The influence of a high level of corn oil on rat serum lipoproteins

Abstract

Although the stated requirement for linoleic acid in humans is less than 2% of the dietary calories, recently there has been considerable emphasis on the necessity to substitute dietary polyunsaturates for saturates in order to reduce serum cholesterol levels. In this study we have sought to determine the nutritional consequences of feeding a very high level of linoleate to rats. Three groups of thirty adult animals each were fed a semipurified diet consisting by weight of casein 17%; mineral mixture 5.5%; vitamin mixture in glucose 2.2%; cellulose fiber 3.0%; and corn oil 0% (group A), 10% (group B) or 40% (group C), which was provided at the expense of glucose. At the end of four weeks on the diets, blood was obtained in the fasting state from 16 rats in each group. The serum was ultracentrifugally fractionated into six classes of lipoproteins and analyzed for lipid composition and protein content. Disc gel electrophoresis using lipid and protein stains established that the various lipoprotein subclasses were reasonably free of adjacent density fractions. Although the total serum cholesterol levels were practically the same in the three groups, the cholesterol moiety of the major low density lipoproteins, LDL 2 ( d 1.019–1.050), but not of very low density lipoproteins, VLDL ( d 1.006) or low density lipoproteins, LDL 1 ( d 1.006–1.019), was substantially and very significantly increased in rats fed the high level of corn oil as compared to the other groups. The concentration of the very low density lipoproteins was significantly lower in group C than in the groups A and B. The LDL 2 concentration but not that of LDL 1 was significantly greater in group C as compared to group A. The cholesterol/total lipid ratio was significantly greater in both LDL 2 and LDL 1 but not in VLDL of group C as compared with group A. The serum high density lipoproteins were relatively less influenced by the ingestion of an excessive level of corn oil at this time period. The serum lipoprotein levels as well as their lipid composition were generally similar in groups A and B and suggested that a moderate level (5%) of dietary linoleate did not cause any untoward changes in rats. On the basis of current information on the metabolism of lipoproteins, it has been proposed that the increase in rat serum LDL 2 of group C reflects the status of the liver and that a large portion of the cholesterol moiety of LDL 2 is perhaps derived from the liver while the protein and phospholipid portions may represent remnants of VLDL catabolism. In view of the magnitude of the changes observed in LDL 2-cholesterol as well as in the liver cholesterol and triglycerides due to the ingestion of a 40% corn oil diet in a usually resistant species, namely the rat, further work along these lines with other species including human and nonhuman primates merits our attention.