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Sex Differences in Effect of Restriction of Time of Access to Food on the Plasma Lipid Components in Rats¹

Department of Nutrition, University of California, Berkeley, California

An investigation of plasma lipid components in young adult rats of both sexes is reported. It was designed to find why part of the cholesterol-fed females given certain fats as 10% of adequate synthetic diets had a higher range of esterified plasma cholesterol values than males or than the rest of the females fed the same diets. Cottonseed and coconut oils were chosen as examples of unsaturated and saturated fats.

Limitation of time of access to food resulted in shifting nearly all of the plasma cholesterol levels in cholesterol-fed females into the higher range. Increases in plasma cholesterol values in the restricted cholesterol-fed males were comparatively small. No significant correlation of high cholesterol levels with any phase of the estrus cycle was found. Data did not eliminate the possibility of differences in eating habits of ad libitum-fed females.

Cholesterol feeding resulted, in rats of both sexes, in significant lowering of the percentage of arachidonic acid in plasma cholesterol ester. The change was greatest in the female rats with high cholesterol values and restricted access to food. When linoleic acid was available from the diet, it took the place of arachidonic acid in the cholesterol ester. Without it, i.e., when fed the coconut oil diet, the percentage of oleic acid was increased.

There was a tendency toward maintenance of a relatively constant and characteristic ratio of unsaturated to saturated fatty acids in each of the plasma lipid fractions regardless of diet, sex or feeding pattern. Approximately 85% of the cholesterol ester fatty acids were unsaturated, those of triglycerides, 60 to 65%, of phospholipids about 55%. The data suggest that rate of esterification of excess dietary cholesterol may differ in males and females and that time restriction of access to food may delay removal of cholesterol ester from plasma to a greater extent in females than in males.

² Post doctoral fellow, National Heart Institute. Data are taken from the thesis submitted in partial fulfillment of the requirements for the Ph.D. degree in Nutrition, University of California by Elaine W. Lis.

Manuscript received 12 August 1960.