EFFECTS OF A DIET RICH IN RESISTANT STARCH ON HEPATIC LIPID-METABOLISM IN THE RAT

The aim of this study was to examine the effects of the replacement of a large part of absorbed glucose by volatile fatty acids on hepatic lipid metabolism. For this purpose, experiments were conducted in rats fed either a diet containing digestible wheat starch or amylase-resistant cornstarch. Compared with the digestible wheat starch diet, plasma insulin was lower in rats fed the resistant cornstarch diet, and the fluctuations of insulinemia during the fed/postabsorptive period were smaller: The marked reduction of hepatic lipogenesis in rats fed the resistant cornstarch diet (-52% compared with the digestible wheat starch diet) resulted from the coordinated inhibition of all major enzymes implicated in the lipogenic pathway except acetyl CoA synthetase activity. This suggests that volatile fatty acids, particularly acetate, constituted the major source of acetyl CoA for lipogenesis, lather than glucose. However these modifications were not accompanied by a significant depressive effect on plasma triglycerides. In rats fed the resistant cornstarch diet, changes in lipogenesis activity were accompanied by a reduction of glycolysis as shown by the net inhibition of glucokinase and pyruvate kinase. In parallel to these modifications, with the resistant cornstarch phosphoenolpyruvate carboxykinase was markedly induced; with this diet, propionate should constitute the major gluconeogenic substrate removed by the liver. HMG CoA reductase was markedly induced in rats adapted to the resistant cornstarch diet (1.6-fold higher than with the digestible wheat starch diet); this could be related to the increased fecal bile acids excretion. A significant hypocholesterolemic effect of the resistant cornstarch diet was only observed during the postabsorptive period. In conclusion, hepatic fatty acid synthesis is tightly controlled by carbohydrate availability, but the possibility that volatile fatty acids exert specific effects on lipogenesis could not be ruled out.