The Influence of Previous DietGrowth and Age Upon the Basal Metabolism of the Rat

Abstract

The basal metabolism of male rats (91 to 196 days old) that received a diet of natural foods was somewhat higher initially (2 weeks after the diet was changed) than the metabolism of other male rats on a so-called synthetic diet. When these diets were fed to rats for a continued length of time, however, they played no significant role in the metabolism. There was practically no difference in the basal metabolism of rats fed diets of high-protein and medium-protein content, but rats on a low-protein diet had a lower metabolism and in these rats post-mortem examinations showed that the diet had begun to have a deleterious physiological effect. The basal heat production of young rats stunted in growth was lower than that of normal rats of the same age but larger weight and lower than that of normal rats of the same weight but different age. With realimentation the stunted rat not only resumed its normal growth but its basal metabolism, when normal growth was attained, was at approximately the same level as that of the normally growing rat of the same age and weight. The rate of growth, whether slow or rapid, did not have any appreciable effect upon the basal metabolism of normal male rats. However, from 40 to 100 days of age the heat production per unit of surface area of the rapidly growing rats was somewhat higher than the metabolism of the slowly growing rats, whereas at 230 to 300 days of age (weight 433 to 664 gm.) the metabolism of the rapid growth rats was slightly lower than that of the slow growth rats (weight 307 to 402 gm.). Male rats in the second and third years of life had a higher metabolism than females of the same age. The basal metabolism of both male and female rats remained at practically a constant level during the second and third years of life. This finding is not in accord with an earlier observation on female rats from a different rat colony, that the heat production increased with increase in age between 2 and 28 months. Analysis of both sets of data indicates a very great difference in the body weights of the two groups of rats. The lack of agreement in the findings may be explained by the supposition that this difference in body size is not ruled out either in the expression of the heat production per unit of weight or that per unit of surface area.