Skeletal muscle composition in dietary obesity-susceptible and dietary obesity-resistant rats.

The intent of this study was to determine whether a relationship exists between susceptibility to high-fat diet (HFD)-induced obesity and skeletal muscle fiber type. Forty-four adult male Wistar rats were given ad libitum access to a HFD (60% of calories from fat) for 4 wk. Rats were then grouped into quartiles for total weight gain, and the top-quartile [obesity prone (OP)] rats were compared with the bottom-quartile [obesity resistant (OR)] rats. OP rats gained 1.5 times as much weight as OR rats. OR rats had a significantly higher proportion of type I muscle fibers in the medial head of the gastrocnemius muscle than OP rats both before (determined from a muscle biopsy) and after the HFD feeding period. A greater proportion of type I fibers may be associated with a greater capacity for fat oxidation, which would favor resistance to body fat accumulation. Preexisting differences in muscle fiber composition may play a role in determining susceptibility to dietary obesity.