Short-term control of feeding: limitation of the glucostatic theory.

In the rat, the short-term effect on spontaneous feeding of intravenous administration of either glucose or glucose plus insulin was investigated. The infusions lasted 4 hours and covered 170% of rats's previously measured spontaneous caloric intake, while control infusions of saline and saline plus insulin were also made. Feeding patterns in subjects' home cages were recorded. Glycaemia and glycosuria were measured in order to asses glucose utilization. When it was infused alone, glucose was utilized at 95% and so covered 160% of oral caloric requirements, while the reduction of oral intake was only 40%. When insulin was co-infused with glucose, utilization reached 100% and oral feeding was reduced by 70%. Saline infusions did not affect oral feeding, and insulin brought about the expected increase in feeding. It is proposed that the mechanism which sustains feeding should depend on multiple macronutrients utilization rather than on one specific chemical family. Furthermore, the fact that insulin has a clear-cut effect, despite its lipogenetic, i.e., metabolite-sequestering properties, favors the ischymetric hypothesis (based on cellular power-production), rather than the energostatic one (based on yields of nutrients).