Eat yourself fitter

Apart from appropriate training, a high carbohydrate diet and an adequate fluid intake are essential for successful sport and exercise. Strategic supplementation of some nutrients may also be of benefit. Clyde Williams discusses

Athletes of the new millennium have access to a far greater range of foods than those who competed in the early Olympic Games and yet they share many of the same misunderstandings about the influence of food on performance. One of the shared myths is that there are "ergogenic supplements" that, when found, will allow athletes to train hard, recover quickly, and compete more successfully than their rivals. The reality is that if they exist, then they are probably pharmacological rather than nutritional supplements. Furthermore, in searching for such "quick fix foods" the contributions of commonly available foods to health and exercise performance are easily overlooked. The first and foremost nutritional need of athletes is for a well balanced diet, made up of a wide range of foods in sufficient quantity to cover their energy expenditure. Thereafter, they can adopt nutritional strategies to ensure that they can train, compete, and recover more successfully than they would if left to follow their own appetites and perceptions about what and when to eat.

Fig 1 Composition of the daily food intake of male distance runners

Fig 2 Composition of the daily food intake of female distance runners

Nutrition
A well balanced diet is one that derives at least 50% of its energy from carbohydrate containing foods, less than 35% from fats, and 12% to 15% from protein. However, within the population at large only endurance athletes and vegetarians have diets that match these recommendations (figs 1 and 2). One of the myths that many modern athletes share with the Olympians of Antiquity is that increased meat (protein) intake helps develop strength. It is mainly the strength and power athletes who cling to this idea about the link between protein intake and increased strength. The reality is that athletes undertaking heavy daily training need only slightly more protein than that recommended for the general population. The daily protein requirements of most people are covered by an intake of about 1 g/kg body weight, whereas a protein intake of between 1.5 and 1.7 g/kg is enough for athletes undergoing heavy training. These amounts are less than the daily protein intakes of power athletes, which are often as high as 3 g/kg body mass.

Energy balance
The amount of food consumed should be sufficient to cover daily energy expenditure, but not be such that it results in a significant increase in body weight. A stable body weight is one indicator that an athlete is in energy balance--that is, that energy intake is equal to energy expenditure.

However, energy balance should be assessed over several days rather than on a daily basis because of the fluctuations in energy intake and expenditure. When energy intake exceeds energy expenditure as a consequence of eating even a well balanced diet, the additional energy is stored as fat and carbohydrate.

"How much should I eat?" is as common a question among sportspeople as it is with less active people. The answer depends on their basal metabolic rate and their daily levels of physical activity. Basal metabolic rate is the minimum amount of energy required to support life and it accounts for about 60% of our daily energy expenditure. It varies with age and can be estimated from body weight. One way of describing the energy cost of daily physical activity is as multiples of basal metabolic rate.

For example, people who are engaged in sedentary occupations have energy expenditures that are about 1.55 times their basal metabolic rate; whereas, in sports involving prolonged heavy exercise, larger energy expenditures are necessary. At the top end of the range of daily energy expenditures are professional cyclists who achieve values of 3.3 times their basal metabolic rate when racing. Therefore, to maintain energy balance these athletes must eat large quantities of energy dense foods. For example, in the Tour de France, the professional cyclists have daily energy intakes of about 6500 kcal (27.17 MJ), increasing to about 9000 kcal (37.6 MJ) during the mountain section of the race. Their average daily energy intake is about double that of active healthy people (2500 kcal to 3000 kcal). The energy cost of individual activities such as walking or running are often described as "mets" where an average basal metabolic rate is assumed and used as a reference value.

Many athletes want to lose body weight as a way of improving their performance either because a low body weight is an advantage--for example, running--or to complete in a lower weight category such as in combat sports--for example, wrestling. We cannot lose weight unless we are in "negative energy balance"--that is, energy intake must be less than expenditure commonly referred to as "dieting." Negative energy balance can also be achieved when energy expenditure is greater than energy intake. Of course, a combination of both methods of achieving a negative energy balance is the most successful way to long term weight reduction. To lose 1 kg of body mass we need to achieve a negative energy balance equivalent to about 7000 kcal. This could be achieved by reducing daily energy intake by 1000 kcal for a week, however this severe reduction in food intake would require a great deal of discipline and may undermine the ability of an athlete to train successfully. A longer term approach is more likely to achieve success without compromising the capacity of an athlete to continue to complete heavy training. Reducing daily energy intake by 250kcal for a month would bring about the desired body weight change of 1kg. It is sustainable and so an athlete could go onto lose a greater amount should it be required. One note of caution is that when body mass is reduced by 1 kg only about three-quarters of the weight loss is fat the rest is lean tissue.

Popular diets only work when a negative energy balance is sustained for an extended period of time. The Atkins Diet has proved to be both popular and effective because the recommendation to eat an almost carbohydrate-free diet leads to undereating--that is, a negative energy balance. This high protein-fat diet is not recommended for anyone who wants or needs to be physically active because the absence of carbohydrates leads to a condition called ketosis. High fat diets lead to low muscle and liver glycogen stores and so the fuel for moderate to high intensity exercise is absent. Fatigue occurs earlier during this type of exercise on high protein-fat diets, and, furthermore, they do not promote optimum responses to endurance training.

