Nutrition Publication

NNIW76 - Limits of Human Endurance

Editor(s): L. van Loon, R. Meeusen. Sports Nutrition Series 76

Nutrition is one of the key factors that modulates exercise performance. In this book, a group of expert scientists discuss the ergogenic properties of various nutritional interventions and present research to show that dietary strategies can be applied to extend the limits of human endurance, lower the risk of illness or injury, and speed recovery rates. More specifically, they discuss recent finding on topics such as caffeine and its effect on the brain, carnitine and fat oxidation, ergogenic properties of beta-alanine, dietary protein and muscle reconditioning, nutrition and immune status, and the importance of proper hydration. 


Related Articles

Caffeine, Exercise and the Brain

Author(s): R. Meeusen, B. Roelands, L. Spriet

Caffeine can improve exercise performance when it is ingested at moderate doses (3–6 mg/ kg body mass). Caffeine also has an effect on the central nervous system (CNS), and it is now recognized that most of the performance-enhancing effect of caffeine is accomplished through the antagonism of the adenosine receptors, influencing the dopaminergic and other neurotransmitter systems. Adenosine and dopamine interact in the brain, and this might be one mechanism to explain how the important components of motivation (i.e. vigor, persistence and work output) and higher-order brain processes are involved in motor control. Caffeine maintains a higher dopamine concentration especially in those brain areas linked with ‘attention’. Through this neurochemical interaction, caffeine improves sustained attention, vigilance, and reduces symptoms of fatigue. Other aspects that are localized in the CNS are a reduction in skeletal muscle pain and force sensation, leading to a reduction in perception of effort during exercise and therefore influencing the motivational factors to sustain effort during exercise. Because not all CNS aspects have been examined in detail, one should consider that a placebo effect may also be present. Overall, it appears that the performance-enhancing effects of caffeine reside in the brain, although more research is necessary to reveal the exact mechanisms through which the CNS effect is established.

Carnitine and Fat Oxidation

Author(s): F. Stephens, S. Galloway

Fat and carbohydrate are the primary fuel sources for mitochondrial ATP production in human skeletal muscle during endurance exercise. However, fat exhibits a relatively low maximal rate of oxidation in vivo, which begins to decline at around 65% of maximal oxygen consumption (VO 2 max) when muscle glycogen becomes the major fuel. It is thought that if the rate of fat oxidation during endurance exercise could be augmented, then muscle glycogen depletion could be delayed and endurance improved. The purpose of the present review is to outline the role of carnitine in skeletal muscle fat oxidation and how this is influenced by the role of carnitine in muscle carbohydrate oxidation. Specifically, it will propose a novel hypothesis outlining how muscle free carnitine availability is limiting to the rate of fat oxidation. The review will also highlight recent research demonstrating that increasing the muscle carnitine pool in humans can have a significant impact upon both fat and carbohydrate metabolism during endurance exercise which is dependent upon the intensity of exercise performed.

Hydration during Intense Exercise Training

Author(s): R. Maughan, N. Meyer

Hydration status has profound effects on both physical and mental performance, and sports performance is thus critically affected. Both overhydration and underhydration – if sufficiently severe – will impair performance and pose a risk to health. Athletes may begin exercise in a hypohydrated state as a result of incomplete recovery from water loss induced in order to achieve a specific body mass target or due to incomplete recovery from a previous competition or training session. Dehydration will also develop in endurance exercise where fluid intake does not match water loss. The focus has generally been on training rather than on competition, but sweat loss and fluid replacement in training may have important implications. Hypohydration may impair training quality and may also increase stress levels. It is unclear whether this will have negative effects (reduced training quality, impaired immunity) or whether it will promote a greater adaptive response. Hypohydration and the consequent hyperthermia, however, can enhance the effectiveness of a heat acclimation program, resulting in improved endurance performance in warm and temperate environments. Drinking in training may be important in enhancing tolerance of the gut when athletes plan to drink in competition. The distribution of water between body water compartments may also be important in the initiation and promotion of cellular adaptations to the training stimulus.

