Because of the limitations in the NBAL method, Young and Bier have been instrumental in devising a conceptual framework from which to determine optimal protein intakes using stable isotopic tracers. They have coined the terms nutrient deficiency, accommodation, adaptation, and nutrient excess.147 148 In a state of protein deficiency, there would be a maximal reduction in amino acid oxidation and a reduction in protein synthesis to all but the essential organs (e.g., brain) that ultimately would result in muscle wasting (negative NBAL). The state of accommodation would be the state where NBAL is achieved with a decrease in a physiologically relevant process. The state of adaptation would be the dietary intake that provided for optimal rates of protein synthesis for growth, interorgan amino acid exchange, and immune function. Finally, the state of protein excess would be defined as that intake where amino acids are oxidized for energy or used in fat storage, and protein synthesis is not further simulated by an increase in intake. These four states can be determined using amino acid tracers during studies at varied protein intakes. The optimal protein intake would be that where amino acid oxidation starts to increase exponentially and protein synthesis starts to plateau. There have only been a few of these studies performed in athletes.10,53,96,97
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