Nutritional strategies

Before exercise
Recognising the central role carbohydrate plays in energy metabolism during exercise, it is not surprising that nutritional strategies have been developed to increase muscle glycogen stores before exercise. The most effective and acceptable method of carbohydrate loading is to decrease training three to four days before competition and increase the consumption of carbohydrate containing foods in each meal. A daily intake of about 9-10 g/kg body weight of carbohydrate is sufficient to increase muscle and liver glycogen stores before competition. Furthermore, a high carbohydrate meal eaten three to four hours before competition also helps to top up glycogen stores. However, the nature of the carbohydrate will influence the amount of fat metabolism before and during exercise. A meal containing high glycaemic index carbohydrates will raise plasma glucose and insulin concentrations and, as a consequence of the elevated insulin concentrations, fatty acid mobilisation from adipose tissue cells is inhibited. The decreased availability of fatty acids will be covered by an increase in carbohydrate metabolism in skeletal muscles with some contribution from intramuscular triglycerides.

The ideal pre-exercise meals would provide enough carbohydrate to top-up fuel stores in the liver and muscle without causing too severe a reduction in the rate of fat oxidation during subsequent exercise. Eating low glycaemic index carbohydrate pre-exercise meal does not depress fatty acid mobilisation, or utilisation to the same extent as high glycaemic index carbohydrate meals. Recent research suggests that eating a low glycaemic index carbohydrate meal three hours before exercise leads to a greater rate of fat oxidation and endurance capacity during prolonged running than eating a high glycaemic index carbohydrate pre-exercise meal.¹ The greater benefit of low glycaemic index carbohydrate pre-exercise meals also extends to recover from exercise. A low glycaemic index carbohydrate diet during 24 hour recovery from prolonged running results in a greater endurance capacity the following day than when the recovery diet consists mainly of high glycaemic index carbohydrates.

During exercise
Dehydration can cause fatigue during prolonged exercise even before the depletion of muscle glycogen stores. Drinking well formulated carbohydrate-electrolyte solutions--that is, several sports drinks--throughout exercise helps prevent severe dehydration and contributes to carbohydrate metabolism in working muscles, delaying the onset of fatigue. Sports drinks that provide carbohydrate at a rate of 30-50 g/hour are effective in increasing endurance performance. A note of caution about overdrinking: athletes should try to relate their drinking to their rate of sweating--that is, when sweating heavily then larger volumes of fluid intake should be ingested than when sweat rates are low. Simply drinking water in excess of sweat rates could dilute the sodium concentration in the blood and lead to hyponatremia and ultimately collapse.

After exercise
Speed of recovery from exercise depends on how quickly muscle glycogen can be replaced and fluid balance restored. Muscle glycogen resynthesis is most rapid immediately after exercise, so drinking a carbohydrate-electrolyte solution (or eating high glycaemic index carbohydrate foods) after exercise will produce the maximum rate of glycogen resynthesis. The optimum amount of carbohydrate is 1 g/kg body weight, which is achieved by drinking about a litre of a sports drink containing 6-7% carbohydrate. However, consuming 50 g of carbohydrate every hour until the next meal appears to be a more practical recommendation. The overall carbohydrate intake during the recovery over 24 hours should be equivalent to about 10 g/kg body weight. Delaying the consumption of carbohydrate for two to three hours after exercise reduces the rate of glycogen resynthesis and delays the return of endurance fitness. When attempting to recover within 24 hours, food intake must be prescribed because a free intake will not provide sufficient carbohydrate to replace muscle glycogen, or restore energy balance.

Nutritional (ergogenic) supplements

Unfortunately, too many sportspeople are vulnerable to the advertising claims that supplements enhance performance. The most popular nutritional supplements are those shown, but there are many others that make claims without any supporting evidence, so these have not been included in this brief overview.

Vitamins and minerals
Vitamin and mineral supplementation is a common practice among many sportspeople, despite there being no good evidence to show that performance is improved after a period of supplementation of a well balanced diet with additional vitamins and minerals. However, the assumption that all athletes consume a diet containing a wide range of foods that provides sufficient energy to cover their energy expenditure is not always true. People who participate in sports with strict weight categories generally attempt to compete in a weight class that is lower than their normal training weight. Low energy intakes and high energy expenditures carry the potential threat of nutrient deficiency. This is particularly true for women, who must pay particular attention to their intake of iron, calcium, and folic acid.

Amino acids
In addition to extra protein containing foods, many strength athletes also supplement their diets with amino acids, especially those such as arginine, that stimulate an increased release of growth hormone. However, the growth hormone releasing effect of these amino acids is far less than that produced by exercise alone and too many strength athletes, and their coaches, mistakenly continue to extol the performance benefits of amino acid supplementation.

