Sex Differences in the Growth of Young Rats and Survival of Adult Rats Fed Protein-Deficient Diets

Abstract

A review of the results of a large number of feeding experiments conducted in this laboratory over a period of several years, involving somewhat over 1,000 young rats approximately equally divided as to sex and ranging in weight at the start from 50 to 60 gm, has revealed that almost invariably the females showed greater weight gains than the males when the diets were inadequate with respect to either the quality or quantity of the protein, but known to be otherwise satisfactory. On the other hand, in similarly conducted experiments in which the animals were provided with diets adequate in all respects, the males consistently surpassed the females in weight gains. Foods supplying protein of high nutritional value included dried whole eggs, casein, cottonseed meal (flotation process), dried skim milk, wheat germ, corn germ, peanut and soybean meals, and lean pork. Protein material of low to medium nutritional values included a number of raw soybean preparations, several cereal grains, patent wheat flour, and commercial corn gluten. The materials were incorporated in the diets to supply protein ranging from 4.5 to 28%. When provided with protein of good nutritional quality the rate of growth for both sexes was greater at intermediate levels (below 20%) than at higher levels. At the higher protein levels the males surpassed the females but at the low level of 4.5% the females invariably equalled or surpassed the males in weight gains. When diets were used supplying protein ranging from low to medium in nutritive value, such as the several preparations of raw soybean meal and patent wheat flour, at 9% protein in the diet, the females fared better than the males. On the other hand, when the diet was enriched with 10 to 15 parts of soybean flour, dried skim milk, or casein, and the mixtures fed at the same protein level (9%), the males exceeded the females in weight gains. With decreasing levels of high quality protein (dried whole egg) in the diet, ranging from 25 to 7.5%, the ratios between the weight gains of the males and those of the females decreased successively in the following order: 1.62, 1.58, 1.36, and 1.14, until at the low level of 4.5% protein the females showed greater gains than the males in the ratio of 1 to 0.87 gm, respectively. Similar decreasing ratios between the sexes were observed when casein supplied the protein in the diet. The number of animals used in the survival experiments was rather small to provide a basis for generalization. However, the results definitely indicate that adult female rats have a greater ability to survive than males when restricted to a diet grossly inadequate with respect to the quality and quantity of its protein.