Contrary to long-held theory, obesity may not increase the risk of breast cancer among older African-American women, according to research performed at Boston University and Howard University in Washington, D.C.

However, the study confirmed previous work that younger women with a high body mass index (BMI) have a reduced risk of breast cancer. The findings were presented at the annual meeting of the American Association for Cancer Research.

The research team, led by Julie R. Palmer, Sc.D., professor of epidemiology at the Boston University School of Public Health, focused on the relationship between breast cancer and various measures of obesity in African-American women, in particular, because the prevalence of obesity in the U.S. is highest in the black population.

There is also a tremendous amount of information available from the Black Women's Health study, which enrolled 59,000 participants in 1995, and followed up on the health and medical status of each with biennial questionnaires.

After eight years of follow-up, 809 women reported having breast cancer – about half of them premenopausal and half postmenopausal.

Whether a breast cancer occurs before or after menopause is an important consideration in light of the association between estrogen-fueled breast cell proliferation and the development and progression of breast cancer. Palmer's group confirmed the findings of previous studies that a higher BMI at age 18 decreases a woman's risk of developing premenopausal breast cancer and, possibly, postmenopausal breast cancer as well.

"Some previous research, but not all, has shown that high body mass index among older women increases breast cancer risk," Palmer explained. "We actually found the opposite in our study. African-American women with a BMI that qualifies them as obese did not have a higher risk of getting breast cancer. In fact, the risk was slightly lower in obese women overall, in agreement with findings from another study of black women."

One possible explanation for the discrepancy between the findings for black and white women has to do with differences in the timing of excess weight gain, with black women, in general, becoming overweight at younger ages than white women. There is some evidence that the adverse effect of overweight on postmenopausal breast cancer risk is strongest in the decades closely following the weight gain. Indeed, in Palmer's study, while there was no increased risk with current BMI overall, there was an association between increasing risk and increasing BMI among postmenopausal women under age 50. Their breast cancer would have developed most closely in time to their maximum weight gain.

The latest report from the American Cancer Society states that the overall rate of breast cancer is 17 percent lower in African-American women than in white women, but the five-year survival rate is 14 percent higher in blacks.

Noting the health benefits of maintaining an optimum weight, Palmer points out that her research does not change that message. "The study results may help explain why breast cancer incidence is lower in black women than white women after age 50 even though the prevalence of obesity is higher," she said.

SOURCE:
Annual Meeting of the American Association for Cancer Research, April 3, 2006, Washington, DC