after Hy Ex(60) and Hy Ex(40) (P < 0.05). HOMA(IR) decreased 24 h after Hy Ex(20) (P < 0.05) and returned to baseline values at 48 h.

CONCLUSIONS

Moderate-intensity exercise in hypoxia (Hy Ex(60) and Hy Ex(40)) stimulates acute- and moderate-term improvements in insulin sensitivity that were less apparent in Hy Ex(20). Results suggest that exercise duration and not total work completed has a greater influence on acute and moderate-term glucose control in type 2 diabetics.

Type 2 diabetes is associated with altered NF-κB DNA binding activity, JNK phosphorylation, and AMPK phosphorylation in skeletal muscle after LPS

Systemic inflammation is often associated with impaired glucose metabolism. We therefore studied the activation of inflammatory pathway intermediates that interfere with glucose uptake during systemic inflammation by applying a standardised inflammatory stimulus in vivo. After ethical approval, informed consent and a thorough physical examination, 10 patients with type 2 diabetes and 10 participants with normal glucose tolerance (NGT) were given an intravenous bolus of E. coli lipopolysaccharide (LPS) of 0.3 ng/kg. Skeletal muscle biopsies and plasma were obtained at baseline and two, four and six hours after LPS. Nuclear factor (NF)-κB p65 DNA binding activity measured by ELISA, tumor necrosis factor-α and interleukin-6 mRNA expression analysed by real time reverse transcription polymerase chain reaction, and abundance of inhibitor of NF-κB (IκB)α, phosphorylated c-Jun-N-terminal kinase (JNK), AMP-activated protein kinase (AMPK), and acetyl-CoA carboxylase measured by Western blotting were detected in muscle biopsy samples. Relative to subjects with NGT, patients with type 2 diabetes exhibited a more pronounced increase in NF-κB binding activity and JNK phosphorylation after LPS, whereas skeletal muscle cytokine mRNA expression did not differ significantly between groups. AMPK phosphorylation increased in volunteers with NGT, but not in those with diabetes. The present findings indicate that pathways regulating glucose uptake in skeletal muscle may be involved in the development of inflammation-associated hyperglycemia. Patients with type 2 diabetes exhibit changes in these pathways, which may ultimately render such patients more prone to develop dysregulated glucose disposal in the context of systemic inflammation.

TRIAL REGISTRATION

ClinicalTrials.gov NCT00412906.

Training in the fasted state improves glucose tolerance during fat-rich diet

A fat-rich energy-dense diet is an important cause of insulin resistance. Stimulation of fat turnover in muscle cells during dietary fat challenge may contribute to maintenance of insulin sensitivity. Exercise in the fasted state markedly stimulates energy provision via fat oxidation. Therefore, we investigated whether exercise training in the fasted state is more potent than exercise in the fed state to rescue whole-body glucose tolerance and insulin sensitivity during a period of hyper-caloric fat-rich diet. Healthy male volunteers (18-25 y) received a hyper-caloric (∼+30% kcal day(-1)) fat-rich (50% of kcal) diet for 6 weeks. Some of the subjects performed endurance exercise training (4 days per week) in the fasted state (F; n = 10), whilst the others ingested carbohydrates before and during the training sessions (CHO; n = 10). The control group did not train (CON; n = 7). Body weight increased in CON (+3.0 ± 0.8 kg) and CHO (+1.4 ± 0.4 kg) (P < 0.01), but not in F (+0.7 ± 0.4 kg, P = 0.13). Compared with CON, F but not CHO enhanced whole-body glucose tolerance and the Matsuda insulin sensitivity index (P < 0.05). Muscle GLUT4 protein content was increased in F (+28%) compared with both CHO (P = 0.05) and CON (P < 0.05). Furthermore, only training in F elevated AMP-activated protein kinase α phosphorylation (+25%) as well as up-regulated fatty acid translocase/CD36 and carnitine palmitoyltransferase 1 mRNA levels compared with CON (∼+30%). High-fat diet increased intramyocellular lipid but not diacylglycerol and ceramide contents, either in the absence or presence of training. This study for the first time shows that fasted training is more potent than fed training to facilitate adaptations in muscle and to improve whole-body glucose tolerance and insulin sensitivity during hyper-caloric fat-rich diet.

