Diet-induced obesity in rats can be produced by high-fat feeding. Comparing high-fat with low-fat feeding, the present study was designed to characterize the phases ofdevelopment ofobesity. In the dynamic phase, male rats were investigated at the age of 9-10 weeks after feeding the diets for 4-5 weeks. In the static phase, the animals at the age of 24-26 weeks were tested after 20-22 weeks ofthe nutritional regime. In this phase, the effects of switching high-fat to low-fat diet for 4 weeks were also examined.
Fractionating lipid extracts by thin layer chromatography the concentrations of several lipids in epididymal adipose tissue, in serum, and in liver were determined. In liver, the enhancement of cholesteryl-ester (CE) concentration after high-fat feeding besides the accumulation of triglycerides (TG) is remarkable. Cell fractionation studies of the livers by differential ultracentrifugation showed the major part of the accumulated CE in the supernatant. In vitro incorporation of (1-l4C)acetate and (2-14C)mevalonate into liver slices indicated that cholesterol synthesis in the liver of the obese rats was not increased. Although the offered fat diet with 0.1 % of cholesterol can not be considered as high in cholesterol, the 2.5-fold higher amount of the highfat diet in comparison with the low-fat diet (0.04% cholesterol) could be responsible for the enlargement of CE in the liver of the fat fed rats. This possibility was proved by measurement of the cholesterol absorption and transport to the liver after oral administeration of (4-14C)cholesterol. Estimation of TG secretion rates of the liver using Triton WR 1339 pointed out higher rates in the obese rats in the dynamic phase. In the static phase, the rates were not different between both feeding groups, while fat restriction in the food produced a striking increase of TG secretion. It is assumed that only in the dynamic phase metabolism is able to compensate the liver TG accumulation by an enhanced transport to the adipose tissue. In the static phase this ability is diminished but not lost.