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The human body requires energy to function. Adenosine triphosphate (ATP) is the cellular currency for energy-requiring processes including mechanical work (i.e., exercise). ATP used by the cells is ultimately derived from the catabolism of energy substrate molecules-carbohydrates, fat, and protein. In prolonged moderate to high-intensity exercise, there is a delicate interplay between carbohydrate and fat metabolism, and this bioenergetic process is tightly regulated by numerous physiological, nutritional, and environmental factors such as exercise intensity and duration, body mass and feeding state. Carbohydrate metabolism is of critical importance during prolonged endurance-type exercise, reflecting the physiological need to regulate glucose homeostasis, assuring optimal glycogen storage, proper muscle fuelling, and delaying the onset of fatigue. Fat metabolism represents a sustainable source of energy to meet energy demands and preserve the 'limited' carbohydrate stores. Coordinated neural, hormonal and circulatory events occur during prolonged endurance-type exercise, facilitating the delivery of fatty acids from adipose tissue to the working muscle for oxidation. However, with increasing exercise intensity, fat oxidation declines and is unable to supply ATP at the rate of the exercise demand. Protein is considered a subsidiary source of energy supporting carbohydrates and fat metabolism, contributing to approximately 10% of total ATP turnover during prolonged endurance-type exercise. In this review we present an overview of substrate metabolism during prolonged endurance-type exercise and the regulatory mechanisms involved in ATP turnover to meet the energetic demands of exercise.…

PMID: 34066984

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.…

PMID: 29498691

Carbohydrates are essential for sustaining performance in most competitive exercise, fueling both anaerobic glycolysis during high-intensity efforts and aerobic metabolism during prolonged activity. Numerous factors contribute to muscle fatigue and exercise performance; still, carbohydrate and muscle glycogen contents are agreed to have an essential role in sustaining prolonged exercise at moderate-to-high intensities. To maintain consistent training and competition performance, elite athletes under certain conditions consume adequate carbohydrates between sessions to restore liver and muscle glycogen and possibly supplement during prolonged workouts to delay depletion. Effective glycogen restoration requires both sufficient carbohydrate intake and adequate recovery time. Understanding how glycogen levels fluctuate during intense or prolonged exercise, the rate at which stores are utilized, and the optimal amount and timing of carbohydrate intake for replenishment is essential. Here we examine the role of carbohydrate availability and utilization in competitive cross-country skiing, which is characterized by exceptionally high whole-body energy turnover with varying loads on the upper- and lower-body muscles as well as fluctuating physiological demands determined by course profile, snow conditions, sub-technique, and race format. This narrative review synthesizes existing evidence on the role of muscle glycogen contents and carbohydrate intake in muscle function and fatigue mechanisms, with a particular focus on cross-country skiing and herein biathlon and Nordic combined. Additionally, we explore how exercise influences glycogen metabolism, the factors regulating glycogen utilization, and training adaptation in order to clarify physiological underpinnings and practical implications for endurance athletes.…

PMID: 41755452

In exercise science, the crossover effect denotes that fat oxidation is the primary fuel at rest and during low-intensity exercise with a shift towards an increased reliance on carbohydrate oxidation at moderate to high exercise intensities. This model makes four predictions: First, >50% of energy comes from carbohydrate oxidation at ≥60% of maximum oxygen consumption (VO…

