Higher acute insulin response to glucose may determine greater free fatty acid clearance in African-American women

Abstract

Context: Obesity and diabetes are more common in African-Americans than whites. Because free fatty acids (FFA) participate in the development of these conditions, studying race differences in the regulation of FFA and glucose by insulin is essential.

Objective: The objective of the study was to determine whether race differences exist in glucose and FFA response to insulin.

Design: This was a cross-sectional study.

Setting: The study was conducted at a clinical research center.

Participants: Thirty-four premenopausal women (17 African-Americans, 17 whites) matched for age [36 ± 10 yr (mean ± sd)] and body mass index (30.0 ± 6.7 kg/m²).

Interventions: Insulin-modified frequently sampled iv glucose tolerance tests were performed with data analyzed by separate minimal models for glucose and FFA.

Main outcome measures: Glucose measures were insulin sensitivity index (S(I)) and acute insulin response to glucose (AIRg). FFA measures were FFA clearance rate (c(f)).

Results: Body mass index was similar but fat mass was higher in African-Americans than whites (P < 0.01). Compared with whites, African-Americans had lower S(I) (3.71 ± 1.55 vs. 5.23 ± 2.74 [×10⁻⁴ min⁻¹/(microunits per milliliter)] (P = 0.05) and higher AIRg (642 ± 379 vs. 263 ± 206 mU/liter⁻¹ · min, P < 0.01). Adjusting for fat mass, African-Americans had higher FFA clearance, c(f) (0.13 ± 0.06 vs. 0.08 ± 0.05 min⁻¹, P < 0.01). After adjusting for AIRg, the race difference in c(f) was no longer present (P = 0.51). For all women, the relationship between c(f) and AIRg was significant (r = 0.64, P < 0.01), but the relationship between c(f) and S(I) was not (r = -0.07, P = 0.71). The same pattern persisted when the two groups were studied separately.

Conclusion: African-American women were more insulin resistant than white women, yet they had greater FFA clearance. Acutely higher insulin concentrations in African-American women accounted for higher FFA clearance.

Fig. 1.

Schematic for glucose and FFA minimal models. G, Plasma glucose concentration; X, insulin action in a remote compartment; F, free fatty acids; I, plasma insulin concentration; l₀, baseline nonsuppressible lipolysis rate; l₂, difference between maximum and nonsuppressible lipolysis rate.

Fig. 2.

Glucose, insulin, and FFA time course during the IM-FSIGT. African-American women are presented by blue lines and white women by green lines. A, Glucose concentrations (mean ± se). B, Insulin concentrations (mean ± se). Inset shows race difference in insulin concentration between 2 and 30 min (all P < 0.05). C, FFA concentrations (mean ± se). Vertical dashed lines demarcate the time period from which data are obtained for entry into the FFA minimal model (i.e. 10–120 min).

Fig. 3.

Box and whiskers plot for S_I, AIRg, and DI. Data are presented by race. A, S_I. B, AIRg. C, DI. Percentiles starting with lowest error bar are: 10th, 25th, median, 75th, and 90th. *, P < 0.05; **, P < 0.01.

Fig. 4.

Hyperbolic curve of the DI calculated as the product of AIRg and S_I. African-American women are represented by blue lines and symbols and white women by green lines and symbols.

Fig. 5.

Correlation between AIRg and c_f (A) and S_I vs. c_f (B). African-American women are represented by blue lines and symbols and white women by green lines and symbols.