Effect of Glucose and Fructose Administration on Lipid Metabolism in the Rat

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Abstract
The effect of glucose and fructose on serum triglycerides was studied after prolonged feeding or under acute loads. In the rat, administration of fructose caused hypertriglyceridemia, but no change in serum triglyceride was found in the guinea pig in which fructose is said to be absorbed mostly in the form of glucose. This finding, as well as the low yield of labeled triglyceride in the chyle after fructose-14C administration, and the lack of stimulation of lipogenesis in the intestine indicate that the absorption of unchanged fructose is operative in the induction of hypertriglyceridemia. In the liver more fructose than glucose was converted to triglycerides and secretion of triglycerides into the serum was higher after fructose than after glucose administration to Triton-treated rats. In addition, fructose, unlike glucose, did not stimulate lipoprotein lipase activity in adipose tissue. The sequence of events occurring in the rat after fructose feeding could be summarized as follows: When owing to low activity of glucose 6-phosphatase in the intestine, fructose is absorbed as such into the portal circulation and reaches the liver, it is converted to α-glycerophosphate. At the same time no repression of the outflow of free fatty acid from adipose tissue occurs, leading to increased triglyceride formation in the liver and its secretion into the serum. As feeding of fructose does not induce lipoprotein lipase activity in adipose tissue the egress of triglyceride from the serum and thus the homeostatic regulation of triglyceride levels is impaired leading to its accumulation in the blood stream.