Intense Exercise Training and Immune Function

Author(s): M. Gleeson, C. Williams

Regular moderate exercise reduces the risk of infection compared with a sedentary lifestyle, but very prolonged bouts of exercise and periods of intensified training are associated with increased infection risk. In athletes, a common observation is that symptoms of respiratory infection cluster around competitions, and even minor illnesses such as colds can impair exercise performance. There are several behavioral, nutritional and training strategies that can be adopted to limit exercise-induced immunodepression and minimize the risk of infection. Athletes and support staff can avoid transmitting infections by avoiding close contact with those showing symptoms of infection, by practicing good hand, oral and food hygiene and by avoiding sharing drinks bottles and cutlery. Medical staff should consider appropriate immunization for their athletes particularly when travelling to international competitions. The impact of intensive training stress on immune function can be minimized by getting adequate sleep, minimizing psychological stress, avoiding periods of dietary energy restriction, consuming a well-balanced diet that meets energy and protein needs, avoiding deficiencies of micronutrients (particularly iron, zinc, and vitamins A, D, E, B 6 and B 12 ), ingesting carbohydrate during prolonged training sessions, and consuming – on a daily basis – plant polyphenol containing supplements or foodstuffs and Lactobacillus probiotics.

Physiological and Performance Adaptations to High-Intensity Interval Training

Author(s): M.Gibala, A. Jones

High-intensity interval training (HIIT) refers to exercise that is characterized by relatively short bursts of vigorous activity, interspersed by periods of rest or low-intensity exercise for recovery. In untrained and recreationally active individuals, short-term HIIT is a potent stimulus to induce physiological remodeling similar to traditional endurance training despite a markedly lower total exercise volume and training time commitment. As little as six sessions of ‘all-out’ HIIT over 14 days, totaling ∼ 15 min of intense cycle exercise within total training time commitment of ∼ 2.5 h, is sufficient to enhance exercise capacity and improve skeletal muscle oxidative capacity. From an athletic standpoint, HIIT is also an effective strategy to improve performance when supplemented into the already high training volumes of well-trained endurance athletes, although the underlying mechanisms are likely different compared to less trained subjects. Most studies in this regard have examined the effect of replacing a portion (typically ∼ 15–25%) of base/normal training with HIIT (usually 2–3 sessions per week for 4–8 weeks). It has been proposed that a polarized approach to training, in which ∼ 75% of total training volume be performed at low intensities, with 10–15% performed at very high intensities may be the optimal training intensity distribution for elite athletes who compete in intense endurance events.

Effect of β-Alanine Supplementation on High-Intensity Exercise Performance

Author(s): R. Harris, T. Stellingwerff

Carnosine is a dipeptide of β-alanine and L -histidine found in high concentrations in skeletal muscle. Combined with β-alanine, the pKa of the histidine imidazole ring is raised to ∼ 6.8, placing it within the muscle intracellular pH high-intensity exercise transit range. Combination with β-alanine renders the dipeptide inert to intracellular enzymic hydrolysis and blocks the histidinyl residue from participation in proteogenesis, thus making it an ideal, stable intracellular buffer. For vegetarians, synthesis is limited by β-alanine availability; for meat-eaters, hepatic synthesis is supplemented with β-alanine from the hydrolysis of dietary carnosine. Direct oral β-alanine supplementation will compensate for low meat and fish intake, significantly raising the muscle carnosine concentration. This is best achieved with a sustained-release formulation of β-alanine to avoid paresthesia symptoms and decreasing urinary spillover. In humans, increased levels of carnosine through β-alanine supplementation have been shown to increase exercise capacity and performance of several types, particularly where the high-intensity exercise range is 1–4 min. β-Alanine supplementation is used by athletes competing in high-intensity track and field cycling, rowing, swimming events and other competitions.

