Dietary lipid, fatty acid synthesis and cholesterol metabolism in aging rats

Male and female rats were fed diets containing 2% of calories as corn oil or that plus 40% of calories as beef tallow or corn oil. After 3, 6, 12 and 18 months groups were given 4‐ 14 C‐cholesterol ip, and feces were collected for 9 days. Just prior to necropsy 3 H‐acetate was administered ip. Samples of serum, liver, heart and carcass were obtained for analysis. Concentrations of fatty acids and cholesterol, synthesis of those and recovery of ring‐labeled steroid are reported. Mortality from acute respiratory disease was very high in male rats fed beef tallow or low fat diets and very low in those fed the corn oil diet. In females, only beef tallow diet resulted in a high mortality rate, and this was lower and at a later age than in males. The most notable effects of age were in relation to fatty acid synthesis and presence of 14 C‐acidic steroid in the carcass. In 3‐month‐old rats both fats depressed fatty acid synthesis in comparison to the low fat diet. At later ages beef fat ceased to depress fatty acid synthesis in both sexes. Corn oil continued to depress fatty acid synthesis up to 12 months in males and 18 months in females. The presence of 14 C‐acidic steroid in carcass was substantial in 6‐month‐old rats and constituted ca. 40% of recovered 14 C in 18‐month‐old rats. The possibility that the increase in acetate incorporation into fatty acids with age in fat feeding is related to chain elongation rather than de novo synthesis is discussed. Both the presence and amount of acidic steroid in the carcass are notable and may be of importance in constructing models of cholesterol turnover.