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Gender differences in the regulation of amino acid metabolism

¹Exercise Science Program, University of Rhode Island, Kingston, Rhode Island 02881; and ²The Schwartz Center for Metabolism and Nutrition, Case Western Reserve University School of Medicine at MetroHealth Medical Center, Cleveland, Ohio 44109

Submitted 11 November 2002 ; accepted in final form 9 June 2003

Exercising men, compared with women, have a greater increase in leucine oxidation but not lysine rate of appearance. The cause for this sexual dimorphism is unknown; however, an inhibition of -adrenoreceptor activity has previously been shown to mediate amino acid metabolism (Lamont LS, McCullough AJ, and Kalhan SC. Am J Physiol Endocrinol Metab 268: E910-E916, 1995; Lamont LS, Patel DG, and Kalhan SC. J Appl Physiol 67: 221-225, 1989). This study was a gender comparison of leucine and lysine kinetics during a -adrenoreceptor blockade (₁,₂-blockade) and a placebo control by using a double-blind crossover protocol. Subjects exercised at 50% of their trial-specific maximal O₂ consumption (1 h) after 7 days of dietary control. During exercise with -blockade, men had an increased nonprotein respiratory exchange ratio (P < 0.001), whereas women had an increased circulation of free fatty acids (P < 0.001). The genders also displayed distinct differences in exercise amino acid kinetics. The men, but not the women, increased leucine oxidation (P < 0.005) and lysine rate of appearance (P < 0.009) when exercising during -adrenergic blockade. This study indicates that during -blockade, exercising men increase their need for amino acids (and carbohydrate) to fuel energy needs, whereas women increase their mobilization of fat, thereby requiring less alternative fuels such as carbohydrate and amino acids. Gender-specific fuel preferences during exercise are regulated by -adrenergic-receptor activity. Substrate availability during exercise appears to modulate the amino acid oxidation differences between genders.

L-[1-¹³C] leucine; L-[-¹⁵N] lysine; prolonged exercise; -adrenergic blockade

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