The effect of meal size on postprandial carbohydrate metabolism in normal and tumor-bearing rats

Doubling the size of a meal causes less than a two-fold increase in the thermic effect of feeding. One possible reason for this is that larger meals may be associated with a change in the pathway of postprandial hepatic glycogen synthesis from the indirect pathway, involving gluconeogenesis, to the more energetically efficient direct pathway. We have therefore investigated the effect of meal size on the relative contributions of those two pathways both in normal rats and in tumor-bearing rats, which have previously been shown to utilize the indirect pathway to a greater extent. Rats bearing a transplantable Leydig cell tumor and freely fed controls were fasted overnight and given a test meal amounting to 12 or 24 kJ of their normal diet. They were then injected with 3H2O and 14C-glycerol and killed one hour later. The total amount of 3H incorporated into liver glycogen was not affected by meal size, although it was greater in tumor-bearing rats than controls. Analysis of the 3H labelling at different positions in the glycogen glucose residues showed that the proportion of glycogen synthesized via pyruvate, which tended to be greater in tumor-bearing rats, was significantly reduced by increasing the size of the meal. Glycogen synthesis from glycerol was not affected by either meal size or tumor growth. Increasing the size of the meal increased the rate of fatty acid synthesis in both the liver and the epididymal fat pad, but not the tumor. Thus increasing the size of the meal appeared to increase the proportion of glycogen synthesized by the direct pathway from glucose in both tumor-bearing and control animals.