Obese premenopausal African-American women with normal and impaired glucose tolerance have a similar degree of insulin resistance but differ in β-cell function

OBJECTIVE - To determine whether insulin resistance and secretion differ in obese pre-menopausal African-American women with and without glucose intolerance. RESEARCH DESIGN AND METHODS - A total of 63 women underwent oral glucose tolerance tests (OGTTs). A total of 48 women underwent frequently sampled intravenous glucose tolerance tests (FSIGTs). Insulin resistance was determined from the insulin sensitivity index (S_I) from the FSIGT. Insulin secretion during the OGTT was determined by (I_{30 min} - I_{0 min})/(G_{30 min} ^-G_{0 min}) and during the FSIGT by the acute insulin response to glucose (AIRg). The disposition index, the product of AIRg and S₁, was used to determine whether AIRg was adequate to compensate for insulin resistance. Statistical analyses included one-way analysis of variance with Bonferroni corrections for multiple comparisons and regression analyses. RESULTS - The women were divided into three groups: nonobese glucose tolerant (n = 32), obese glucose tolerant (n = 17), and obese glucose intolerant (n = 14). The BMI of the three groups were 24.8 ± 2.3, 37.8 ± 5.5, and 42.0 ± 7.6 kg/m² (mean ± SD), respectively (P < 0.0001). The ages of the three groups were 34.9 ± 8.4, 32.1 ± 5.0, and 41.1 ± 6.3 years (P = 0.011). S_I was higher in the nonobese women than in the obese glucose-tolerant women (3.99 ± 1.44 vs. 2.66 ± 2.14 l · mU^-1 · min^-1, P = 0.03). S_I was similar in the obese glucose-intolerant and obese glucose-tolerant women (2.12 ± 1.27 vs. 2.66 ± 2.14 l · mU^-1 · min^-1, P = 0.9). OGTT showed that insulin secretion was lower in the glucose-intolerant than the obese glucose-tolerant women (1.73 ± 1.38 vs. 3.62 ± 2.11, P = 0.005). FSIGT showed that AIRg was not significantly lower in glucose-intolerant than in obese glucose-tolerant women (807 ± 665 vs. 1,253 ± 655 mU · l^-1 · min, P = 0.078). The disposition index was lower in glucose-intolerant than in obese glucose-tolerant women (1,324 ± 1,061 vs. 2,656 ± 1,415, P = 0.014). CONCLUSIONS - Obese premenopausal African-American women with and without glucose intolerance have a similar degree of insulin resistance but differ in insulin secretion.