Amino acid pharmacokinetics and safety assessment

Tracer kinetic studies of amino acid metabolism during periods of high amino acid intake should allow insights into adaptive or maladaptive regulatory mechanisms controlling amino acid catabolic or disposal events before clinically evident effects. The principles of amino acid tracer kinetics have been well defined, but their application to establishing upper safe intake levels has been essentially nonexistent. Similarly, the pharmacology field has well-established disciplines of toxicokinetics (the relationship of toxicant dose and delivery to its site of action) and toxicodynamics (the relationship of toxicant at its site of action and downstream functional consequences), but these principles have not been transferred to the field of amino acid metabolism. In this context, a theoretical framework is presented for tracer kinetic experiments to help establish upper tolerable levels of amino acid infusion and/or ingestion. In addition, experiments to couple specific amino acid intake levels with their consequent physiological dynamic effects are suggested to lead to the construction of benefit-risk curves that may permit definition of safe amino acid intake ranges for the population. J. Nutr. 133: 2034S-2039S, 2003.