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Tissue Composition Affects Measures of Postabsorptive Human Skeletal Muscle Metabolism: Comparison across Genders¹

Department of Internal Medicine and Diagnostic Radiology and General Clinical Research Center, University of Virginia Health Sciences Center, Charlottesville, Virginia 22908

Address all correspondence and requests for reprints to: Linda A. Jahn, Department of Internal Medicine, MR-4 Box 5116, University of Virginia Health Sciences Center, Charlottesville, Virginia 22908.

Despite clear anthropomorphic differences, gender differences in human skeletal muscle protein and carbohydrate metabolism have not been carefully examined. We compared postabsorptive forearm glucose, oxygen, and lactate balances and forearm protein kinetics between 40 male and 36 female subjects. Forearm composition was measured in a subset of 17 subjects (8 males and 9 females) using multislice magnetic resonance imaging. Oxygen uptake, net phenylalanine release, and estimated rates of forearm protein synthesis and degradation were greater in male than in female subjects when expressed as the rate per 100 mL forearm volume (P < 0.05). In males, however, muscle accounted for 58% of forearm volume, compared with 46% in females (P < 0.001). When phenylalanine balance, protein degradation and synthesis, and glucose and oxygen uptake were expressed per 100 mL forearm muscle, there were no significant differences across gender. Likewise, the extraction fractions for oxygen, glucose, phenylalanine, and labeled phenylalanine were comparable in males and females. We conclude that cross-gender comparisons of metabolic variables must accommodate differences in tissue composition. These data indicate that in the postabsorptive state, skeletal muscle metabolism of glucose, protein, and oxygen do not differ by gender in healthy young humans.

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