The effects of three dietary fats, corn oil (CO), tallow (T) and marine oil (MO), on serum triglycerides, hepatic lipogenic enzyme activity and lipogenesis in vivo using 3H2O were measured in fed and fasted rats that had been consuming diets in which the carbohydrate was either glucose or fructose. Hepatic triglyceride secretion was also measured in fasted rats fed the same diets. In both the fed and fasted state, hepatic enzyme activity and lipogenesis in vivo were greater in fructose-fed rats than in glucose-fed rats and less in both CO- and MO-fed rats than in T-fed rats. In rats fed glucose, serum triglycerides were lower in fasted rats fed MO than in fasted rats fed CO. In rats fed glucose or fructose, hepatic triglyceride secretion was lower in rats fed MO than in those fed CO or T.
We have studied hepatic and extrahepatic lipogenesis in mice fed diets containing 0, 5, 10, or 15% corn oil in which the carbohydrate was one of fructose, sucrose, glucose, or wheat starch. Meal-fed animals were trained to eat a quantity of food such that at a given level of fat ail animals consumed the same amount of carbohydrate. Fatty acids synthesis in vivo, acetyl CoA carboxylase, fatty acid synthetase, ATP citrate lyase, glucose-6-phosphate dehydrogenase, and malic enzyme were determined. At 0 and 5% corn oil the values generally decreased in the order fructose > sucrose > glucose > wheat starch. At 10 and 15% corn oil differences due to the type of carbohydrate were less pronounced. The effect of dietary carbohydrate on glucokinase activity followed the reverse pattern with the values decreasing in the order wheat starch > glucose > sucrose > fructose. The results obtained demonstrate that hepatic lipogenesis was increased in mice consuming fructose or sucrose independent of carbohydrate intake or the level of dietary fat. However, total capacity for lipogenesis was greater in glucose-fed mice than those consuming wheat starch, fructose, or sucrose. Regardless of the carbohydrate in the diet, as dietary fat increased all the lipogenic parameters measured declined.Increasing corn oil from 0 to 15% caused a greater change in lipogenic capacity than the change due to the various carbohydrates. The percentage of total synthesis which occurred in liver was greater in mice fed fructose or sucrose. This increased contribution by the liver was unaltered by dietary fat which may contribute to the hypertriglyceridemia in such animals. Compared with diets based on glucose or wheat starch, diets based on fructose or sucrose do not cause increased lipogenesis in meal-fed mice if the diets contain more than 30 calorie percent fat. Our results suggest that when diets contain fat a a level similar to that in North American diets, the effects of the type of dietary carbohydrate on lipogenesis are minimal.
The activity of lipoprotein lipase (LPL) in the adipose tissue and skeletal muscle of rats fed glucose- or fructose-based diets containing fish oil, corn oil or tallow was examined. In addition, heart LPL activity was measured in rats fed a glucose-based diet containing either corn oil or fish oil. Adipose tissue LPL activity was unaffected by dietary fat. In both heart and skeletal muscle, LPL activity was higher in rats fed the fish oil diet. These results suggest that increased removal of triglyceride by muscle may contribute to the blood triglyceride lowering effect of dietary fish oil.
5. The results demonstrate a dissociation between hepatic fatty acid synthesis and ME and suggest that when rats consume low-protein diets the NADPH needed for fatty acid synthesis is generated primarily by ME but that as the level of dietary protein is increased the contribution of ME is reduced while that of the phosphogluconate pathway becomes more important.
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