PMID: 37057184

Fatty acids are the most abundant source of endogenous energy substrate. They can be mobilized from peripheral adipose tissue and transported via the blood to active muscle. During higher intensity exercise, triglyceride within the muscle can also be hydrolyzed to release fatty acids for subsequent direct oxidation. Control of fatty acid oxidation in exercise can potentially occur via changes in availability, or via changes in the ability of the muscle to oxidize fatty acids. We have performed a series of experiments to distinguish the relative importance of these potential sites of control. The process of lipolysis normally provides free fatty acids (FFA) at a rate in excess of that required to supply resting energy requirements. At the start of low intensity exercise, lipolysis increases further, thereby providing sufficient FFA to provide energy substrates in excess of requirements. However, lipolysis does not increase further as exercise intensity increases, and fatty acid oxidation becomes approximately equal to the total amount of fatty acids available at 65% of VO2 max. When plasma FFA concentration is increased by lipid infusion during exercise at 85% VO2 max, fat oxidation is significantly increased. Taken together, these observations indicate that fatty acid availability can be a determinant of the rate of their oxidation during exercise. However, even when lipid is infused well in excess of requirements during high-intensity exercise, less than half the energy is derived from fat. This is because the muscle itself is a major site of control of the rate of fat oxidation during exercise. We have demonstrated that the mechanism of control of fatty acid oxidation in the muscle is the rate of entry into the mitochondria. We hypothesize that the rate of glycolysis is the predominant regulator of the rate of carbohydrate metabolism in muscle, and that a rapid rate of carbohydrate oxidation caused by mobilization of muscle glycogen during high intensity exercise inhibits fatty acid oxidation by limiting transport into the mitochondria. During low intensity exercise, glycogen breakdown and thus glycolysis is not markedly stimulated, so the increased availability of fatty acids allows their oxidation to serve as the predominant energy source. At higher intensity exercise, stimulation of glycogen breakdown and glycolysis cause increased pyruvate entry into the TCA cycle for oxidation, and as a consequence the inhibition of fatty acid oxidation by limiting their transport into the mitochondria.…

PMID: 9781322

Increased muscle mitochondrial mass is characteristic of elite professional endurance athletes (PAs), whereas increased blood lactate levels (lactatemia) at the same absolute submaximal exercise intensities and decreased mitochondrial oxidative capacity are characteristics of individuals with low aerobic power. In contrast to PAs, patients with metabolic syndrome (MtS) are characterized by a decreased capacity to oxidize lipids and by early transition from fat to carbohydrate oxidation (FATox/CHOox), as well as elevated blood lactate concentration [La The aim of this study was to assess metabolic flexibility across populations with different metabolic characteristics. We used indirect calorimetry and [La FATox was significantly higher in PAs than MAs and patients with MtS (p < 0.01), while [La Blood lactate accumulation is negatively correlated with FATox and positively correlated with CHOox during exercise across populations with widely ranging metabolic capabilities. Because both lactate and fatty acids are mitochondrial substrates, we believe that measurements of [La…

PMID: 28623613

Position Statement: The International Society of Sports Nutrition (ISSN) bases the following position stand on a critical analysis of the literature regarding the effects of diet types (macronutrient composition; eating styles) and their influence on body composition. The ISSN has concluded the following. 1) There is a multitude of diet types and eating styles, whereby numerous subtypes fall under each major dietary archetype. 2) All body composition assessment methods have strengths and limitations. 3) Diets primarily focused on fat loss are driven by a sustained caloric deficit. The higher the baseline body fat level, the more aggressively the caloric deficit may be imposed. Slower rates of weight loss can better preserve lean mass (LM) in leaner subjects. 4) Diets focused primarily on accruing LM are driven by a sustained caloric surplus to facilitate anabolic processes and support increasing resistance-training demands. The composition and magnitude of the surplus, as well as training status of the subjects can influence the nature of the gains. 5) A wide range of dietary approaches (low-fat to low-carbohydrate/ketogenic, and all points between) can be similarly effective for improving body composition. 6) Increasing dietary protein to levels significantly beyond current recommendations for athletic populations may result in improved body composition. Higher protein intakes (2.3-3.1 g/kg FFM) may be required to maximize muscle retention in lean, resistance-trained subjects under hypocaloric conditions. Emerging research on very high protein intakes (>3 g/kg) has demonstrated that the known thermic, satiating, and LM-preserving effects of dietary protein might be amplified in resistance-training subjects. 7) The collective body of intermittent caloric restriction research demonstrates no significant advantage over daily caloric restriction for improving body composition. 8) The long-term success of a diet depends upon compliance and suppression or circumvention of mitigating factors such as adaptive thermogenesis. 9) There is a paucity of research on women and older populations, as well as a wide range of untapped permutations of feeding frequency and macronutrient distribution at various energetic balances combined with training. Behavioral and lifestyle modification strategies are still poorly researched areas of weight management.…

