The Composition of Gains Made by Rats on Diets Promoting Different Rates of Gain

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Abstract
Rates of gain including comparatively rapid early gain of rapidly growing and “normal” rats, the slow later increase of the same animals, the retarded early growth of two stunted groups and a more rapid increase in weight made by the stunted animals at a mature age, were produced by dietary adjustment. Results relating to the rapidly growing and “normal” groups show that during early growth gains were rapid and contained a preponderance of protein over fat, while later gains were made considerably more slowly and were accompanied by the accumulation of large quantities of fat. The performance of the stunted animals, on the other hand, demonstrates conclusively that ratio of protein to fat in the gains does not, as has been suggested, run parallel to the rate of increase of body weight. During early life, when gains were slight, the stunted animals added much more protein than fat. After realimentation, when their most rapid increase in weight took place at a rate greater than that of the heavier groups of the same age, these animals added considerably more fat than previously. No single generality, therefore, relates rate of growth and composition of gains, but other factors such as age and nature of the diet appear to influence the composition of the gains. The composition of the whole bodies of the four groups reflected the trends described for gains. The fat-free matter of the bodies showed smaller variations. At the end of the experiment, although the whole bodies differed considerably in size and composition, the fat-free material of the four groups was of practically identical composition. The iodine number of the body fat rose sharply between the first and fourteenth days. Thereafter this value did not differ significantly for animals of different age and dietary history.