Sports Nutrition Overview
Reilly T., W. Waterhouse, and L. M. Burke. Nutrition for travel. Journal of Sports Sciences 25 Suppl 1: 125-134, 2007.
Burke L. M., G. Millet, and M. A. Tarnopolsky. Nutrition for distance events. Journal of Sports Sciences 25 Suppl 1: 29-38, 2007.
Burke L. M., R. Maughan, and S. Shirreffs. The 2007 IAAF Consensus conference on Nutrition for Athletics. Journal of Sports Sciences 25 Suppl 1: S1, 2007.
Sawka, M. N., L. M. Burke, E. R. Eichner, R. J. Maughan, S. J. Montain, and N. S. Stachenfield. American College of Sports Medicine Position Stand: Exercise and Fluid Replacement. Medicine & Science in Sports & Exercise 39: 377-390, 2007.
Burke L. M. Energy needs of athletes. Canadian Journal of Applied Physiology 26 Suppl: S202-19, 2001.
Burke L. M. Nutritional practices of male and female endurance cyclists. Sports Medicine 31: 521-32, 2001.
Burke L. M. Nutritional needs for exercise in the heat. Comparative Biochemistry and Physiology Part A Molecular Integrated Physiology 128: 735-48, 2001.
Maughan, R., and L. M. Burke. Sports nutrition: an historical perspective. In: Clinical Sports Nutrition (2nd ed.), edited by L. Burke and V. Deakin. Sydney: McGraw-Hill, 2000, p. 1-13.
Hopkins, W. G., J. A. Hawley, and L. M. Burke. Design and analysis of research on sport performance enhancement. Medicine & Science in Sports & Exercise 31: 472-485, 1999.
Hopkins, W. G., J. A. Hawley, and L. M. Burke. Researching worthwhile performance enhancements. Sportscience 3, 1999.
Burke, L. Practical issues in nutrition for athletes. Journal of Sports Sciences 13: S83-S90, 1995.
The training and competitive programmes of elite athletes incorporate travel schedules, often long journeys, across multiple time zones. In such cases, travel causes both transient fatigue and a malaise known as “jet-lag” that persists for some days. Jet-lag is due to the disturbance of the body's circadian rhythms: diurnal and performance rhythms are displaced, depending on the direction of travel and the number of time zones crossed in flight. Attention to diet and hydration is relevant during the flight and following disembarkation until adjustment to the new meridian is complete. The consequences of jet-lag on rhythms in digestion may be compounded if food preparation and hygiene are inadequate in training camps or competitive venues overseas. The irony of travel is that it often places athletes at a greater risk of failing to meet their specific nutrition goals or succumbing to illness, at a time when the demands or outcomes of performance are of greatest importance. In addition, gastrointestinal infections related to travelling are frequent among athletes. Fastidious planning and organization among the support staff is recommended before the journey to prevent any such problems arising. Equally, athletes often need special education initiatives to assist them to cope with the challenges of a new and unusual food supply, or altered access to food.
The goal of training is to prepare the distance athlete to perform at his or her best during major competitions. Whatever the event, nutrition plays a major role in the achievement of various factors that will see a runner or walker take the starting line in the best possible form. Everyday eating patterns must supply fuel and nutrients needed to optimize their performance during training sessions and to recover quickly afterwards. Carbohydrate and fluid intake before, during, and after a workout may help to reduce fatigue and enhance performance. Recovery eating should also consider issues for adaptation and the immune system that may involve intakes of protein and some micronutrients. Race preparation strategies should include preparation of adequate fuel stores, including carbohydrate loading for prolonged events such as the marathon or 50-km walk. Fluid and carbohydrate intake during races lasting an hour or more should also be considered. Sports foods and supplements of value to distance athletes include sports drinks and liquid meal supplements to allow nutrition goals to be achieved when normal foods are not practical. While caffeine is an ergogenic aid of possible value to distance athletes, most other supplements are of minimal benefit.
