In the National Heart, Lung, and Blood Institute's Growth and Health Study, the researchers examined the association of obesity, puberty, and race with fasting insulin, glucose, and homeostasis model assessment of insulin resistance (HOMA-IR) measures at baseline (9 to 10 years of age) and 10 years later.

Baseline and year-10 BMI was significantly greater in black girls than in white girls. The prevalence of obesity in black girls at baseline and year 10 was 17.6% and 28.8%, respectively, compared with 6.2% and 11.2%, respectively, for white girls (p < 0.0001). Black girls had a 2.5-fold increased 10-year incidence of obesity compared to white girls (13.2% versus 5.2%).

"BMI-insulin correlations were positive in both black and white girls at both visits, but insulin remained higher in black girls after controlling for BMI," Dr. Morrison and colleagues explain.

Insulin and HOMA-IR were higher in the prepubertal period, before the emergence of racial differences in BMI, in black girls compared to white girls. Insulin and HOMA-IR increased more during puberty and decreased less after completion of puberty in black girls.

In black girls, baseline BMI was predictive of year-10 glucose (p < 0.005) and the development of impaired fasting glucose (p = 0.02). The rate of BMI increase predicted these outcomes in white girls (p < 0.0001 and p = 0.002, respectively).

The 10-year incidence of type 2 diabetes in black girls was 1.4%, while no white girls became overtly diabetic.

The investigators say the findings "strengthen the possibility that there are underlying ethnic differences in gene polymorphisms that influence insulin sensitivity or insulin production or metabolism."