Effects of long-term feeding of high-protein or high-fat diets on the response to exercise in the rat

The aim of this work was to find by which mechanisms an increased availability of plasma free fatty acids (FFA) reduced carbohydrate utilization during exercise. Rats were fed high-protein medium-chain triglycerides (MCT), high-protein long-chain triglycerides (LCT), carbohydrate (CHO) or high-protein low-fat (HP) diets for 5 weeks, and liver and muscle glycogen, gluconeogenesis and FFA oxidation were studied in rested and trained runner rats. In the rested state the hepatic glycogen store was decreased by fat and protein feeding, whereas soleus muscle glycogen concentration was only affected by high-protein diets. The percentage decrease in liver and muscle glycogen stores, after running, was similar in fat-fed, high-protein and CHO-fed rats. The fact that plasma glucose did not drastically change during exercise could be explained by a stimulation of hepatic gluconeogenesis: the activity of phosphoenolpyruvate carboxykinase (PEPCK) and liver phosphoenolpyruvate (PEP) concentration increased as well as cyclic adenosine monophosphate (AMPc) while liver fructose 2,6-bisphosphate decreased and plasma FFA rose. In contrast, the stimulation of gluconeogenesis in rested HP-, MCT- and LCT-fed rats appears to be independent of cyclic AMP.