Fat Accretion and Growth in the Rat

Abstract

The normal course of fat accretion in male rats from birth to 3 years is described. The relative amount of fat and relative size of fat pads increased steadily throughout life (except for a brief cessation in fat accretion at weaning), in a manner closely related to body weight and independent of age. Several new rat strains with extremes of growth or fat accretion, fed the same diet, were used to define and illustrate courses of fat accretion: very large and very small non-obese strains and a hereditarily obese rat. Rats of different known strains fed a variety of more refined diets show a course of fat growth different from that obtained with stock diets. Several types of obesity are discussed: insulin obesity, nutritional obesity, and hereditary obesity. Hereditary size and growth of the rat were readily (in a few generations) altered up or down, by consistently using the largest or the smallest 25% of the population for breeders. It is probable that the large increase in size of some laboratory rats over the past 40 years has been accomplished not only by improvement in stock diets and colony conditions, but also by selection for increased hereditary size.