Re-evaluation of effects of meal feeding on lipogenic activation by glucose in rats

Based upon analyses of epididymal fat pads, gorging rats have been reported to synthesize fatty acids from glucose carbon 200 times faster than nibbling rats. This contrasts with our earlier study in mice in which no such adaptation was found in gorgers. Three methods were used to re-evaluate lipogenesis from glucose carbon in fasted-refed nibbling and gorging rats. Two methods in which [U-14C]glucose was injected intraperitoneally before or after different test-meals confirmed an apparent 100- to 200-fold increase in lipogenesis (14C incorporation into fatty acids) in epididymal fat pads of gorgers; however, incorporation of 14C into total fatty acids in the whole body of gorgers was only five times greater than in nibblers. Quantitative tracer techniques (intravenous and oral [U-14C]-glucose) were used to evaluate glucose carbon flux (22-hour fasted) and lipogenic activation following the ingestion of a labeled glucose test-meal. Glucose carbon conversion to total fatty acids (whole rat) increased from 2.7 (24-hour fasted) to 11 microng C/minute/200 g body weight, a fourfold activation, within 15 minutes after feeding the test-meal to nibbling rats. The corresponding increase in gorging rats was from 3.7 to 53 micrgong C/minute/200 g body weight, a 14-fold activation. These data indicate a species difference exists between rats and mice during adaptation to a gorging food-intake pattern.