Branched chain amino acids have also been suggested as a way of delaying "central fatigue". The central fatigue concept proposes that the rise in plasma-free tryptophan during prolonged exercise increases serotonin, one of the brain's neurotransmitters, which in turn decreases the drive to continue exercising. Branched chain amino acids (valine, leucine, and isoleucine) share the same carrier mechanism for crossing the blood-brain barrier with tryptophan. Thus, the hypothesis is that raising the plasma concentration of branched chain amino acids will result in competitive inhibition of tryptophan's transport across the blood-brain barrier and avoid large increases in serotonin. Ingesting branched chain amino acids during prolonged exercise increases their plasma concentration and reduces the ratio of free tryptophan to branched chain amino acids. Although the rate of perceived exertion is reduced, endurance performance is not improved with this amino acid supplementation.

Bicarbonate loading
During brief high intensity exercise, such as sprinting, there is a rapid rate of glycogen degradation with an accompanying increase in lactate and hydrogen ion accumulation. The resulting acidity in muscle cells is associated with the onset of fatigue. The increased blood buffering capacity encourages the transfer of hydrogen ions out of the muscle cells and so helps restore the pH more quickly to pre-exercise values. Ingestion of a bicarbonate solution increases the capacity to perform high intensity exercise for short periods of time. For example, bicarbonate loading has been shown to significantly improve performance times of non-elite runners in 400, 800, and 1500 metre races. The dose of bicarbonate is 300 mg/kg body mass taken an hour or more before exercise. Higher doses cause gastrointestinal disturbances and, in some people, nausea and diarrhoea.

Creatine
In multiple sprint sports, the rate of ATP resynthesis is more important than the capacity for energy production. The rapid resynthesis of ATP occurs as a result of the contributions from the degradation of phosphocreatine and glycogenolysis. Performance during a series of maximum sprints, separated by recovery periods of no more than 30 seconds, decreases because there is not enough time for resynthesis of phosphocreatine. Creatine supplementation before exercise delays this decrease in performance by increasing the resynthesis of phosphocreatine. The main source of creatine is from foods containing meat or fish. However, creatine monohydrate is the supplement used and one gram is equivalent to the creatine content in 1 kg of fresh meat.

DPA/PA

In laboratory studies, the effective dose of creatine is 20-30 g/day for five to six days. Creatine supplementation at this level increases the pre-exercise phosphocreatine concentration, but about half is lost in urine. Lower doses of creatine also seem to influence performance during repeated sprints when recovery between sprints is longer than 60 seconds.

Caffeine
Consuming caffeine in the form of strong coffee or in tablet form was suggested as a way of increasing the mobilisation of fatty acids from fat cells. The increased availability of fatty acids should then increase the amount of fat metabolism in skeletal muscles and so reduce the contribution of the limited muscle glycogen stores to energy production. However, caffeine has a much more powerful effect on the central nervous system than on fat cells. It is the result of caffeine's impact on the brain that results in improvements in performance following caffeine ingestion rather than the result of a caffeine induced increase in fatty acid mobilisation. Even relatively small doses of caffeine have a significant ergogenic effect. As little as 9 mg/kg body mass improves endurance capacity without resulting increasing urine concentration above the former International Olympic Committee limit of 12 mg/l. Although the ban on caffeine intake has been removed, great care should be taken when using caffeine as an ergogenic aid because "more doesn't mean better," from a health perspective.

Clyde Williams, professor of sports science, Loughborough University
Email: C.Williams@lboro.ac.uk


studentBMJ 2004;12:265-308 July ISSN 0966-6494

1. Wu CL, Nicholas C, Williams C, Took A, Hardy L. The influence of high-carbohydrate meals with different glycaemic indices on substrate utilisation during subsequent exercise. Br J Nutr 2003;90:1049-56.

   

           

Responses published this month Articles Responses EDUCATION Eat yourself fitter       Clyde Williams (July 2004) Catherine Pereira (July 1, 2004) Read this response EDUCATION Eat yourself fitter       Clyde Williams (July 2004) Catherine Pereira (July 1, 2004)       Dietetics student, final year University of Cape Townuct_samsa@yahoo.com After having read this article in the August 2003 issue of The article entitled "The influence of high carbohydrate meals with different glycaemic indices on substrate utilization during subsequent exercise." briefly mentions the Atkins Diet as "...popular and effective...". This is true, and everything else in the relevant paragraph that is stated about the advert is true. It is, however, felt that when reference is made to the Atkins Diet, that due reference should be made to its potential negative side effects. Due to the extremely low carbohydrate content, the Atkins Diet is very high in both protein and fat, including saturated fat. The high protein content may have adverse effects on renal function, and the relationship between total fat and saturated fat content of the diet and high blood cholesterol, and coronary heart disease has been well established. Although the reference to the Atkins Diet is not the main focal point of the article, and, in the article, the Atkins Diet is not advocated for sports persons to follow, it is felt that too much positive publicity is given to the Atkins Diet, and not enough consideration is given to the implications of following such a diet. Otherwise, the article is a very informative one on nutrition in exercise. Catherine Pereira Student Dietitian (final year) University of Cape Town (South Africa)