Carbohydrate availability and training adaptation: effects on cell metabolism

Several markers of endurance training adaptation are enhanced to a greater extent when individuals undertake selected training sessions with low compared with normal muscle glycogen content or with low exogenous carbohydrate availability. The potential mechanisms underlying the cellular responses arising from such nutrient-exercise interactions are discussed in the context of promoting training adaptation.

Exercise induces a marked increase in plasma follistatin: evidence that follistatin is a contraction-induced hepatokine

Follistatin is a member of the TGF-β super family and inhibits the action of myostatin to regulate skeletal muscle growth. The regulation of follistatin during physical exercise is unclear but may be important because physical activity is a major intervention to prevent age-related sarcopenia. First, healthy subjects performed either bicycle or one-legged knee extensor exercise. Arterial-venous differences were assessed during the one-legged knee extensor experiment. Next, mice performed 1 h of swimming, and the expression of follistatin was examined in various tissues using quantitative PCR. Western blotting assessed follistatin protein content in the liver. IL-6 and epinephrine were investigated as drivers of follistatin secretion. After 3 h of bicycle exercise, plasma follistatin increased 3 h into recovery with a peak of 7-fold. No net release of follistatin could be detected from the exercising limb. In mice performing a bout of swimming exercise, increases in plasma follistatin as well as follistatin mRNA and protein expression in the liver were observed. IL-6 infusion to healthy young men did not affect the follistatin concentration in the circulation. When mice were stimulated with epinephrine, no increase in the hepatic mRNA of follistatin was observed. This is the first study to demonstrate that plasma follistatin is increased during exercise and most likely originates from the liver. These data introduce new perspectives regarding muscle-liver cross talk during exercise and during recovery from exercise.

Carbohydrate administration and exercise performance: what are the potential mechanisms involved?

It is well established that carbohydrate (CHO) administration increases performance during prolonged exercise in humans and animals. The mechanism(s), which could mediate the improvement in exercise performance associated with CHO administration, however, remain(s) unclear. This review focuses on possible underlying mechanisms that could explain the increase in exercise performance observed with the administration of CHO during prolonged muscle contractions in humans and animals. The beneficial effect of CHO ingestion on performance during prolonged exercise could be due to several factors including (i) an attenuation in central fatigue; (ii) a better maintenance of CHO oxidation rates; (iii) muscle glycogen sparing; (iv) changes in muscle metabolite levels; (v) reduced exercise-induced strain; and (vi) a better maintenance of excitation-contraction coupling. In general, the literature indicates that CHO ingestion during exercise does not reduce the utilization of muscle glycogen. In addition, data from a meta-analysis suggest that a dose-dependent relationship was not shown between CHO ingestion during exercise and an increase in performance. This could support the idea that providing enough CHO to maintain CHO oxidation during exercise may not always be associated with an increase in performance. Emerging evidence from the literature shows that increasing neural drive and attenuating central fatigue may play an important role in increasing performance during exercise with CHO supplementation. In addition, CHO administration during exercise appears to provide protection from disrupted cell homeostasis/integrity, which could translate into better muscle function and an increase in performance. Finally, it appears that during prolonged exercise when the ability of metabolism to match energy demand is exceeded, adjustments seem to be made in the activity of the Na+/K+ pump. Therefore, muscle fatigue could be acting as a protective mechanism during prolonged contractions. This could be alleviated when CHO is administered resulting in the better maintenance of the electrical properties of the muscle fibre membrane. The mechanism(s) by which CHO administration increases performance during prolonged exercise is(are) complex, likely involving multiple factors acting at numerous cellular sites. In addition, due to the large variation in types of exercise, durations, intensities, feeding schedules and CHO types it is difficult to assess if the mechanism(s) that could explain the increase in performance with CHO administration during exercise is(are) similar in different situations. Experiments concerning the identification of potential mechanism(s) by which performance is increased with CHO administration during exercise will add to our understanding of the mechanism(s) of muscle/central fatigue. This knowledge could have significant implications for improving exercise performance.

