Effect of isoleucine supplementation on the growth of rats fed zein or corn diets

Isoleucine was found to be limiting for the growth of rats fed corn diets supplemented with lysine, tryptophan, valine, and threonine or 24% zein diets supplemented with most of the essential amino acids. Since phenylalanine, which is known to be present in zein at quite a high level, was also found to be limiting for the growth of rats fed 21% of zein, it was suggested that all of the essential amino acids in zein were utilized poorly. This was further substantiated when rats fed diets containing alcohol-extracted corn showed a much greater growth response to a level of isoleucine equivalent to that supplied by the zein extracted than to the zein itself. When the level of leucine supplied by the zein was also added, no growth depression was obtained.