Circadian variations in plasma neutral and basic amino acid concentrations in young men on an ordinary Taiwanese diet

Background and purpose: Plasma levels of circulating free amino acids reflect the net status of protein breakdown and synthesis, and may be linked to various disease states. We studied circadian variations in plasma concentrations of neutral and basic amino acids during a 24-hour period in healthy young men who consumed ordinary Taiwanese test meals. Methods: Ten subjects ingested the test diet (protein intake, 1.5 g · kg^-1 · d^-1) which was offered in three meals and two light snacks during the day. Thirteen heparinized blood samples were collected from each subject to analyze plasma amino acid concentrations during the experimental period, at 1- to 3-hour intervals. Results: The plasma concentrations of all neutral amino acids, including the large neutral amino acids (leucine, isoleucine, valine, tryptophan, tyrosine, and phenylalanine) and methionine, as well as the small neutral amino acids (glycine, serine, threonine, and proline) and the basic amino acids (histidine, arginine, lysine), varied significantly as a function of the time of day (p < 0.001). Except for glycine and proline, all of the neutral amino acids exhibited a marked evening elevation after dinner, with the highest plasma concentration at 23:00. Proline showed peak concentrations at 09:00, while glycine and the basic amino acids exhibited peak concentrations at 21:00. Most of the plasma amino acids exhibited the lowest concentrations at 12:00. Conclusion: Plasma neutral and basic amino acid concentrations exhibited significant circadian variations. The present study also provided the mean fasting plasma levels of amino acids in healthy young men consuming an ordinary Taiwanese diet.