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

Bridging the gap: Evidence-based practice guidelines for sports nutritionists

Evidence-based practice is a systematic approach to decision-making developed in the 1990s to help healthcare professionals identify and use the best available evidence to guide clinical practice and patient outcomes amid a plethora of information in often challenging, time-constrained circumstances. Today’s sports nutrition practitioners face similar challenges, as they must assess and judge the quality of evidence and its appropriateness to their athlete, in the often chaotic, time-pressed environment of professional sport. To this end, we present an adapted version of the evidence-based framework to support practitioners in navigating their way through the deluge of available information and guide their recommendations to athletes whilst also reflecting on their practice experience and skills as evidence-based practitioners, thus, helping to bridge the gap between science and practice in sport and exercise nutrition

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

Perceptions of Current Issues in Female Sport Nutrition From Elite Athletes, Practitioners, and Researchers

In response to the ongoing sex data gap, the present study provides a qualitative exploration of females’ nutritional experiences in elite sporting environments. Semistructured interviews were conducted with multiple participant groups (n = 18), including athletes (n = 7), practitioners (n = 6), and researchers (n = 5) across differing disciplines within professional sporting organiza-tions and/or national governing bodies. Combined content and thematic analysis provided an insight into the specific factors influencing current sport nutrition practices. A common theme highlighted among all participant groups was the paradoxical struggle between adequate fueling for training and competition demands, and the fear this may impact body mass and body composition goals. This tension was identified as being rooted within athletes’ perceptions of body image and driven by other participant groups and wider societal ideals. Each participant group also highlighted influences on cravings and approaches to food and dietary supplementation, centered around individual perceptions and challenges driven by symptomology associated with the female menstrual cycle and contraceptive use. To address these challenges, all participant groups called for more research to inform future change and continuing education pathways. In summary, this study contributes to providing a more complete understanding of elite female athlete sport nutrition experiences than currently exists. Multiple perspectives highlight the complexity of providing sport nutrition support to elite female athlete populations and directs future research, and practice, to reconsider one-size-fits-all approaches and acknowledge unique individual contexts which may influence these areas

Perceptions of Current Issues in Female Sport Nutrition From Elite Athletes, Practitioners, and Researchers

In response to the ongoing sex data gap, the present study provides a qualitative exploration of females’ nutritional experiences in elite sporting environments. Semistructured interviews were conducted with multiple participant groups (n = 18), including athletes (n = 7), practitioners (n = 6), and researchers (n = 5) across differing disciplines within professional sporting organiza-tions and/or national governing bodies. Combined content and thematic analysis provided an insight into the specific factors influencing current sport nutrition practices. A common theme highlighted among all participant groups was the paradoxical struggle between adequate fueling for training and competition demands, and the fear this may impact body mass and body composition goals. This tension was identified as being rooted within athletes’ perceptions of body image and driven by other participant groups and wider societal ideals. Each participant group also highlighted influences on cravings and approaches to food and dietary supplementation, centered around individual perceptions and challenges driven by symptomology associated with the female menstrual cycle and contraceptive use. To address these challenges, all participant groups called for more research to inform future change and continuing education pathways. In summary, this study contributes to providing a more complete understanding of elite female athlete sport nutrition experiences than currently exists. Multiple perspectives highlight the complexity of providing sport nutrition support to elite female athlete populations and directs future research, and practice, to reconsider one-size-fits-all approaches and acknowledge unique individual contexts which may influence these areas

Graded reductions in pre-exercise muscle glycogen impair exercise capacity but do not augment cell skeletal muscle signalling: implication for CHO periodisation

We examined the effects of graded muscle glycogen on exercise capacity and modulation of skeletal muscle signalling pathways associated with the regulation of mitochondrial biogenesis. In a repeated measures design, eight males completed a sleep-low, train-low model comprising an evening glycogen depleting cycling protocol followed by an exhaustive exercise capacity test (8 x 3 min at 80% PPO, followed by 1 min efforts at 80% PPO until exhaustion) the subsequent morning. Following glycogen depleting exercise, subjects ingested a total of 0 g kg-1 (L-CHO), 3.6 g kg-1 (M-CHO) or 7.6 g kg-1 (H-CHO) of carbohydrate during a 6 h period prior to sleeping, such that exercise was commenced the next morning with graded (P < 0.05) muscle glycogen concentrations (Mean ± SD) (L-CHO: 88 ± 43, M-CHO: 185 ± 62, H-CHO: 278 ± 47 mmol kg-1 dw). 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) post- and 3 h post-exercise, respectively. In contrast, exercise nor CHO availability affected the phosphorylation of p38MAPKThr180/Tyr182, CaMKIIThr268 or mRNA expression of p53, Tfam, CPT-1, CD36 or PDK4. Data demonstrate that when exercise is commenced with muscle glycogen below 300 mmol kg-1 dw, further graded reductions of 100 mmol kg-1 dw impair exercise capacity but do not augment skeletal muscle cell signaling

