Adiposity and β-cell function: relationships differ with ethnicity and age.

The prevalence of type 2 diabetes is higher among African Americans (AA) vs European Americans (EA), is highest at middle age, and is related to obesity. This study was conducted to test the hypothesis that the association of adiposity (percent body fat; %fat) with indices of insulin sensitivity and β-cell function would differ with ethnicity and age. Subjects were 168 healthy, normoglycemic AA and EA girls and women aged 7–12 yr, 18–32 yr, and 40–70 yr. An intravenous glucose tolerance test was used to assess indices of insulin secretion and action: Insulin sensitivity (SI), acute C-peptide secretion (X0); basal, first-phase, second-phase, and total β-cell responsivity to glucose (PhiB, Phi1, Phi2, and PhiTOT, respectively); and the disposition index (DI = SI × PhiTOT). %fat was assessed with dual-energy X-ray absorptiometry. Adiposity was significantly associated with insulin sensitivity among EA (−0.57 P<0.001) but not AA (−0.20, P=0.09). Adiposity appeared stimulatory to β-cell function in the two groups of younger subjects and in EA, but inhibitory in postmenopausal women, particularly AA postmenopausal women. Among AA postmenopausal women, %fat was inversely associated with Phi1 (r = −0.57, P<0.05) and PhiTOT (r = −0.68, P<0.01). These results suggest that the impact of adiposity on insulin secretion and action differs with age and ethnicity.