The influence of diet and exercise on mammalian muscle metabolism

• Main Author: Maughan, Ronald John
• Published: University of Aberdeen 1978
• Subjects: 571.1
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.465296

The aim of these experiments was to investigate the interrelationships of fat and carbohydrate (CHO) metabolism in mammalian muscle. In particular, it was hoped to clarify the mechanisms regulating the integration of the supply and utilisation of metabolic substrates in skeletal muscle. This was achieved by studying the response to a perturbation of normal metabolic processes. Administration of a low CHO diet following exereise-induced glycogen depletion resulted in a situation where the muscle and liver glycogen stores were lower than normal, and the availability of plasma FFA was greater than normal. Administration of a high CHO diet immediately following the low CHO diet resulted in the achievement of greater than normal glycogen stores and a restricted availability of FFA. Subjects were studied at rest and during exercise of different intensities at each stage of this dietary regime Measurements were made of blood metabolites and cardiovascular and respiratory parameters. Following the low CHO phase of the diet, fat metabolism was enhanced, as evidenced by a higher than normal plasma concentration of FFA and glycerol and a lower than normal respiratory exchange ratio (R). CHO metabolism was depressed; the blood concentrations of glucose, lactate and pyruvate were lower than normal. Following the high CHO phase of the diet, plasma FFA and glycerol concentrations were lower than normal, and the R value was higher than normal, indicating a decreased contribution of fat to oxidative metabolism. Blood glucose, lactate and pyruvate concentrations were higher than normal under these conditions. In a second series of experiments, subjects were studied before and after a twenty-four hours' fast. Pasting is known to result in a decreased rate of CHO utilisation accompanied by a compensatory increase in fat availability and oxidation, but has little effect on the muscle glycogen store. Blood and muscle metabolite concentrations were measured in resting and exercising subjects before and after fasting. In addition, skeletal and cardiac muscle as well as blood metabolite concentrations were measured in fed and fasted rats. In fasted human subjects and rats, the blood glucose concentration was lower than normal; an elevation of circulating FFA and ketone concentrations occurred in response to fasting. It appears that the inter-relationships of fat and CHO metabolism may be regulated by several different mechanisms. An increased rate of fat oxidation may result in an intra-cellular accumulation of citrate, resulting in turn in an inhibition of glycolysis at the level of phosphofructokinase; an increased rate of fat oxidation may also decrease glucose utilisation by an inhibition of pyruvate dehydrogenase activity. There was also some evidence to support the proposal that high lactate concentrations may exert an inhibitory influence on fat metabolism. Finally, although hormone levels were not measured, it appears that, during fasting, a decreased circulating insulin level may result in a decreased rate of glucose uptake and utilisation by human skeletal muscle.