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Response of Adult Rats to Low Dietary Levels of Essential Amino Acids¹

Department of Nutrition, Harvard School of Public Health, Boston, Massachusetts 02115

The response of adult female rats to diets in which each essential amino acid was varied from zero to slightly above maintenance requirements has been examined. The estimates of the amino acid requirements for maintenance of adult female rats are presented. Only the animals fed diets free of threonine, isoleucine or methionine-cystine responded in a manner similar to animals fed a protein-free diet. Thus, in these instances the net protein utilization (NPU) values were zero as expected from the amino acid score. However, animals fed lysine- and leucine-free diets lost little tissue and the NPU values were much above those predicted from the amino acid score. Other amino acid deficiencies gave intermediate responses. Therefore, it is concluded that the biologic measures of protein quality, such as biological value and NPU, do not correlate with amino acid score and are not entirely a function of protein composition but depend also upon the level of intake tested. The proportions of amino acids required for growth and maintenance in rats are different. The question is raised whether estimates of nutritional quality determined with young growing rats are applicable in human nutrition.

Manuscript received 25 May 1970.

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