Muscle fiber characteristics in postmenopausal women with normal or impaired glucose tolerance

Muscle fiber characteristics are altered in type 2 diabetes. We studied whether these alterations also exist in impaired glucose tolerance (IGT) and whether they are determinants of insulin sensitivity and glucose tolerance in postmenopausal women.

Percutaneous muscle biopsies from the vastus lateralis muscle were obtained from 77 postmenopausal women aged 57-59 years: 50 women with normal glucose tolerance (NGT) and 27 with IGT. The IGT group had a reduced insulin sensitivity compared with the NGT group (euglycemic-hyperinsulinemic clamp) (P = 0.003).

The groups did not differ in muscle fiber composition, as judged by the percentage of type I, IIa, or IIx fibers. In contrast, the IGT group had increased size of the IIa (mean +/-SD 3,776+/-987 vs. 3,078+/-862 microm2, P = 0.002) and IIx fibers (2,730+/-1,037 vs. 2,253+/-672 microm2, P = 0.017). There was a trend for the capillary diffusion areas (the muscle area supplied by each capillary) to be larger in the IGT group for the IIa (1,132+/-286 vs. 1,013+/-240 microm2, P = 0.061) and IIx fibers (1,020+/-246 vs. 906+/-240 microm2, P = 0.058). In the entire group, insulin sensitivity correlated with the size of the type IIa fibers (r = -0.28, P = 0.013), but not with the percentages of muscle fiber types. In a multiple regression, insulin sensitivity was determined by body fat content and HDL cholesterol level, while the size of the IIa fibers was not included in the model. Glucose tolerance was independently predicted by the number of capillaries/type I fiber, as well as by insulin sensitivity and triglyceride levels.

We conclude that although muscle fiber composition is not altered, women with IGT have larger type IIa and IIx muscle fibers and a trend for increased capillary diffusion areas for these fibers, compared with women with NGT. In the entire group, insulin sensitivity was determined mainly by body fat content, while muscle fiber capillarization may be of importance for glucose tolerance.