Sawka, M. N., L. M. Burke, E. R. Eichner, R. J. Maughan, S. J. Montain, and N. S. Stachenfield. American College of Sports Medicine Position stand: Exercise and Fluid Replacement. Medicine & Science in Sports & Exercise 39: 377-390, 2007.
This Position Stand provides guidance on fluid replacement to sustain appropriate hydration of individuals performing physical activity. The goal of prehydrating is to start the activity euhydrated and with normal plasma electrolyte levels. Prehydrating with beverages, in addition to normal meals and fluid intake, should be initiated when needed at least several hours before the activity to enable fluid absorption and allow urine output to return to normal levels. The goal of drinking during exercise is to prevent excessive (>2% body weight loss from water deficit) dehydration and excessive changes in electrolyte balance to avert compromised performance. Because there is considerable variability in sweating rates and sweat electrolyte content between individuals, customized fluid replacement programs are recommended. Individual sweat rates can be estimated by measuring body weight before and after exercise. During exercise, consuming beverages containing electrolytes and carbohydrates can provide benefits over water alone under certain circumstances. After exercise, the goal is to replace any fluid electrolyte deficit. The speed with which rehydration is needed and the magnitude of fluid electrolyte deficits will determine if an aggressive replacement program is merited.
Each athlete has unique energy requirements, which underpin their ability to meet total nutritional goals. For everyday dietary planning and evaluation, energy requirements can be predicted via estimations of RMR and activity levels. Research methods such as indirect calorimetry and DLW allow energy requirements to be measured, and may be useful to confirm situations in which an athlete has a true energy balance anomaly. There is some evidence that individual athletes may have reduced energy requirements, although this occurs less frequently than is reported. Most self-reports of food intake substantially under-estimate energy intake, due to under-reporting or under-eating during the period of record keeping. Many athletes are over-focused on reducing body mass and body fat below levels that are consistent with long-term health and performance. Restrained eating can cause significant detrimental outcomes to body function. Leptin may be involved in modulating or mediating some of these changes. Athletes should use their energy budget to choose foods that provide macronutrient and micronutrient needs for optimal health and performance. Practical advice may help athletes to achieve energy intake challenges.
The nutritional requirements of the training and competition programmes of elite endurance cyclists are challenging. Notwithstanding the limitations of dietary survey techniques, studies of high-level male road cyclists provide important information about nutrient intake and food practices during training and major stage races. Typically, male cyclists undertaking intensive training programmes report a high energy intake (> or = 250 kJ/kg/day) and carbohydrate (CHO) intakes of 8 to 11 g/kg/day. Intakes of protein and micronutrients are likely to meet Recommended Dietary Intake levels, because of high energy intakes. Data on female cyclists are scarce. Stage racing poses an increased requirement for energy and CHO, with daily energy expenditure often exceeding 25 MJ. This must be achieved in the face of practical constraints on the time available for eating, and the suppression of appetite after exhausting exercise. However, studies show that male cyclists riding for professional teams appear to meet these challenges, with the assistance of their medical/scientific support crews. Current dietary practices during cycle tours appear to favour greater reliance on pre-stage intake and post-stage recovery meals to achieve nutritional goals. Recent reports suggest that current riding tactics interfere with previous practices of consuming substantial amounts of fluid and CHO while cycling. Further study is needed to confirm these practices, and to investigate whether these or other dietary strategies produce optimal cycling performance. Other issues that should receive attention include dietary practices of female cyclists, beliefs and practices regarding bodyweight control among cyclists, and the use of supplements and sports foods.
