Whole-body protein utilization in humans.

Recently, several authors have recommended protein intakes as high as 2 to 3 g X kg body weight-1 X d-1 for physically active individuals. Review of the literature cited to support this recommendation, as well as other published and unpublished work, suggests that there are at least three variables which have not been sufficiently controlled or investigated in much of the research done, making such a recommendation premature. These variables are timing of sampling relative to initiation of an exercise program (training), energy and protein intake associated with activity, and intensity of exercise performed. Several investigators report a 12- to 14-d adaptation period following initiation of an activity program during which nitrogen (N) balance falls and then returns near to equilibrium. Data collected during this adaptation period can be expected to reflect a more negative N balance than data collected after that time. Butterfield and co-workers have found that exercise at 40 to 50% of maximal oxygen consumption actually promotes N retention in previously untrained men given sufficient time to adjust to the new exercise regimen, allowing maintenance of N equilibrium on a marginal protein intake. Energy intake, energy balance, and quantity of protein ingested inter-act in this effect. Finally, recent experiments suggest that, at high intensity activities, energy balance becomes even more critical in the utilization of protein: when energy balance is negative, an intake of protein as high as 2 g X kg body weight-1 X d-1 may be inadequate to maintain N equilibrium in previously trained individuals exercising at 64% of maximal oxygen consumption.(ABSTRACT TRUNCATED AT 250 WORDS)