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Studies in Amino Acid Utilization

IV. The Minimum Requirements of the Indispensable Amino Acids for Maintenance of the Adult Well-nourished Male Albino Rat¹

One Figure

Department of Pathology, University of Chicago

Quantitative requirements for the 9 amino acids indispensable for maintenance of weight, appetite and nitrogen equilibrium of the adult well-nourished male albino rat have been measured. The measurements were made under a standardized set of conditions, including adequate caloric, vitamin and mineral intakes. Total nitrogen intake was more than sufficient to supply the maintenance needs of the animals. The level of the amino acid under consideration was varied, while all other factors of the diet were maintained constant. Weight changes and nitrogen balance were determined. The minimum quantity of each amino acid necessary to maintain nitrogen equilibrium and weight balance was estimated from the curves relating amino acid intake to nitrogen balance or weight change, respectively. From the data presented the following conclusions were drawn:

1. The findings substantiate those previously reported from this laboratory that tryptophan, lysine, histidine, threonine, methionine, valine, phenylalanine, leucine and isoleucine are all essential for the maintenance of weight and nitrogen equilibrium in the adult well-nourished rat, since in the complete absence of, or in the presence of an insufficient quantity of, any one of these amino acids neither nitrogen equilibrium nor weight was maintained.

2. The minimum levels for each of the above-mentioned amino acids necessary for the maintenance of weight and nitrogen equilibrium under standard conditions are presented.

3. Mixtures of amino acids lacking an essential amino acid, while they cannot establish nitrogen or weight equilibrium, do have some nitrogen sparing action.

4. Reasons for believing that the maintenance functions of the amino acids differ in certain important respects from their functions in growth or in the rehabilitation of the depleted organism are briefly discussed.

5. A comparison is made between the minimum essential amino acid requirements for man as found in the literature and for the rat as derived from the present experiments. There is reasonable agreement between the two sets of estimates. This suggests once more that there is a fundamental similarity among the mammalian organisms, and reaffirms the value of lessons learned from the animals for use as guides in human nutritional planning and studies.

The work has been aided, also, by the National Livestock and Meat Board and the Douglas Smith Foundation for Medical Research of the University of Chicago.

Manuscript received 28 July 1949.