Dietary Protein for Muscle Hypertrophy

Author(s): K. Tipton, S. Phillips

Skeletal muscle hypertrophy is a beneficial adaptation for many individuals. The metabolic basis for muscle hypertrophy is the balance between the rates of muscle protein synthesis (MPS) and muscle protein breakdown (MPB), i.e. net muscle protein balance (NMPB = MPS – MPB). Resistance exercise potentiates the response of muscle to protein ingestion for up to 24 h following the exercise bout. Ingestion of many protein sources in temporal proximity (immediately before and at least within 24 h after) to resistance exercise increases MPS resulting in positive NMPB. Moreover, it seems that not all protein sources are equal in their capacity to stimulate MPS. Studies suggest that ∼ 20–25 g of a high-quality protein maximizes the response of MPS following resistance exercise, at least in young, resistance-trained males. However, more protein may be required to maximize the response of MPS with less than optimal protein sources and/or with older individuals. Ingestion of carbohydrate with protein does not seem to increase the response of MPS following exercise. The response of inactive muscle to protein ingestion is impaired. Ingestion of a high-quality protein within close temporal proximity of exercise is recommended to maximize the potential for muscle growth.

The Role of Amino Acids in Skeletal Muscle Adaptation to Exercise

Author(s): N. Aguirre, L. van Loon, K. Baar

The synthesis of new protein is necessary for both strength and endurance adaptations. While the proteins that are made might differ, myofibrillar proteins following resistance exercise and mitochondrial proteins and metabolic enzymes following endurance exercise, the basic premise of shifting to a positive protein balance after training is thought to be the same. What is less clear is the contribution of nutrition to the adaptive process. Following resistance exercise, proteins rich in the amino acid leucine increase the activation of mTOR, the rate of muscle protein synthesis (MPS), and the rate of muscle mass and strength gains. However, an effect of protein consumption during acute post-exercise recovery on mitochondrial protein synthesis has yet to be demonstrated. Protein ingestion following endurance exercise does facilitate an increase in skeletal MPS, supporting muscle repair, growth and remodeling. However, whether this results in improved performance has yet to be demonstrated. The current literature suggests that a strength athlete will experience an increased sensitivity to protein feeding for at least 24 h after exercise, but immediate consumption of 0.25 g/kg bodyweight of rapidly absorbed protein will enhance MPS rates and drive the skeletal muscle hypertrophic response. At rest, ∼ 0.25 g/kg bodyweight of dietary protein should be consumed every 4–5 h and another 0.25–0.5 g/kg bodyweight prior to sleep to facilitate the postprandial muscle protein synthetic response. In this way, consuming dietary protein can complement intense exercise training and facilitate the skeletal muscle adaptive response.

National Nutritional Programs for the 2012 London Olympic Games: A Systematic Approach by Three Different Countries

Author(s): L. Burke, N. Meyer, J. Pearce

Preparing a national team for success at major sporting competitions such as the Olympic Games has become a systematic and multi-faceted activity. Sports nutrition contributes to this success via strategic nutritional interventions that optimize the outcomes from both the training process and the competitive event. This review summarizes the National Nutrition Programs involved with the 2012 London Olympic Games preparation of the Australian, British and American sports systems from the viewpoints of three key agencies: the Australian Institute of Sport, the English Institute of Sport and the United States Olympic Committee. Aspects include development of a nutrition network involving appropriately qualified sports dietitians/nutritionists within a multi-disciplinary team, recognition of continual updates in sports nutrition knowledge, and a systematic approach to service delivery, education and research within the athlete’s daily training environment. Issues of clinical nutrition support must often be integrated into the performance nutrition matrix. Food service plays an important role in the achievement of nutrition goals during the Olympic Games, both through the efforts of the Athlete Dining Hall and catering activities of the host Olympic Games Organizing Committees as well as adjunct facilities often provided by National Olympic Committees for their own athletes.