Relevance and Safe Utilization of Amino Acids in Supplements for Human Nutrition: Lessons from Clinical and Preclinical Studies

Amino acid availability is central for the synthesis of macromolecules and numerous bioactive compounds. Amino acids are also involved in ATP production, cell signaling, and the epigenetic regulation of gene expression in human cells. From clinical and experimental studies, it appears that supplementation with specific amino acids may be relevant to correct for amino acid deficiency in the case of insufficient supply from dietary proteins with regards to the amounts needed for optimal metabolism and physiological functions. Clinical and experimental arguments suggest that amino acid supplementation may be indicated in specific situations under a specific nutritional context. However, it is essential not to overdose with excessive quantities of amino acids in supplements beyond the upper levels of safe intake (ULSI). In this narrative review, I recapitulate the protein and amino acid requirements for the general population and for subgroups of the population, and these requirements are compared to the usual consumption. Typical examples of clinical trials showing the benefits from amino acid supplementation in different physiological and pathophysiological contexts are presented together with results obtained from experimental studies. Parameters such as the no-observed-adverse-effect-level (NOAEL) values used to determine the ULSI for amino acid supplementation are defined, and values determined in clinical trials are given and discussed. Finally, prospects for future research in the field are proposed.