Metering of intravenous versus oral nutrients and regulation of energy balance.

Unrestrained rats received continuous and discontinuous intravenous infusions of nutritive substances over long periods, and the effects on energy regulation were examined. Oral food intake was decreased by all glucidic infusions, but by less than the theoretical (caloric) value of the infused substances. A residual oral food intake thus persisted when the energy needs were supplied intravenously, and rats with the greater residual intakes gained excessive body weight. The less nutritionally balanced the infusion, the less the reduction in food intake, which suggests that some of the residual oral feeding may be due to a specific appetite for missing elements. The reduction of oral intake became equal to the calories infused when exogenous insulin was coinfused, and results from discontinuous infusions showed a similar trend. A model for the metering of nutrients at the systemic level was proposed; only when substances are metabolized is there a direct effect on feeding control mechanisms.