Dietary manipulation of the disappearance of trans-octadecenoates in rat tissues

The effects of dietary manipulations on the fate of trans-octadecenoates deposited in the tissues of rats were examined. Male rats were fed on a 15% fat diet containing trans-octadecenoic acids (46.6% of total fatty acids) for 35 days followed by various diets free of trans-fatty acids. After removal of trans-fatty acids from the diet, there were phased disappearances of trans-octadecenoates from the circulation; a rapid and broad reduction in one day and a slow and gradual reduction thereafter. The rate of the initial reduction in serum trans-octadecenoates was highest on a high fat (20%) diet in relation to low fat (1 or 5%) diets. However, the disappearance rate at the later stage was apparently the same among the various groups and trans-fatty acid contents in the serum declined to about 10% of the initial value in 2 weeks and thereafter. After 35 days, the concentration of trans-octadecenoates remaining in the adipose tissue was markedly lower in rats fed on a high protein (40%) diet. The effects of dietary fat type and cholesterol on the fate of serum trans-octadecenoates were virtually the same, but livers from rats fed on the cholesterol-free safflower oil diet contained more trans-fatty acids than those from rats fed on the corresponding olive oil diet. Thus, the amounts of trans-fatty acids stored in the tissues cannot be merely predicted from serum levels. It seems that both dietary fat and protein affect the metabolic rate of trans-octadecenoates in rats.