Leg citrate metabolism at rest and during exercise in relation to diet and substrate utilization in man

Previously we have demonstrated a lower rate of carbohydrate utilization in skeletal muscle after a fat than after a carbohydrate rich diet both at rest and during exercise. To test the hypothesis of citrate as a regulator of glycolysis (1) arterial-femoral venous (a-fv) differences for oxygen, citrate, pyruvate and lactate and (2) muscle citrate and glucose-6-phosphate (G-6-P) were determined at rest and after 5 and 25 min of submaximal bicycle exercise. Citrate release and muscle citrate concentration were higher after fat than after carbohydrate diet at rest and 5 min exercise, but did not differ between diets at 25 min exercise. Lactate release and muscle lactate concentration were lower after fat diet at 5 and 25 min exercise. Pyruvate release at 5 min exercise was higher after fat diet. The G-6-P concentration was higher at rest, insignificantly higher after 5 min exercise and lower at 25 min exercise after the fat diet. The findings support the notion that at rest and 5 min exercise fat diet induced inhibition of glycolysis might be mediated through increased intramuscular citrate acting on phosphofructokinase. In addition, the greater pyruvate release at 5 min exercise after fat diet in spite of a smaller lactate release, indicates a decreased pyruvate dehydrogenase activity and/or NADH to NAD ratio after fat diet. The inhibition of glycolysis at 25 min exercise after fat diet on the other hand does not seem to be citrate mediated.