Nutrition and exercise--a consensus view.

The ability to perform exercise is impaired if the diet is inadequate. Conversely, performance may be improved by appropriate dietary manipulation. The primary need for the diet of athletes in training is to meet additional nutrient requirements imposed by the training load. Many athletes consider that a high protein diet is essential to stimulate muscle growth and promote repair. Evidence shows that hard exercise increases the protein requirement, but athletes eating a varied diet in sufficient quantity to meet their energy demands will obtain adequate protein. Carbohydrate is the main fuel used by the muscle in hard exercise, and carbohydrate intake must be sufficient to enable the training load to be sustained. During each strenuous training session, depletion of the glycogen stores in the exercising muscles and in the liver takes place. If this carbohydrate reserve is not replenished before the next training session, training intensity must be reduced, leading to corresponding decrements in the training response. It is recommended for athletes in training that carbohydrate should account for 60-70% of total energy intake, but the type of carbohydrate consumed is not crucial. With regular training, there must be an increased total energy intake to balance the increased energy expenditure. Provided that a reasonably normal diet is consumed this will supply more than adequate amounts of protein, minerals, vitamins and other micronutrients. Possible exceptions are iron and calcium, especially when energy intake is restricted to control body weight. There is no good evidence to suggest that specific supplementation with any of these dietary components is necessary or that it will improve performance. Attention must be paid, however, to ensure an adequate water intake during training: dehydration will reduce performance. The body does not adapt to dehydration. Consumption of a high-carbohydrate diet for the few days before competition with a reduction in the training load can double the muscle glycogen content and is generally known to be effective in increasing endurance performance. There is some evidence that the muscle glycogen content may also influence performance in events lasting only a few minutes. A high muscle glycogen content may be important when repeated sprints at near maximum speed have to be made. There is scope for nutritional intervention during exercise only when the duration of events is sufficient to allow absorption of drinks or foods ingested and where the rules of the sport permit. The primary aims must be to ingest a source of energy, usually carbohydrate, and fluid for replacement of water lost as sweat. Carbohydrate-electrolyte (sodium) drinks are the most effective way of achieving this. Each athlete must establish by trial and error 1000 the most suitable dietary programme for training and competition.