Variations of plasma lipid fractions in relation to age in two models of obesity

The Zucker rat (ten weeks old) is characterized by a large increase in plasma triglyceride, which is accompanied by a smaller augmentation of phospholipids and tri-unsaturated cholesteryl ester levels. On the other hand, lipid increases in hyperphagic animals are characterized by less hypertriglyceridemia but significant hypercholesterolemia involving several lipoprotein fractions. Variation in time and start of these changes were observed, using a thin layer chromatography method. In goldthioglucose (GTG) mice variations in plasma lipids of controls were irregular, consequently the influence of obesity was expressed as the ratio obese/control as functions of time. The levels of total lipids, cholesteryl ester, triglyceride, lecithin and lysolecithin had a linear progression with time in contrast to sphingomyelin, saturated and tri-unsaturated cholesteryl ester. Two phases can be distinguished in the variation of lipids in GTG mice: the first corresponding to onset, up to the seventh week, and the second corresponding to definite obesity. In Zucker rats lipid levels of lean rats decrease with time while they increased in obese rats. Triglyceride increase was predominant at each period of the study. Changes in total lipids, triglyceride, free fatty acids, cholesteryl ester, lecithin and lysolecithin were also linear. These results make it evident that the lipid disorder as described in the adult begins very early. Because the variations in lipid levels are more regular, the Zucker rat is preferable to GTG mice as a model of obesity.