Intra-individual and inter-individual variations in plasma free amino acids in normal adults

Over periods of several months three serial determinations of plasma free amino acids have been performed in each of 14 individuals. Large intra-individual variations in levels of most of these amino acids were observed. Since normal values for each plasma free amino acid may vary as much as twofold or more among individuals, any study of plasma free amino acids in groups of patients must encompass a large population sample, unless profound differences from normal exist. Other considerations in experimental design of a study of this type have been discussed. Although the concept of biochemical individuality holds for all the amino acids except lysine and phenylalanine, it is diluted in importance by the large intra-individual variations observed.