Beneficial metabolic adaptations due to endurance exercise training in the fasted state

Training with limited carbohydrate availability can stimulate adaptations in muscle cells to facilitate energy production via fat oxidation. Here we investigated the effect of consistent training in the fasted state, vs. training in the fed state, on muscle metabolism and substrate selection during fasted exercise. Twenty young male volunteers participated in a 6-wk endurance training program (1-1.5 h cycling at ∼70% Vo(₂max), 4 days/wk) while receiving isocaloric carbohydrate-rich diets. Half of the subjects trained in the fasted state (F; n = 10), while the others ingested ample carbohydrates before (∼160 g) and during (1 g·kg body wt⁻¹·h⁻¹) the training sessions (CHO; n = 10). The training similarly increased Vo(₂max) (+9%) and performance in a 60-min simulated time trial (+8%) in both groups (P < 0.01). Metabolic measurements were made during a 2-h constant-load exercise bout in the fasted state at ∼65% pretraining Vo(₂max). In F, exercise-induced intramyocellular lipid (IMCL) breakdown was enhanced in type I fibers (P < 0.05) and tended to be increased in type IIa fibers (P = 0.07). Training did not affect IMCL breakdown in CHO. In addition, F (+21%) increased the exercise intensity corresponding to the maximal rate of fat oxidation more than did CHO (+6%) (P < 0.05). Furthermore, maximal citrate synthase (+47%) and β-hydroxyacyl coenzyme A dehydrogenase (+34%) activity was significantly upregulated in F (P < 0.05) but not in CHO. Also, only F prevented the development exercise-induced drop in blood glucose concentration (P < 0.05). In conclusion, F is more effective than CHO to increase muscular oxidative capacity and at the same time enhances exercise-induced net IMCL degradation. In addition, F but not CHO prevented drop of blood glucose concentration during fasting exercise.

Combining short-term metformin treatment and one bout of exercise does not increase insulin action in insulin-resistant individuals

Results from the Diabetes Prevention Program highlight the effectiveness of metformin or regular physical activity in the prevention of type 2 diabetes. Independently, metformin and exercise increase insulin sensitivity, but they have not been studied in combination. To assess the combined effects, insulin-resistant subjects (n = 9) matched for weight, body fat, and aerobic fitness were studied before any treatment (B), after 2-3 wk of 2,000 mg/day metformin (MET), and after metformin plus 40 min of exercise at 65% Vo(2peak) (MET + Ex). A second group (n = 7) was studied at baseline and after an identical bout of exercise with no metformin (Ex). Biopsies of the vastus lateralis were taken at B, after MET, immediately after MET + Ex (group 1), or immediately after Ex (group 2). Insulin sensitivity was assessed 4 h postexercise with a euglycemic hyperinsulinemic (40 mU.m(2).min(-1)) clamp enriched with [6,6-(2)H]glucose. Insulin sensitivity was 54% higher after Ex (P < 0.01), but there was no change with Met + Ex. Skeletal muscle AMPKalpha2 activity was elevated threefold (P < 0.01) after Ex, but there was no increase with MET + Ex. These findings suggest that the combination of short-term metformin treatment and an acute bout of exercise does not enhance insulin sensitivity, and the addition of metformin may attenuate the well-documented effects of exercise alone.

