Effects of Altered Free Fatty Acid Mobilization on the Metabolic Response to Exercise*

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Abstract
This study was designed to evaluate the metabolic response to limitation of fat mobilization during severe exercise in man. To this end, we assessed the relationship between fat mobilization, lipid-ketone physiology, and exercise-induced elevations in the counter-regulatory hormones. Ten well trained athletes ran a 10–mile premeasured course in as short atime as possible during both a placebo and an experimental study. In the experimental study, fat mobilization was inhibited by the oral administrationof 2 g of nicotinic acid. Blood samples for multiple substrate and hormonalassay were obtained 2 h before exercise, immediately before exercise, and immediately after exercise. In the experimental study, nicotinic acid effectively blocked the expected rise in plasma free fatty acid concentration observed in the placebo study. Although blunted, the exercise-induced rise in plasma ketone concentration persisted in spite of the reduced free fatty acid levels. The significance of the concommitant exag-erated rise in glucagon and GH in stimulating ketogen-esis is discussed. The inhibition of fat mobilization did not reduce the mean time required to run 10 miles. Since muscle uptake of free fatty acid is directly proportional to the plasma concentration of this substrate, our study suggests that the inability to mobilize fat does not prevent severe exercise for 70 min. (J Clin Endocrinol Metab46: 254, 1978)