Hormonal contraceptive use, menstrual cycle characteristics and training/nutrition related profiles of elite, sub-elite and amateur athletes and exercisers: One size is unlikely to fit all

The aim of the present study was to simultaneously audit hormonal contraceptive (HC) use, menstrual cycle characteristics and training/nutrition related profiles in pre-menopausal women from varying athletic and exercise backgrounds. Elite (n = 51), sub-elite (n = 118) and amateur (n = 392) female athletes and exercisers were examined via an anonymous quantitative/qualitative survey tool. All analyses for ratio data were conducted utilising one- and two-way ANOVA/ANCOVA and odds ratio models, with ordinal data analysed via Pearson's Chi-squared tests. HC use was similar across elite, sub-elite and amateurs (34–44%). Menstrual cycle length was not different (P = 0.08) between competitive levels (28 ± 13 days), but 66% of respondents reported cycle variability (10 ± 11 days). Training profiles were not different based on contraceptive status (P > 0.05) yet were across competitive groups (P 0.05), though elite and sub-elite groups reported a higher daily meal consumption compared to amateurs (P < 0.01). Forty percent of all respondents skipped meals, attributed to lack of time, schedule and alterations in appetite, with reported changes in taste preferences during pre-menses (14–35%) and menses (15–25%) towards sweet foods (60%), with cravings for chocolate and other confectionary (25%). Prevalence of dietary supplement use was not influenced by contraceptive status (P = 0.31), though elites (76%) reported higher use (P = 0.04) than amateurs (63%). Data demonstrates that in female athletes and exercisers, competitive level appears to affect simple markers of training and nutrition practice, yet contraceptive status does not

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

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 0.05). 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). 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

Plant Protein Blend Ingestion Stimulates Post-Exercise Myofibrillar Protein Synthesis Rates Equivalently to Whey in Resistance-Trained Adults

Purpose Whey protein ingestion is typically considered an optimal dietary strategy to maximize myofibrillar protein synthesis (MyoPS) following resistance exercise. While single source plant protein ingestion is typically less effective, at least partly, due to less favorable amino acid profiles, this could theoretically be overcome by blending plant-based proteins with complementary amino acid profiles. We compared the post-exercise MyoPS response following the ingestion of a novel plant-derived protein blend with an isonitrogenous bolus of whey protein. Methods Ten healthy, resistance trained, young adults (male/female: 8/2; age: 26 ± 6 y; BMI: 24 ± 3 kg·m-2) received a primed continuous infusion of L-[ring-2H5]-phenylalanine and completed a bout of bilateral leg resistance exercise before ingesting 32 g protein from whey (WHEY) or a plant protein blend (BLEND; 39.5% pea, 39.5% brown rice, 21.0% canola) in a randomized, double-blind crossover fashion. Blood and muscle samples were collected at rest, and 2 and 4 h after exercise and protein ingestion, to assess plasma amino acid concentrations, and postabsorptive and post-exercise MyoPS rates. Results Plasma essential amino acid availability over the 4 h postprandial post-exercise period was ~44% higher in WHEY compared with BLEND (P = 0.04). From equivalent postabsorptive values (WHEY, 0.042 ± 0.020%·h−1; BLEND, 0.043 ± 0.015%·h−1) MyoPS rates increased following exercise and protein ingestion (time effect; P 0.05). Conclusions Ingestion of a novel plant-based protein blend stimulates post-exercise MyoPS to an equivalent extent as a whey protein, demonstrating the utility of plant protein blends to optimize post-exercise skeletal muscle reconditioning

Nutrition Strategies for Triathlon

Contemporary sports nutrition guidelines recommend that each athlete develop a personalised, periodised and practical approach to eating that allows him or her to train hard, recover and adapt optimally, stay free of illness and injury and compete at their best at peak races. Competitive triathletes undertake a heavy training programme to prepare for three different sports while undertaking races varying in duration from 20 min to 10 h. The everyday diet should be adequate in energy availability, provide CHO in varying amounts and timing around workouts according to the benefits of training with low or high CHO availability and spread high-quality protein over the day to maximise the adaptive response to each session. Race nutrition requires a targeted and well-practised plan that maintains fuel and hydration goals over the duration of the specific event, according to the opportunities provided by the race and other challenges, such as a hot environment. Supplements and sports foods can make a small contribution to a sports nutrition plan, when medical supplements are used under supervision to prevent/treat nutrient deficiencies (e.g. iron or vitamin D) or when sports foods provide a convenient source of nutrients when it is impractical to eat whole foods. Finally, a few evidence-based performance supplements may contribute to optimal race performance when used according to best practice protocols to suit the triathlete’s goals and individual responsiveness