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Effect of Weight Loss on Muscle Fiber Type, Fiber Size, Capillarity, and Succinate Dehydrogenase Activity in Humans¹

Department of Medicine, Division of Endocrinology, University of Arkansas for Medical Sciences, and The Central Arkansas Veterans Health Care System (P.A.K.), Little Rock, Arkansas 72205; and the Department of Medicine, Cedars-Sinai Medical Center, University of California School of Medicine (R.B.S., M.F.), Los Angeles, California 90048

Address all correspondence and requests for reprints to: Philip A. Kern, M.D., ACOS-Research, 151/LR, Central Arkansas Veterans Healthcare System, 4300 West 7th Street, Little Rock, Arkansas 72205. E-mail: kernphilipa{at}exchange.uams.edu

To examine the effects of weight loss on muscle oxidative properties, nine obese subjects (body mass index, 34 ± 1.5) had muscle biopsies before and after weight loss and weight stabilization. Weight loss ranged from 13–32 kg and represented 20.8 ± 2.1% of initial weight. After weight loss, there was no change in the proportions of oxidative (type I and type IIa) fibers and also no change in mean fiber cross-sectional area, whereas there was a small, but significant, decrease in the relative interstitial space (P < 0.05). However, weight loss resulted in a 32 ± 6% (mean ± SEM) increase in capillary/fiber ratio and a 54% increase in capillary density (P < 0.05). In addition, there was a 41 ± 13% increase in succinate dehydrogenase (SDH) activity (P < 0.05). This increase in muscle capillarization and SDH activity was seen in all fiber types, even the relatively lower oxidative type IIx fibers. There was a strong correlation between the change in capillary/fiber ratio and the change in SDH activity (r = 0.82; P < 0.02). Thus, weight loss resulted in no change in muscle fiber type or cross-sectional area, but produced increases in capillary/fiber ratio, capillary density, and SDH activity, suggesting an increase in muscle oxidative capacity.

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