The BCAAs, leucine, isoleucine, and valine, represent 3 of the 20 amino acids that are used in the formation of proteins. Thus, on average, the BCAA content of food proteins is about 15% of the total amino acid content. If we take an average man with a sedentary lifestyle, his daily energy intake will be about 10 MJ/day, and let us say that 15% of this will come from protein. Thus, he consumes about 1500 kJ as protein, which is equivalent to about 63 g. Therefore, his intake of BCAAs is about 9.5 g. Contrast this with a Tour de France cyclist: the energy intake of these elite athletes has been measured as averaging around 25 MJ/day over a 2- to 3-week period.44,45 The proportion of protein in the diet may be a little less, as much of the extra energy consumed is in the form of carbohydrates, but even so, the protein content of the diet is still about 12% of the total energy content. Thus, the elite cyclist consumes about 3000 kJ as protein, which is equivalent to 126 g of protein, which in turn represents about 19 g of BCAAs, or twice the amount of the sedentary individual. The requirement for protein may actually be higher in the endurance athlete because some amino acids, including the BCAAs, are oxidized in increased amounts during exercise compared with rest.36-9 On the other hand, the efficiency of protein utilization appears to be increased with exercise training, and thus the dietary protein requirement may not be very different in athletes compared with less active or even sedentary individuals.46,47
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