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The Utilization by the Adult Rat of Amino Acid Mixtures Low in Leucine¹

One Figure

Department of Physiology and Vital Economics, University of Rochester School of Medicine and Dentistry, Rochester, New York

Nitrogen balance and energy metabolism studies were performed on adult rats receiving diets containing mixtures of pure amino acids as the source of nitrogen. Each experiment included a 7-day period on a N-free diet, a 7-day period on an amino acid mixture at approximately half the maintenance level of nitrogen, and a 7-day period on the same amino acid mixture in about full maintenance quantity. These diets were fed by stomach tube twice daily so that each rat received the same quantity of diet every day.

Stepwise reduction in the amount of leucine to one-sixth that in egg protein failed to alter the heat production significantly.

The nitrogen balance index of dietary nitrogen (K' of Allison and Anderson, '45) was computed for each amino acid mixture in two ways; first, using the N-free period as a reference and second, using the half-N period as a reference.

The half-N reference method of determining K' seems preferable to the N-free reference method for the following reasons: (1) the former method gives K' values of reasonable magnitude, i.e., slightly less than unity, as compared with the latter method which gives some K' values well above unity; (2) rats will not voluntarily eat a N-free diet in full quantity for 7 days. Further experiments are needed to prove which method gives the more consistent measure of the quality of an amino acid mixture or protein.

The half-N reference method of computation leads to the following two conclusions which are not confirmed by the N-free reference method: (1) the leucine content of the one-third L-leucine amino acid mixture, which is 21.1 mg L-leucine nitrogen per gram of total nitrogen (one-third the proportion in whole egg protein), is adequate to permit complete utilization of the whole mixture; (2) as a supplement to the one-sixth L-leucine amino acid mixture L-leucine is more efficiently utilized than D-leucine.

Both criteria support the following conclusions: (1) the quantity of leucine in the one-sixth L-leucine mixture (10.6 mg L-leucine nitrogen per gram of total nitrogen, which is one-sixth the proportion in whole egg protein) is suboptimum; (2) the one-third DL-leucine amino acid mixture, which contains in addition to the above-mentioned quantity of L-leucine an equal quantity of D-leucine, is significantly better than the one-sixth L-leucine mixture. D-leucine is utilizable, therefore, to replace part of the L-leucine required for N equilibrium in the adult rat.

Manuscript received 21 November 1949.