Background: Epidemiologic studies, largely of white women, have found that recent long-term female hormone use, particularly use of estrogen with progestin, is associated with an increased risk of breast cancer. Some studies suggest that the increase is greater among leaner women. Our aim was to assess the relation of female hormone use to incidence of breast cancer in black women, with attention to differences in effect according to body mass index.
Methods: Data on female hormone use, breast cancer risk factors, and the occurrence of breast cancer were collected through biennial questionnaires from 1995 through 2003 in the Black Women's Health Study, a follow-up study of US black women. During 182 629 person-years of follow-up of 32 559 women 40 years or older, 615 cases of breast cancer were reported.
Results: The incidence rate ratio for breast cancer in women recently using female hormone supplements relative to those who had never used female hormones, with control for confounding factors, increased with duration of use and was 1.58 (95% confidence interval [CI], 1.12-2.23) for 10 or more years of use; the incidence rate ratios were 1.41 (95% CI, 0.95-2.10) for 10 or more years of use of estrogen alone, and 1.45 (95% CI, 0.94-2.23) for 5 or more years of use of estrogen with progestin. The association of breast cancer with female hormone use was stronger among leaner women (body mass index [calculated as weight in kilograms divided by the square of height in meters] <25) than among heavier women. Among the leaner women who recently used female hormone supplements for durations of 10 or more years, the incidence rate ratio was 3.08 (95% CI, 1.70-5.56); the corresponding estimates among women with body mass indexes of 25 to 29 and 30 or greater were 1.43 and 0.91, respectively, and neither was statistically significant.
Conclusion: These results based on data from US black women strengthen the evidence that use of estrogen alone and estrogen with progestin increases the risk of breast cancer and that the association is stronger among leaner women.