Comparative Value of L-, DL-, and D-Methionine Supplementation of an Oat-based Diet for Humans

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Abstract
Total sulfur-containing amino acids have been found to be the first limiting amino acid in several foods in comparison with human amino acid requirements. Addition of methionine in appropriate amounts to these foods might be expected to improve protein value. Economically, DL-methionine would be preferable to L-methionine for this purpose. However, the comparative utilization of L-, DL-, and D-methionine is unclear. The objective of the current project was to compare the effectiveness of L-, DL-, and D-methionine supplementation of diets based on a food product known to be low in methionine value for human subjects. “Instant” oatmeal was fed to adult subjects to provide 4.0 g of nitrogen/day. In randomly arranged periods, these diets were supplemented with L-, DL-, or D-methionine at two levels (0.58 and 1.16 g of methionine/day). An unsupplemented diet was used in a control period. Diets were adequate in vitamins, minerals, and energy. Mean nitrogen balances of subjects while receiving the L-methionine supplements at the 0.58 and 1.16 g levels were -0.10 and +0.06 g of nitrogen, respectively. At similar levels of DL-methionine supplementation, nitrogen balances were -0.12 and -0.15 g of nitrogen, respectively, and -0.24 and -0.18 g of nitrogen with D-methionine supplementation. The mean nitrogen balance when no supplement was used was -0.22 g of nitrogen. Thus, D-methionine is seemingly poorly utilized by the human. Urinary methionine excretion data supported these results.