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 shows 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. It is often recommended that athletes 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. Those who take no exercise have an estimated average requirement of about 0.6 g kg-1 day and the recommended intake for these individuals is about 0.81.0 g kg-1 day.
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 2-4 g-1 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. This means that individuals such as strength and power athletes who consume a highprotein diet over many years will find that any reduction in protein intake will result in a loss of muscle mass. This is because of an upregulation of the activity of the enzymes involved in protein oxidation to cope with the high intake: activity of these enzymes remains high when there is a sudden decrease in intake, leading to a net catabolic effect.
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 high (30%) protein, high (30%) fat, low (40%) carbohydrate diets have been promoted for weight loss, and some diets even suggest almost complete elimination of carbohydrate from the diet. Some of these diets have been specifically targeted at athletes, accompanied by impressive claims and celebrity endorsements. Proposed mechanisms of action of these diets include reduced circulating insulin levels, increased fat catabolism, and altered prostaglandin metabolism, but it seems more likely that these diets achieve weight loss simply by restricting dietary choice. These diets can be effective in promoting short-term weight loss, primarily by restricting energy intake (typically to 1000-2000 kcal day-1). There is no evidence to support improvements in exercise performance, and what evidence there is does not support the concept.
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