PMID: 28630601

It has been hypothesized that performing aerobic exercise after an overnight fast accelerates the loss of body fat. The purpose of this study was to investigate changes in fat mass and fat-free mass following four weeks of volume-equated fasted versus fed aerobic exercise in young women adhering to a hypocaloric diet. Twenty healthy young female volunteers were randomly assigned to 1 of 2 experimental groups: a fasted training (FASTED) group that performed exercise after an overnight fast (n = 10) or a post-prandial training (FED) group that consumed a meal prior to exercise (n = 10). Training consisted of 1 hour of steady-state aerobic exercise performed 3 days per week. Subjects were provided with customized dietary plans designed to induce a caloric deficit. Nutritional counseling was provided throughout the study period to help ensure dietary adherence and self-reported food intake was monitored on a regular basis. A meal replacement shake was provided either immediately prior to exercise for the FED group or immediately following exercise for the FASTED group, with this nutritional provision carried out under the supervision of a research assistant. Both groups showed a significant loss of weight (P = 0.0005) and fat mass (P = 0.02) from baseline, but no significant between-group differences were noted in any outcome measure. These findings indicate that body composition changes associated with aerobic exercise in conjunction with a hypocaloric diet are similar regardless whether or not an individual is fasted prior to training.…

PMID: 25429252

<h4>Background</h4>Preserving fat-free mass (FFM) during weight loss is critical for preventing sarcopenia and maintaining metabolic health. This study examined the effects of resistance training (RT), aerobic exercise (AR), and no exercise (NO) on body composition during a calorie-restricted diet.<h4>Methods</h4>This retrospective cohort study included 304 adults (183 men, 121 women; aged 20-74 years; BMI: 18.5-45 kg/m²) who followed a hypocaloric diet and self-selected RT, AR, or NO. The diet was designed to provide an individualized energy deficit of approximately 500 kcal/day, calculated relative to each participant's measured resting metabolic rate (RMR) and total estimated daily energy expenditure. Body composition was assessed using dual-energy X-ray absorptiometry (DXA), and abdominal circumference (ABC) was measured as a marker of central obesity.<h4>Results</h4>Mean follow-up was 5.1 months ± 0.42 months. In men, total weight loss was similar across groups (NO: - 8.5 kg ± 3.2 kg; AR: - 9.0 kg ± 4.2 kg; RT: - 7.7 kg ± 4.2 kg). However, RT produced the greatest reduction in fat mass (RT: - 8.9 kg ± 4.1 kg; AR: - 7.8 kg ± 3.2 kg; NO: - 5.8 kg ± 2.5 kg) and was the only modality associated with an increase in fat-free mass (RT: + 0.8 kg ± 5.0 kg vs. AR: - 1.1 kg ± 2.0 kg and NO: - 2.8 kg ± 1.4 kg). ABC declined in all groups, with larger reductions in RT (- 9.0 cm ± 3.7 cm) and AR (- 8.0 cm ± 3.2 cm) compared with NO (- 6.1 cm ± 2.4 cm). Among women, weight loss was also comparable between groups (NO: - 7.13 kg ± 3.27 kg; AR: - 6.43 kg ± 3.53 kg; RT: - 5.42 kg ± 3.76 kg). RT produced the greatest fat-mass reduction (RT: - 6.36 kg ± 3.82 kg; NO: - 5.47 kg ± 2.64 kg; AR: - 4.10 kg ± 3.17 kg) and was the only modality that increased fat-free mass (RT: + 0.90 kg ± 1.24 kg). Both NO (- 2.94 kg ± 1.40 kg) and AR (- 0.37 kg ± 1.45 kg) experienced FFM loss. The fat mass (FM)-to-weight loss ratio was lowest in the NO group (0.7 ± 0.2), higher in AR (0.86 ± 0.2), and highest in RT (1.1 ± 0.7; <i>p</i> = 0.0002 vs. NO, <i>p</i> = 0.0051 vs. AR). ABC reduction correlated strongly with FM loss (<i>r</i> = 0.84; <i>p</i> = 0.0001), highlighting its utility as a marker of high-quality weight loss.<h4>Conclusion</h4>RT enhances weight-loss quality by maximizing FM reduction while preserving or increasing FFM. Incorporating RT into weight-loss programs may improve long-term weight maintenance and mitigate FFM loss.…

PMID: 41625248