Isotopic studies of protein and amino acid requirements

The quantification of protein and amino acid requirements in health and disease is still an incompletely resolved issue, despite its importance to our knowledge of nutrition, to the clinical management of most health disorders and to food policy. However, the dynamic and adaptive features of protein metabolism render this determination difficult. The first nitrogen balance studies performed have demonstrated their limitations in providing accurate protein and amino acid requirements. Isotopic methods developed over the past 15 years have considerably enhanced the quantification of amino acid and protein requirements and our knowledge of the physiological phenomena underlying these needs. These methods are consistently being improved and producing new estimates for protein and amino acid requirements, together with a clearer understanding of this complex issue.