Nitrogen retention in men fed isolated soybean protein supplemented with L-methionine, D-methionine, N-acetyl-L-methionine, or inorganic sulfate.

The ability of various sulfur-containing compounds to replace L-methionine (L-Met) was investigated by metabolic balance studies in man. N-acetyl-L-Methionine (AcMet), D-methionine (D-Met), and sodium sulfate (Na2SO4) were used to supplement a diet deficient in sulfur amino acids. The daily diet contained 4.5 g nitrogen (N) from isolated soybean protein (SB) and 4.5 g from glycine and alanine (9 g total N). SB diet was given alone or supplemented to six adult men for periods of 9 days after a standardization period with an equal N eggwhite diet, preceded by a 2-day zero N adaptation period. Supplements provided equivalent amounts of sulfur to that present in 420 ml L-Met, the amount added to SB to bring the total sulfur amino acid content to 900 mg/day. AcMet was as benfeficial as L-Met in improving N balance but D-Met was not as effective as L-Met. Difference between balances obtained with L-Met and Na2SO4 was not significant due to large variation in response to Na2SO4. While addition of D-Met to SB did not result in significantly greater N retention than unsupplemented SB, NA2SO4 addition did cause increased N retention.