Glucose ingestion during endurance training in men attenuates expression of myokine receptor

Glucose ingestion during exercise attenuates the release of the myokine interleukin-6 (IL-6) from working skeletal muscle, which results in a diminished increase in plasma IL-6. Interleukin-6 receptor alpha (IL-6Ralpha) expression in skeletal muscle is induced by acute exercise, mediated in part by an increased IL-6 concentration in the bloodstream. We hypothesized that endurance training would increase the density of IL-6Ralpha in skeletal muscle and that glucose ingestion would attenuate the effect. Nine subjects performed 10 weeks of one-legged knee-extensor training. They trained one leg (Glc-leg) while ingesting a glucose solution (Glc) and ingested a placebo (Plc) while training the other leg (Plc-leg). Endurance training increased peak power by 14% and reduced the exercise-induced gene expression of IL-6 and IL-6Ralpha in skeletal muscle and IL-6 plasma concentration. The IL-6Ralpha density increased to a lesser extent in the Glc-leg, suggesting that glucose ingestion attenuates the effect of training on IL-6Ralpha by blunting the IL-6 response. We conclude that glucose ingestion during endurance training attenuates the increase in IL-6Ralpha density.

Glucose ingestion during endurance training does not alter adaptation

Glucose ingestion during exercise attenuates activation of metabolic enzymes and expression of important transport proteins. In light of this, we hypothesized that glucose ingestion during training would result in 1) an attenuation of the increase in fatty acid uptake and oxidation during exercise, 2) lower citrate synthase (CS) and β-hydroxyacyl-CoA dehydrogenase (β-HAD) activity and glycogen content in skeletal muscle, and 3) attenuated endurance performance enhancement in the trained state. To investigate this we studied nine male subjects who performed 10 wk of one-legged knee extensor training. They trained one leg while ingesting a 6% glucose solution (Glc) and ingested a sweetened placebo while training the other leg (Plc). The subjects trained their respective legs 2 h at a time on alternate days 5 days a week. Endurance training increased peak power (Pmax) and time to fatigue at 70% of Pmax ∼14% and ∼30%, respectively. CS and β-HAD activity increased and glycogen content was greater after training, but there were no differences between Glc and Plc. After training the rate of oxidation of palmitate (Rox) and the % of rate of disappearance that was oxidized (%Rdox) changed. %Rdox was on average 16.4% greater during exercise after training whereas, after exercise %Rdox was 30.4% lower. Rox followed the same pattern. However, none of these parameters were different between Glc and Plc. We conclude that glucose ingestion during training does not alter training adaptation related to substrate metabolism, mitochondrial enzyme activity, glycogen content, or performance.

Physiological roles of muscle-derived interleukin-6 in response to exercise

PURPOSE OF REVIEW

To discuss recent findings with regard to the regulation of muscle-derived interleukin-6 as well as the possible physiological and metabolic roles of interleukin-6 in response to exercise.

RECENT FINDINGS

Contraction-induced transcription and release of interleukin-6 is primarily regulated by an altered intramuscular milieu in response to exercise. Accordingly, changes in calcium homeostasis, impaired glucose availability and increased formation of reactive oxygen species are all associated with exercise and capable of activating transcription factors known to regulate interleukin-6 synthesis. Acute interleukin-6 administration to humans increases lipolysis, fat oxidation and insulin-mediated glucose disposal. Adenosine monophosphate-activated protein kinase activation by interleukin-6 appears to play an important role in modulating some of these metabolic effects. Interleukin-6 facilitates an antiinflammatory milieu and may exert some of its biological effects via inhibition of the proinflammatory cytokine tumor necrosis factor-alpha.

SUMMARY

The discovery of contracting muscle as a cytokine-producing organ opens a new paradigm: skeletal muscle is an endocrine organ that in response to contractions produces and releases 'myokines', which subsequently can modulate the metabolic and immunological response to exercise in several tissues. In our view, interleukin-6 may be one of several myokines.

