The nutritive value of bread protein as influenced by the level of protein intake, the level of supplementation with L-lysine and L-threonine, and the addition of egg and milk proteins.

Over 40 years ago Osborne & Mendel (1914) showed that the protein of wheat is deficient in lysine. This limitation of the nutritive value of wheat protein was emphasized when Rose’s (1937) classical work established that the requirement for lysine of the weanling rat is higher than for any of the other indispensable amino acids. If the amino-acid composition of wheat and flour proteins is considered together with Rose’s data on the minimum levels of the essential amino acids required for optimum growth (Rose, Smith, Womack & Shane, 1949), it is now apparent not only that lysine is the amino acid primarily limiting the nutritive value of the protein, but also that the content of the amino acids is such that the nutritive value of the protein may be substantially improved by a small addition of lysine. In recent years this presumption from the data of the protein chemist has been amply confirmed in investigations on the growth of rats, by Rosenberg & Rohdenburg (1952), Sure (1953)’ Hutchinson, Moran & Pace (19566) and Deshpande, Harper & Elvehjem (1957); on net protein utilization in the rat by Bender (1957); on nitrogen balance in dogs by Allison (1949); and on human adults in a state of protein depletion by Hoffman & McNeil(1949). With this general agreement, that it is lysine that primarily limits the nutritive value of wheat protein, firmly established, several investigators have subsequently examined the effect of supplementing wheat protein with other amino acids in addition to lysine. Some of the results obtained have been apparently contradictory. For example, Sure (1953) and Hundley, Ing & Krauss (1956) found a supplement of lysine with threonine better than one of lysine alone, whereas Rosenberg, Rohdenburg & Baldini (1954) found no additional effect with threonine, valine or methionine and, as Bender (1958) has pointed out, there has been some confusion in interpreting the results. In our opinion the discrepancies in the results recorded in the literature when the rat-growth method has been used have arisen primarily because different investigators have used different levels of protein in the diet and have frequently studied the effect of supplementation at only one level of protein. Furthermore, in some of the work an amino acid has been added only at one level which sometimes has been excessively high. For these reasons the interrelated aspects of protein level and food consumption, particularly important with an unbalanced protein, and the possible effect of the level of addition of the amino acid upon palatability and appetite have not