Studies on lipogenesis in vivo: Comparison of cholesterol and fatty acid synthesis in rats and mice

1. The importance of fatty acid synthesis as a pathway for the disposal of ingested glucose has been evaluated in rats and mice given a purified diet high in glucose and low in fat. [U-(14)C]Glucose was either added to the diet and fed for 24hr. or given by stomach tube as a 250mg. (mice) or 1000mg. (rats) meal. The two methods of isotope administration gave similar results. 2. Under the conditions employed fatty acid synthesis appeared to be a more important pathway for glucose disposal in mice than in rats. In mice 15.3% of ingested [U-(14)C]glucose was converted into fatty acid and in rats the corresponding value was 8.6%. In contrast, the conversion of [U-(14)C]glucose into cholesterol, as a percentage of dose, was twice as high in rats as in mice. 3. The effect of 20% of corn oil in the diet on the conversion of dietary [U-(14)C]glucose into fat was also investigated. Mice given diets containing 1% or 20% of corn oil converted 14.6% or 7.0% respectively of dietary [U-(14)C]glucose into fatty acid over a 24hr. period. There was no effect of fat on the incorporation of the isotope into cholesterol. 4. In mice given diets containing 1% or 20% of corn oil approx. 10% and 2% respectively of newly synthesized fatty acids were found in the liver. Hepatic fatty acid synthesis appears to be more sensitive to dietary fat than is extrahepatic synthesis.