Although hot conditions are not typically conducive to optimal sports performance, nutritional strategies play an important role in assisting an athlete to perform as well as possible in a hot environment. A key issue is the prevention of hypohydration during an exercise session. Fluid intake strategies should be undertaken in a cyclical sequence: hydrate well prior to the workout, drink as much as is comfortable and practical during the session, and rehydrate aggressively afterwards in preparation for future exercise bouts. There is some interest in hyperhydration strategies, such as hyperhydration with glycerol, to prepare the athlete for a situation where there is little opportunity for fluid intake to match large sweat losses. Recovery of significant fluid losses after exercise is assisted by the simultaneous replacement of electrolyte losses. Carbohydrate (CHO) requirements for exercise are increased in the heat, due to a shift in substrate utilization towards CHO oxidation. Daily food patterns should focus on replacing glycogen stores after exercise, and competition strategies should include activities to enhance CHO availability, such as CHO loading for endurance events, pre-event CHO intake, and intake of sports drinks in events lasting longer than 60 min. Although CHO ingestion may not enhance the performance of all events undertaken in hot weather, there are no disadvantages to the consumption of beverages containing 4-8% CHO and electrolytes. In fact, the palatability of these drinks may enhance the voluntary intake of fluid. Although there is some evidence of increased protein catabolism and cellular damage due to production of oxygen radicals during exercise in the heat, there is insufficient evidence to make specific dietary recommendations to account for these issues.
The performances of today's athletes are far superior to those achieved in earlier times, and as with improvements in the health of the general population, nutritional advances have played a role. The increased percentage of the total gene pool which now participates in sport is a major factor in the continuing improvement of world records. For the individual, however, training, diet and the avoidance of illness and injury will remain the most important elements. This book provides a summary of the considerable knowledge of sports nutrition that is available to athletes in the new millennium. It describes the rigorous science behind the principles of sports nutrition and offers guidelines for best practice in the sporting arena. Before starting our contemporary understanding of sports nutrition, it is interesting to consider how its science and practice have evolved over the past decades and centuries. An awareness of the history of sport, and of the evolution of training and nutritional practices, is not only of interest in itself, but can help the understanding of current practices. This chapter highlights some key steps that have led us to where we are today.
Further details on Clinical Sports Nutrition can be found in the Publications section of our site.
PURPOSE: The purpose of this study was to assess research aimed at measuring performance enhancements that affect success of individual elite athletes in competitive events.
ANALYSIS: Simulations show that the smallest worthwhile enhancement of performance for an athlete in an international event is 0.7-0.4 of the typical within-athlete random variation in performance between events. Using change in performance in events as the outcome measure in a crossover study, researchers could delimit such enhancements with a sample of 16-65 athletes, or with 65-260 in a fully controlled study. Sample size for a study using a valid laboratory or field test is proportional to the square of the within-athlete variation in performance in the test relative to the event; estimates of these variations are therefore crucial and should be determined by repeated-measures analysis of data from reliability studies for the test and event. Enhancements in test and event may differ when factors that affect performance differ between test and event; overall effects of these factors can be determined with a validity study that combines reliability data for test and event. A test should be used only if it is valid, more reliable than the event, allows estimation of performance enhancement in the event, and if the subjects replicate their usual training and dietary practices for the study; otherwise the event itself provides the only dependable estimate of performance enhancement. Publication of enhancement as a percent change with confidence limits along with an analysis for individual differences will make the study more applicable to athletes. Outcomes can be generalized only to athletes with abilities and practices represented in the study.
CONCLUSION: estimates of enhancement of performance in laboratory or field tests in most previous studies may not apply to elite athletes in competitive events.
Hopkins, W. G., J. A. Hawley, and L. M. Burke. Researching worthwhile performance enhancements . Sportscience 3, 1999.
For an athlete at the top of the field, a performance enhancement makes a difference to the chance of winning when it is about half the athlete's typical between-event variation in performance. Measuring enhancements of this magnitude with adequate precision requires much bigger sample sizes than researchers normally use. To avoid confusion over interpretation of their findings, researchers should therefore publish and explain the precision of their estimates of performance enhancement.
Many athletes do not achieve sound nutritional practices to optimise their sports performance. Factors include poor nutrition knowledge, dietary extremism, poor practical skills in choosing or preparing meals, and reduced access to food due to a busy lifestyle and frequent travel. Education in nutrition for the athlete needs to be practical, so as to address eating strategies and key food and fluid choices that will help to achieve the goals of sound nutrition. Strategies that can achieve a number of nutritional goals simultaneously are most useful, since athletes often find it difficult to integrate separate issues. Athletes with extreme nutrient requirements, or with nutritional problems, should seek individual assessment and counselling from a sports nutrition expert.