Effect of Level of Total Nitrogen Intake on Second Limiting Amino Acid in Corn for Humans

Abstract

The objective of the project was to determine the second limiting amino acid in corn protein for maintenance of nitrogen retention in human adults as affected by level of total dietary nitrogen. In the first study, various proportions of lysine-supplemented corn meal to diammonium citrate-glycine nitrogen (total dietary nitrogen constant at 13.0 g N/subject/day) were fed to 10 adult men in order to determine minimum lysine-supplemented corn protein requirements. Nitrogen balances of the subjects fed a slightly inadequate lysine-diammonium citrate-glycine-supplemented corn diet plus selected purified essential amino acids were improved when methionine was included. Tryptophan additions were ineffective. Thus, methionine rather than tryptophan was indicated as being the second limiting amino acid in corn under conditions of high total nitrogen intake. In a second study, lysine-supplemented corn was fed to nine adult men to supply 3.0 g N/subject/day. Addition of methionine again resulted in increased nitrogen retention when the total nitrogen intake level was maintained at 12.0 g N/day (3.0 g N/day from corn, 9.0 g N/day from urea and the basal diet), with tryptophan additions being less effective. However, when total dietary nitrogen was maintained at 4.0 g N/day, tryptophan additions resulted in increased nitrogen retention and methionine additions had no effect. Thus, it is concluded that level of total dietary nitrogen can influence the order in which amino acids become limiting in the dietary protein source. By inference, this suggests that level of total dietary nitrogen also influences quantitative requirements of specific amino acids to varying degrees.