Growth and Metabolism Studies with Rats Fed Rations Containing Purified Amino Acids

Abstract

Experiments have been conducted to compare the rate of growth of rats fed a casein ration with that of rats fed rations containing various mixtures of purified amino acids. Apparently a period of adaptation to the amino acid diets was required by the rat, since during the first 7 days on experiment the rats receiving the optimum amino acid diet grew at a significantly slower rate than the control rats receiving the casein ration. After this adaptation period the two groups grew at nearly the same rate. The addition of monosodium glutamate to an amino acid diet does not improve the palatability of the diet for the rat. Vitamin B₁₂ is necessary for optimum growth and food efficiency for rats, but an exogenous supply of this factor as an adjunct to all diets is not always required. This may be due to the presence of this factor in the diet, per se, as a contaminant; to sufficient storage of the vitamin in the livers of the test animals; or to its synthesis in the intestinal tract. The addition of either streptomycin or aureomycin to the optimum amino acid diet produced a significant increase in the growth rate of rats. The resultant stimulation was comparable to that obtained by the addition of intact casein to the amino acid diet. Therefore growth factors such as strepogenin are not required, per se, in the diet of the rat. The use of the body balance technique in conjunction with paired feeding has clearly shown, in the case of the rat, that the utilization of nitrogen and energy derived from free amino acids is less efficient than when the nitrogen is fed in the form of intact casein. This is shown by a lower average weight gain, less efficient utilization of the ration, decreased deposition of nitrogen and fat in the body, increased urinary excretion of energy, and a significantly lower metabolizable energy value for the amino acid diet.