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Nitrogenous Factors Affecting the Adequacy of Rice to Meet the Protein Requirements of Human Adults¹

Department of Food and Nutrition, School of Home Economics, University of Nebraska, Lincoln, Nebraska

A study was made to determine the first-limiting nitrogenous component in rice protein for nitrogen retention in human adults. Six subjects fed diets providing 1) 6 g nitrogen from rice or 6 g nitrogen from rice supplemented with 2) 2 g amino acid nitrogen combined in the rice pattern, 3) 2 g nonspecific nitrogen, and 4) 6 g nonspecific nitrogen, achieved average nitrogen balances of – 0.01, + 0.23, – 0.13, and + 0.25 g/day, respectively. The study was 43 days in length and included three 10-day and one 5-day experimental periods. A second study was made to determine the first-limiting essential amino acid in rice protein for nitrogen retention when the diet contained adequate amounts of total nitrogen (12 g/day). The study was 55 days in length with experimental periods of 5 days. In the first part of the experiment, reducing the amount of rice to provide from 6 to 2 g nitrogen/day resulted in highly significant differences in nitrogen balances of 6 adult subjects. Mean nitrogen balances were: + 0.45 (6 g rice N), – 0.31 (4 g rice N), – 0.70 (3 g rice N), and – 1.13 g/day (2 g rice N). Subsequently, various essential amino acids or amino acid combinations, suspected as being limited in rice protein, were tested in subjects fed minimal levels of rice nitrogen and a high level of total nitrogen (12 g/day). Mean nitrogen balances of subjects fed rice supplemented with amino acids as provided by 0.5 g rice nitrogen were + 0.15 (EAA plus cystine and tyrosine), + 0.26 (lysine plus threonine), + 0.14 (lysine), – 0.32 (threonine), and – 0.03 g/day (methionine plus cystine). Nitrogen retentions were significantly higher in response to the lysine-containing supplements than to those containing threonine alone or methionine-cystine, indicating that lysine is the first-limiting amino acid in rice protein for supporting nitrogen retention when the diet provides a high level of total nitrogen.

² Present address: Department of Food Science and Nutrition, Stout State University, Menomonie, Wisconsin.

Manuscript received 10 February 1967.