Post-prandial tracer studies of protein and amino acid utilisation: what can they tell us about human amino acid and protein requirements?

Nitrogen-balance, the principal methodology used to derive recommendations for human protein and amino-acid requirements, has been widely criticised, and calls for increased protein and amino-acid requirement recommendation have been made, often on the basis of post-prandial amino-acid tracer-kinetic studies of muscle protein synthesis, or of amino-acid oxidation. In each case, for such studies to inform about requirements over 24hrs or longer, it is arguable that their design and interpretation should take into account an understanding of the impact of varying protein intakes on homeostatic regulation of the FFM throughout the diurnal cycle of feeding and fasting. This narrative review considers our knowledge of this regulation and what can and has been learnt from post-prandial amino-acid tracer studies, about amino-acid and protein requirements. It appears that the FFM in well fed weight-stable adults with healthy lifestyles is fixed at a phenotypic level within a wide range of habitual protein intakes, with muscle-mass likely to be unresponsive to increased intakes. However homeostatic regulation occurs in response to variation in habitual protein intake, with adaptive changes in amino-acid oxidation which influence the magnitude of diurnal losses and gains of body protein. Post-prandial amino-acid tracer studies have been introduced as an alternative to the logistically complex, expensive and difficult to execute 24h [13 C-1] amino-acid balance studies, the recognised valid alternative to nitrogen-balance for assessment of amino acid requirements. However, a detailed examination of the most widely used methodology, indicator amino-acid oxidation studies of human amino-acid and protein requirements, shows that their design and interpretation takes no account of the homeostatic regulation of amino acid oxidation in response to varying habitual protein intakes and also involves design factors and analyses which seriously constrain their ability to provide reliable values. New ideas and a much more critical approach to existing work is needed if real progress is to be made in the area.