Studies on lipogenesis in vivo. Lipogenesis during extended periods of re-feeding after starvation

Abstract

Lipo-genesis was studied in mice re-fed for up to 21 days after starvation. At appropriate times [U-14c]glucose was given by stomach tube and incorporation of 14C into various lipid fractions measured. In mice starved for 48 hr. and then re-fed for 4 days with a diet containing 1% of corn oil, incorporation of 14c from [U-14c]glucose into liver fatty acids and cholesterol was respectively 3-fold and 8-fold higher than in controls fed ad libitum. The percentages by weight of fatty acids and cholesterol in the liver also increased and reached peaks after 7 days. Both the radioactivity and weights of the fractions returned to control values after 10-14 days'' re-feeding. These changes could be diminished by re-feeding the mice with a diet containing 20% of corn oil. Incorporation of 14c from [U-14C]glucose into extra-hepatic fatty acids (excluding those of the epididymal fat pads) was not elevated during re-feeding with a diet containing either 1% or 20% of corn oil. However, incorporation of 14c from [U-14C]glucose into the fatty acids of the epididymal fat pads was increased in mice re-fed with either diet, as compared with non-starved controls. Lipogenesis was also studied in mice alternately fed and starved. Mice given a diet containing 1% of corn oil for 6 hr./day for 4 weeks lost weight initially and never attained the weight or carcass fat content of controls fed ad libitum. Incorporation of 14c from dietary [U-14C]glucose into the fatty acids of the epididymal fat pads was elevated 3-fold in the mice allowed limited access to food, although the incorporation into the remainder of the extrahepatic fatty acids was not different from that found for controls. Mice given a diet containing 20% of corn oil for 6 hr./day adapted to the limited feeding regimen quicker and in 4 weeks did attain the weight and carcass fat content of controls. Incorporation of 14C from [U-14c]glucose into the fatty acids of the epididymal fat pads and the remainder of the extrahepatic fatty acids was respectively 5-fold and 3-fold higher than in controls fed ad libitum. The elevation in liver lipogenesis during re-feeding was greatest on a diet containing 1% of corn oil, whereas in extrahepatic tissues the increase in lipogenesis was greater when the mice were re-fed or were allowed limited access to a diet containing 20% of corn oil. These results suggest that the causes of the increased rate of incorporation of 14c from [U-14c]glucose into fatty acids during re-feeding may be different in liver from that in extrahepatic tissues.