The Development With Age of Hypothalamic Restraint Upon the Appetite of the Rat

SUMMARY During a brief period after birth, while they are kept warm by the mother, young rats employ the major part of the food available to them for growth. Variations in milk supply lead to great differences in weaning weight. The amount of food the rats eat subsequently, and the rate at which they grow, depend upon the weight at weaning, that is, it is roughly exponential. In spite of their large calorie intake, heavy weanlings do not become obese, but their sexual development is markedly accelerated. Lesions in the ventromedial nuclei of the hypothalamus of the adult rat are immediately followed by a twofold increase in the intake of food and the rapid development of obesity. Weanling rats, which already eat twice as much as adults in relation to their body weight, are unaffected by hypothalamic lesions until the age at which the intake of food normally falls. They resemble in this respect lactating adults, in whom the intake of food, already high, is not raised by hypothalamic lesions. At any age, the maximum calorie intake which has been recorded bears the same relation to body weight, about 45 kcal/100 g/day. The existence of this extra-hypothalamic restraint upon calorie intake probably explains the failure to saturate the capacity for growth, and so to produce obesity, in normal young rats. In a minority of weanling rats, hypothalamic lesions retard somatic growth, and in such rats obesity develops rapidly.