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1 Exercise Science, University of Rhode Island, Kingston, RI, USA
2 Schwartz Center for Metabolism and Nutrition, Case-Western Reserve University, Cleveland, OH, USA
* To whom correspondence should be addressed. E-mail: lla4983u{at}postoffice.uri.edu.
Exercising men when compared with women have a greater increase in leucine oxidation, but not lysine rate of appearance. The cause for this sexual dimorphism is unknown; but an inhibition of 
-adrenoreceptor activity has previously been shown to mediate amino acid metabolism. This study was a gender comparison of leucine and lysine kinetics during a -adrenoreceptor blockade (1, 2-blockade) and a placebo control using a double-blind cross-over protocol. Subjects exercised at 50% of their trial-specific VO2max (1-hour) after 7-days of dietary control. During exercise with beta-blockade, men had an increased nonprotein RER (P < 0.001) while women had an increased FFA circulation (P < 0.001). The genders also displayed distinct differences in 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.
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