Effect of acute exercise on AMPK signaling in skeletal muscle of subjects with type 2 diabetes: a time-course and dose-response study

Activation of AMP-activated protein kinase (AMPK) by exercise induces several cellular processes in muscle. Exercise activation of AMPK is unaffected in lean (BMI approximately 25 kg/m(2)) subjects with type 2 diabetes. However, most type 2 diabetic subjects are obese (BMI >30 kg/m(2)), and exercise stimulation of AMPK is blunted in obese rodents. We examined whether obese type 2 diabetic subjects have impaired exercise stimulation of AMPK, at different signaling levels, spanning from the upstream kinase, LKB1, to the putative AMPK targets, AS160 and peroxisome proliferator-activated receptor coactivator (PGC)-1alpha, involved in glucose transport regulation and mitochondrial biogenesis, respectively. Twelve type 2 diabetic, eight obese, and eight lean subjects exercised on a cycle ergometer for 40 min. Muscle biopsies were done before, during, and after exercise. Subjects underwent this protocol on two occasions, at low (50% Vo(2max)) and moderate (70% Vo(2max)) intensities, with a 4-6 week interval. Exercise had no effect on LKB1 activity. Exercise had a time- and intensity-dependent effect to increase AMPK activity and AS160 phosphorylation. Obese and type 2 diabetic subjects had attenuated exercise-stimulated AMPK activity and AS160 phosphorylation. Type 2 diabetic subjects had reduced basal PGC-1 gene expression but normal exercise-induced increases in PGC-1 expression. Our findings suggest that obese type 2 diabetic subjects may need to exercise at higher intensity to stimulate the AMPK-AS160 axis to the same level as lean subjects.

Predominant alpha2/beta2/gamma3 AMPK activation during exercise in human skeletal muscle

5'AMP-activated protein kinase (AMPK) is a key regulator of cellular metabolism and is regulated in muscle during exercise. We have previously established that only three of 12 possible AMPK alpha/beta/gamma-heterotrimers are present in human skeletal muscle. Previous studies describe discrepancies between total AMPK activity and regulation of its target acetyl-CoA-carboxylase (ACC)beta. Also, exercise training decreases expression of the regulatory gamma3 AMPK subunit and attenuates alpha2 AMPK activity during exercise. We hypothesize that these observations reflect a differential regulation of the AMPK heterotrimers. We provide evidence here that only the alpha2/beta2/gamma3 subunit is phosphorylated and activated during high-intensity exercise in vivo. The activity associated with the remaining two AMPK heterotrimers, alpha1/beta2/gamma1 and alpha2/beta2/gamma1, is either unchanged (20 min, 80% maximal oxygen uptake ) or decreased (30 or 120 s sprint-exercise). The differential activity of the heterotrimers leads to a total alpha-AMPK activity, that is decreased (30 s trial), unchanged (120 s trial) and increased (20 min trial). AMPK activity associated with the alpha2/beta2/gamma3 heterotrimer was strongly correlated to gamma3-associated alpha-Thr-172 AMPK phosphorylation (r(2) = 0.84, P < 0.001) and to ACCbeta Ser-221 phosphorylation (r(2) = 0.65, P < 0.001). These data single out the alpha2/beta2/gamma3 heterotrimer as an important actor in exercise-regulated AMPK signalling in human skeletal muscle, probably mediating phosphorylation of ACCbeta.

Role of AMPK in skeletal muscle metabolic regulation and adaptation in relation to exercise

The 5'-AMP-activated protein kinase (AMPK) is a potent regulator of skeletal muscle metabolism and gene expression. AMPK is activated both in response to in vivo exercise and ex vivo contraction. AMPK is therefore believed to be an important signalling molecule in regulating muscle metabolism during exercise as well as in adaptation of skeletal muscle to exercise training. The first part of this review is focused on different mechanisms regulating AMPK activity during muscle work such as alterations in nucleotide concentrations, availability of energy substrates and upstream AMPK kinases. We furthermore discuss the possible role of AMPK as a master switch in skeletal muscle metabolism with the main focus on AMPK in metabolic regulation during muscle work. Finally, AMPK has a well established role in regulating expression of genes encoding various enzymes in muscle, and this issue is discussed in relation to adaptation of skeletal muscle to exercise training.