The idea that protein requirements are increased by physical activity is intuitively attractive, and highprotein diets are a common feature of the diets of sportsmen and women. The available evidence does show an increased rate of oxidation of the carbon skeletons of amino acids during exercise, especially when carbohydrate availability is low. Protein contributes only about 5% of total energy demand in endurance exercise, but the absolute rate of protein breakdown is higher than at rest (where protein contributes about the same fraction as the protein content of the diet, i.e., typically about 12-16%) because of the higher energy turnover. Most recommendations suggest that individuals engaged in endurance activities on a daily basis should aim to achieve a protein intake of about
1.2-1.4 g kg-1 day-1, whereas athletes engaged in strength and power training may need as much as 1.6-1.7gkg-1 day-1. In strength and power sports such as weightlifting, sprinting, and bodybuilding, the use of high-protein diets and protein supplements is especially prevalent, and daily intakes in excess of 4 g kg-1 are not unusual. Scientific support for such high intakes is generally lacking, but those involved in these sports are adamant that such high levels of intake are necessary, not only to increase muscle mass, but also to maintain muscle mass. This apparent inconsistency may be explained by Millward's adaptive metabolic demand model, which proposes that the body adapts to either high or low levels of intake, and that this adjustment to changes in intake occurs only very slowly.
Protein synthesis and degradation are both enhanced for some hours after exercise, and the net effect on muscle mass will depend on the relative magnitude and duration of these effects. Several recent studies have shown that ingestion of small amounts of protein (typically about 35-40 g) or essential amino acids (about 6g) either before or immediately after exercise will result in net protein synthesis in the hours after exercise, whereas net negative protein balance is observed if no source of amino acids is consumed. These observations have led to recommendations that protein should be consumed immediately after exercise, but the control condition in most of these studies has involved a relatively prolonged (6-12 h) period of fasting, and this does not reflect normal behavior. Individuals who consume foods containing carbohydrate and proteins in the hour or two before exercise may not further increase protein synthesis if additional amino acids or proteins are ingested immediately before, during, or after exercise.
Various low-(40%) carbohydrate, high-(30%) fat, high-(30%) protein diets have been promoted for weight loss and athletic performance. Proposed mechanisms include reduced circulating insulin levels, increased fat catabolism, and altered prosta-glandin metabolism. These diets can be effective in promoting short-term weight loss, primarily by restricting energy intake (to 1000-2000 kcal day-1) and by restricting dietary choice. There is no evidence to support improvements in exercise performance, and what evidence there is does not support the concept.
Carbohydrate is stored in the body in the form of glycogen, primarily in the liver (about 70-100 g in the fed state) and in the skeletal muscles (about
300-500 g, depending on muscle mass and preceding diet). These stores are small relative to the rate of carbohydrate use during exercise. Fat and carbohydrate are the main fuels used for energy supply in exercise. In low-intensity exercise, most of the energy demand can be met by fat oxidation, but the contribution of carbohydrate, and especially of the muscle glycogen, increases as the energy demand increases. In high-intensity exercise, essentially all of the energy demand is met by carbohydrate metabolism, and carbohydrate oxidation rates of 3-4 g min-1 may be sustained for several hours by athletes in training or competition. When the glycogen content of the exercising muscles reaches very low levels, the work rate must be reduced to a level that can be accommodated by fat oxidation. Repeated short sprints will also place high demands on the muscle glycogen store, most of which can be converted to lactate within a few minutes.
Carbohydrate supplies about 45% of the energy in the typical Western diet: this amounts to about 200300 g day-1 for the average sedentary individual, and is the amount that is necessary to get through normal daily activities. In an hour of hard exercise, up to 200 g of carbohydrate can be used, and sufficient carbohydrate must be supplied by the diet to replace the amount used. Replacement of the glycogen stores is an essential part of the recovery process after exercise; if the muscle glycogen content is not replaced, the quality of training must be reduced, and the risks of illness and injury are increased. Low muscle glyco-gen levels are associated with an increased secretion of cortisol during exercise, with consequent negative implications for immune function.
Replacement of carbohydrate should begin as soon as possible after exercise with carbohydrate foods that are convenient and appealing, and at least 50100 g of carbohydrate should be consumed within the first 2 h of recovery. Thereafter, the diet should supply about 5-10 g of carbohydrate per kg body mass, including a mixture of different carbohydrate-rich foods. For athletes preparing for competition, a reduction in the training load and the consumption of a high carboydrate diet in the last few days are recommended: this will maximize the body's carbohydrate stores, and should ensure optimum performance, not only in endurance activities, but also in events involving short-duration high-intensity exercise and in field games involving multiple sprints.
The high-carbohydrate diet recommended for the physically active individual coincides with the recommendations of various expert committees that a healthy diet is one that is high in carbohydrate (at least 55% of energy) and low in fat (less than
30% of energy). However, where energy intake is either very high or very low, it may be inappropriate to express the carbohydrate requirement as a fraction of energy intake. With low total energy intakes, the fraction of carbohydrate in the diet must be high, but the endurance athlete with a very high energy intake may be able to tolerate a higher fat intake.
Fat is an important metabolic fuel in prolonged exercise, especially when the availability of carbohydrate is low. One of the primary adaptations to endurance training is an enhanced capacity to oxidize fat, thus sparing the body's limited carbohydrate stores. Studies where subjects have trained on high-fat diets, however, have shown that a high-carbohydrate diet during a period of training brings about greater improvements in performance, even when a high-carbohydrate diet is fed for a few days to allow normalization of the muscle glycogen stores before exercise performance is measured. It must be recognized, though, that these short-term training studies usually involve relatively untrained individuals and may not reflect the situation of the highly trained elite endurance athlete where the capacity of the muscle for oxidation of fatty acids will be much higher. For the athlete with very high levels of energy expenditure in training, the exercise intensity will inevitably be reduced to a level where fatty acid oxidation will make a significant contribution to energy supply and fat will provide an important energy source in the diet. Once the requirements for protein and carbohydrate are met, the balance of energy intake can be in the form of fat.
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