Age, Obesity, and Breast Cancer Risk
Extra Weight May Lower Risk for Younger Women
Nov. 27, 2006 -- Obesity is a well-known risk factor for breast cancer after
menopause, but it also appears to help protect women from developing the
disease earlier in life.
Now, new research takes aim at the prevailing theory for why that might
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
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
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
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,
"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.