Is the observed association between dairy intake and fibroids in African Americans explained by genetic ancestry?

Abstract

Uterine leiomyomata are a major source of gynecological morbidity and are 2-3 times more prevalent in African Americans than European Americans. In an earlier report, we found that dairy intake was inversely associated with uterine leiomyomata among African Americans. Because African Americans are more likely to have lactose intolerance and avoid dairy products, the observed association might have been confounded by genetic ancestry. This report reevaluates the dairy-uterine leiomyomata association after accounting for genetic ancestry among 1,968 cases and 2,183 noncases from the Black Women's Health Study (1997-2007). Dairy intake was estimated by using food frequency questionnaires in 1995 and 2001. Percent European ancestry was estimated by using a panel of ancestry informative markers. Incidence rate ratios and 95% confidence intervals were estimated by using Cox regression, with adjustment for potential confounders and percent European ancestry. Incidence rate ratios comparing 1, 2, 3, and ≥4 servings/day with <1 serving/day of dairy products were 0.95 (95% confidence interval (CI): 0.85, 1.06), 0.75 (95% CI: 0.61, 0.92), 0.77 (95% CI: 0.57, 1.04), and 0.59 (95% CI: 0.41, 0.86), respectively (Ptrend = 0.0003). These effect estimates were similar to those obtained without control for ancestry. The findings suggest that the observed inverse association between dairy consumption and uterine leiomyomata in African Americans is not explained by percent European ancestry.

Keywords: African Americans; dairy products; genetic ancestry; leiomyoma; prospective studies.

Figure 1.

Relation of percent European ancestry to incidence of uterine leiomyomata, Black Women's Health Study, 1997–2007. The graph was generated by using restricted cubic splines for percent European ancestry as a continuous variable, with 4 knots located at 10%, 15.9%, 22.5%, and 38.1% European ancestry. European ancestry was estimated by 22 ancestry informative markers in common between cases and noncases. The solid line represents the incidence rate ratio; dotted lines represent upper and lower confidence bounds.