Metabolic markers, regional adiposity, and adipose cell size: relationship to insulin resistance in African-American as compared with Caucasian women

Abstract

Background/objectives: African-American women have the greatest prevalence of obesity in the United States, and higher rates of type 2 diabetes than Caucasian women, yet paradoxically lower plasma triglycerides (TG), visceral fat and intrahepatic fat, and higher high-density lipoprotein (HDL)-cholesterol. Visceral fat has not been evaluated against insulin resistance in African-American women, and TG/HDL-cholesterol has been criticized as a poor biomarker for insulin resistance in mixed-sex African-American populations. Adipocyte hypertrophy, reflecting adipocyte dysfunction, predicts insulin resistance in Caucasians, but has not been studied in African-Americans. Our goal was to assess whether traditional correlates of insulin resistance, measures of adiposity and adipocyte characteristics similarly predict peripheral insulin resistance in African-American and Caucasian women.

Subjects/methods: Thirty-four healthy African-American (n = 17) and Caucasian (n = 17) women, matched for age (mean = 53.0 yrs) and body mass index (BMI) (mean = 30 kg/m²), underwent a steady-state plasma glucose test to measure insulin sensitivity; computed tomography (fat distribution); and a periumbilical scalpel biopsy (adipocyte characterization). By-race analyzes utilized analysis of covariance; linear regressions evaluated relationships between metabolic/adipose variables. All analyses adjusted for BMI and menopausal status.

Results: Insulin sensitivity did not differ between groups (p = 0.65). Neither BMI, nor %body fat or thigh fat predicted insulin resistance in African-American women. Fasting TG (p = 0.046), HDL-cholesterol (p = 0.0006) and TG/HDL-cholesterol ratio (p = 0.009) strongly predicted insulin resistance in African-American women. Despite being lower in African-American women, hepatic fat and visceral adipose tissue (VAT) correlated with insulin resistance in both groups, as did fasting glucose, VAT/SAT (subcutaneous adipose tissue) ratio, and %SAT (inverse).

Conclusions: Total adiposity measures and adipocyte hypertrophy did not predict insulin resistance in African-American women, but did in Caucasian women. Plasma TG and HDL-cholesterol were significant predictors of insulin resistance in African-American women. Our findings demonstrate the need to identify race and sex-specific biomarkers for metabolic risk profiling.

Figure 1:

Relationships between insulin sensitivity and metabolic characteristics in African-American women compared to Caucasian women. African-Americans shown on left (●). Caucasians shown on right (○). Data shown are based on stepwise multiple linear regression analyses with adjustments for BMI and menopausal status.

Figure 2:

Relationships between insulin sensitivity and simple measures of obesity in African-American women compared to Caucasian women. African-Americans shown on left (●). Caucasians shown on right (○). Data shown are based on stepwise multiple linear regression analyses with adjustments for BMI and menopausal status.

Figure 3:

Relationships between insulin sensitivity and measures of fat distribution in African-American women compared to Caucasian women. African-Americans shown on left (●). Caucasians shown on right (○). Data shown are based on stepwise multiple linear regression analyses with adjustments for BMI and menopausal status.

Figure 4:

Peak diameter versus SSPG in African-American and Caucasian women. A) African-American women, solid circles (n=14); B) Caucasian women, open circles (n=14); Data shown are based on stepwise multiple linear regression analyses with adjustments for BMI and menopausal status. C) African-American women categorized by insulin sensitivity; D) Caucasian women categorized by insulin sensitivity. Insulin sensitive individuals are identified by squares. Insulin resistant individuals are identified by circles. Data shown based on linear regression analysis separated by insulin sensitivity.