Now, new research takes aim at the prevailing theory for why that might be.
Being overweight or obese has been linked to menstrual cycle irregularities and other medical conditions that limit ovulation. Less ovulation means lower circulating levels of the breast-cancer-promoting hormones estradiol and progesterone. The thinking has been that obesity helps protect against breast cancer prior to menopause, but not after it, by reducing circulating levels of these sex hormones.
But the largest study ever to explore this hypothesis found little evidence to back it up.
Researchers from the Harvard Medical School and Brigham and Women's Hospital looked for markers of decreased ovulation among more than 113,000 premenopausal female nurses participating in an ongoing health study.
These markers included specific menstrual cycle characteristics, certain types of infertility, or having polycystic ovary syndrome (PCOS).
The participating nurses were followed from 1989 to 2003, during which time 1,398 cases of invasive breast cancer were diagnosed.
Women who were obese at the time of diagnosis had a 19% lower breast cancer risk than normal-weight women, after researchers adjusted for other disease risk factors such as family history and lifestyle and estimated ovulation history.
The protection was strongest among very young women. Being overweight or obese at age 18 was associated with a 43% lower risk of developing breast cancer prior to menopause.
"The earlier in life you look, the stronger the [protective] association between obesity and early breast cancer gets," researcher Karin B. Michels, ScD, PhD, tells WebMD.
The finding was puzzling, Michels says, because early obesity is increasingly believed to increase a woman's risk for developing breast cancer after menopause.
Risks Outweigh Benefits
Obesity remains one of the strongest risk factors for postmenopausal breast cancer. Around 80% of breast cancers are diagnosed in women who are 50 and over.
Because of this, the slight protective effect of obesity against disease early in life should not be taken as license to pack on the pounds at any point in life, Michels warns.
American Cancer Society epidemiologist Heather Spencer Feigelson, PhD, agrees.
"This small benefit doesn't come close to counterbalancing the risks associated with obesity, including postmenopausal breast cancer, diabetes, and heart disease," she tells WebMD.
The latest research is important, Spencer Feigelson says, because a better understanding of the mechanisms associated with premenopausal breast cancer may lead to better ways to prevent or treat the disease.
Michels and her Harvard colleagues will continue to study the impact of obesity on breast cancers diagnosed early in life.
"The hope is that we can prevent this very tragic illness among young women if we know more about its causes," she says.
SOURCES: Michels, K.B. Archives of Internal Medicine, Nov. 27, 2006. Karin B. Michels, ScD, PhD, associate professor, obstetrics, gynecology and reproductive biology, Harvard Medical School and Brigham and Women's Hospital. Heather Spencer Feigelson, PhD, strategic director of genetic epidemiology, American Cancer Society.
By Salynn Boyles
Reviewed by Louise Chang