Metabolic response to moderate exercise in obese man during prolonged fasting

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Abstract
Exercise is associated with mobilization of fuel substrates to meet muscle requirements. To study the effect of exercise in a nutritional state with decreased hepatic glycogen and increased fat mobilization, 5 obese nondiabetic subjects were exercised for 45 min on a vertical cycle ergometer at 60% of maximum in the postabsorptive state (PA) and again following a 2 wk fast (F). Glycemia, glucose turnover, substrates, hormones and respiratory quotient (RQ) were measured during rest, exercise and recovery. Fasting resulted in a decrease in glycemia, glucose turnover, RQ, pyruvate, alanine and insulin, an increase in free fatty acid (FFA) and 3-hydroxybutyrate, and no change in lactate or glucagon. The metabolic responses to exercise in PA and F were as follows: glycemia was constant in both but more glucose was produced and utilized in PA compared to F; RQ increased in PA but did not change in F; pyruvate and alanine increased less in PA than F; lactate increases similarly in PA and F; FFA was unchanged in PA but increased in F; and 3-hydroxybutyrate was unchanged in PA but decreased in F. Insulin decreased in each subject with exercise in PA and F whereas minimal change occurred in glucagon. Apparently, glucoregulation is preserved in F during exercise and the glucose utilized during exercise in F is oxidized to a lesser extent than PA; adaptation probably occurred in F to spare glucose and utilize fat-derived substrate during exercise.