Estimates of protein requirements for athletes and all other populations are based on the concept that the adaptations owing to protein ingestion depend solely on the amount of protein ingested on a daily basis given the training demands for a given group (eg, endurance or resistance-trained athletes). The influence that other dietary factors, such as type of protein being consumed, and that other nutrients in the diet and timing of protein ingestion may have on the use of the ingested protein and the adaptations stemming from intake of the protein is not taken into account. In recent years, a growing body of evidence based on acute metabolic studies suggests that the metabolic response to protein and amino acid ingestion, particularly in muscle, is far more complex than is implied simply by consideration of the amount of protein ingested on a daily basis. For any given protein intake, the metabolic response—and presumably the adaptations in the muscle—would vary and depend on a variety of factors involved in the form and process of nutrient intake.
The composition of the ingested protein would influence the response to a given diet. The impact of protein quality on protein requirements has long been recognized as an important consideration for making nutritional recommendations. On a whole-body level, studies suggest that although vegetarian diets may be sufficient for positive nitrogen balance, reliance on animal proteins results in superior balance [63-65]. The purported superiority of animal proteins may not be as clear, however, as some studies indicate .
Whole-body studies may not give a clear picture of the importance of protein intake to other tissues, particularly muscle. In a series of experiments involving modeling based on stable isotopes, the complexities of use of amino acids from meals including different types of proteins has been examined. In general, use of amino acids from animal proteins (eg, milk) is greater than plant proteins (eg, wheat) [60-62], but differences exist even among different plant proteins [60-62,89,90]. These data suggest that amino acids from different protein sources may be preferentially used by different tissues. Amino acids ingested as milk proteins are taken up in greater amounts by peripheral (ie, muscle) rather than splanchnic tissues [61,90]. There is an interaction of protein type and the amount of protein ingested, such that use of amino acids from ingested animal proteins is diminished less than plant proteins at higher protein intake levels . Although these investigations were performed in resting subjects, and the relevancy to athletes may be questioned, these data make it clear that use of amino acids from ingested proteins may be handled differently depending on the type of protein that is ingested. These results may be interpreted to support the idea that adaptations to diets with different types of proteins during training may be different even if similar amounts of proteins are ingested.
Data on amino acid use from various proteins after exercise are limited. Consistent with the data based on modeling in resting adults, Phillips and colleagues  reported that uptake of amino acids from milk proteins into muscle is greater than from soy protein after resistance exercise. In resting volunteers, casein may provide a superior anabolic response compared with whey proteins on a whole-body level . On a muscle level after resistance exercise, however, the differences in amino acid uptake between casein and whey proteins are less clear .
Other nutrients ingested concurrently with protein also influence use ofthe ingested amino acids. At rest, whole-body amino acid retention is increased when proteins are consumed with carbohydrates [93,94]. Although the total retention of ingested amino acids is greater with carbohydrate than fat ingestion [93,94], the uptake into body regions seems to be differentially affected. Concurrent fat ingestion resulted in greater retention of ingested amino acids in peripheral tissues than did sucrose ingestion . Consistent with these results in resting subjects, it has been shown that carbohydrate ingestion increases the use ofamino acids ingested concomitantly after resistance exercise [95-98], an effect likely mediated by the insulin response . Preliminary evidence suggests that lipid increases amino acid use of milk proteins ingested during recovery from resistance exercise . The mechanism for this effect remains to be elucidated. The results from several studies examining use of ingested proteins after exercise are summarized in Fig. 2. Taken together, these results show that ingestion of a particular amount of protein stimulates metabolic processes that are influenced by the nutrients ingested concurrently. These acute responses suggest that adaptations in athletes could be independent of the amount of protein ingested.
In addition to other nutrients and the type of protein, the metabolic response of muscle may be affected by the timing of the ingestion of amino acids or protein in relation to the exercise bout. Timing of ingestion of a mixture of carbohydrate, fat, and protein ; carbohydrates alone ; and EAA plus carbohydrates  would influence the anabolic response to resistance exercise. It seems that different sources of amino acids do not engender the same response to varied timing of ingestion. In a previous study, the anabolic response to ingestion of a solution of EAA and carbohydrates immediately before exercise was approximately three times that of the response when the solution was ingested after exercise . In a more recent study using an identical protocol, however, the response to ingestion of whey proteins immediately before exercise was similar to that after exercise . It seems that not only timing of ingestion, but also the interaction of the type of protein with the timing determines the anabolic response in muscle.
Taken together, the anabolic response of muscle depends not only on the form of the ingested amino acids, but also on the nutrients ingested in association with the amino acids and the timing of the ingestion in relation to exercise—not to mention the interaction of all these factors. The complexity involved in assessing the relationship of the anabolic response to exercise and nutrition is readily apparent. Consideration of only the amount of protein ingested on a daily basis does not provide a complete picture of the metabolic situation that would influence the adaptations to training and nutrition. Broad recommendations for a particular amount of protein for all athletes or even subgroups of those involved in various types of sport without consideration of many other factors seems nonsensical.
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