Changes in Tissue Lipids in Response to Diet

Abstract

Liver and plasma data were reported for young rats at intervals following change from a diet containing 10% of a largely saturated fat (coconut oil) to one containing the same amount of a linoleate-rich oil (safflower oil) and return to the saturated fat. Diets were semisynthetic, adequate to support normal growth, and identical except for the kind of fat. All contained 1% of cholesterol. The lipid extracts from liver and plasma were fractionated on silicic acid columns, and the methylated fatty acids of each fraction were analyzed by gas liquid chromatography. Fatty acid composition of mesenteric, subcutaneous and interscapular fat of the same animals are reported in the preceding paper. In contrast to adipose tissues, which contained largely triglycerides, the liver and plasma lipids of all groups contained relatively large proportions of cholesterol esters and phospholipids, and appreciable amounts of the fraction containing mono- and diglycerides and free fatty acids as well as free cholesterol. The lipids of liver and plasma contained little lauric or myristic acid, regardless of diet, whereas the adipose tissue of the animals fed conconut oil sometimes contained as much as 30% of lauric and myristic acids. But there was always considerable arachidonic acid in liver and plasma lipids and little or none in adipose tissue. Although the linoleic acid content of liver and plasma lipids was increased by feeding safflower oil and decreased by feeding conconut oil, there seemed to be some tendency toward retention of linoleic acid in liver lipids, following change of the rats back to the conconut oil diet. If phospholipid fatty acids were included, however, the variations of the liver lipid fatty acids with diet stayed within narrower ranges than those of the adipose tissue. Characteristic sex differences were observed both in the concentration of the different liver and plasma lipids, and in their fatty acid patterns. Plasma cholesterol levels for males fed coconut oil averaged about 10 mg per 100 ml higher than those for males fed safflower oil. Females did not show this difference. For the females fed safflower oil for 9 weeks, the plasma cholesterol values of most doubled during the period of safflower oil feeding. Females fed coconut oil had higher cholesterol ester arachidonic acid values than males. Also, when females were changed back from the safflower oil diet to coconut oil, they responded with greater increases in percentages of plasma ester arachidonate than the males. But the males fed safflower oil showed more liver enlargement and more liver storage of both cholesterol ester and linoleic acid. In neither sex did the proportion of linoleic acid in the glyceride fraction of liver and plasma return to its original level within 6 weeks after return to the coconut oil diet following three weeks of safflower feeding.