Nitrogen retention and plasma amino acids of adults who consumed isonitrogenous diets containing rice and milk or wheat versus their constituent amino acids.

Nitrogen retention and concentrations of plasma amino acids were compared when young adults consumed isonitrogenous diets containing proteins (3.0 g of N from rice plus 3.0 g of N from milk or 3.0 g of N from rice plus 3.0 g of N from wheat flour) or mixtures of their constituent amino acids. Diets containing proteins induced greater nitrogen retention than did those containing corresponding amounts of amino acids in crystalline form, and the difference between the combinations of proteins was delineated more sharply. Concentrations of several amino acids were elevated by substituting crystalline amino acids for proteins, especially in postprandial plasma. Subjects responded differently to addition of the limiting amino acids, lysine and tryptophan, to the diets containing amino acids instead of rice plus wheat. Therefore, data obtained by means of combinations of cereals and by mixtures of their constituent amino acids cannot always be used interchangeably.