TUESDAY, Nov. 28, 2006 (HealthDay News) -- Women who are heavier in young adulthood have a lower risk of developing breast cancer before they reach menopause, new research shows.
The finding, published in the Nov. 27 issue of the Archives of Internal Medicine, confirms and expands on previous research suggesting that larger body size before menopause helps fend off breast cancer.
That's not a license for women to gain weight, however, experts stressed.
"We have to remind women that obesity is pretty much bad for everything else and, as soon as you become postmenopausal, obesity is one of the strongest predictors of breast cancer," said lead researcher Karin B. Michels, an associate professor of obstetrics, gynecology and reproductive biology at Harvard Medical School and Brigham and Women's Hospital in Boston.
"It's never a good idea to be obese because of other health effects," added Dr. Jay Brooks, chairman of hematology/oncology at Ochsner Health System in Baton Rouge, La. "The problem is that women who are postmenopausal, which is where the largest number of breast cancers are, have a very hard time losing weight."
Recent research has shown that postmenopausal women can cut their breast cancer risk by shedding extra pounds.
However, the situation is somewhat different for younger, premenopausal females. In this group, prior studies had shown that as body-mass index (BMI) went up, breast cancer risk declined.
Some had speculated that this was because a higher BMI is associated with irregular or long menstrual cycles and the development of polycystic ovary syndrome --- all of which are associated with problems in ovulation. Ovulatory dysfunction, in turn, decreases levels of estrogen and progesterone. Lower levels of these sex hormones -- long linked to breast cancer -- could account for a lowered risk of malignancy.
For this study, Michels and her colleagues tracked outcomes for more than 113,000 premenopausal women who were part of the Nurses' Health Study II, a group of female registered nurses who have been followed by researchers since 1989. The women were followed until 2003, or until they developed breast or other cancer, died or reached menopause, whichever came first.
Women with a current BMI of 30 or higher (considered obese) had a 19 percent lower risk of breast cancer compared with those who had BMIs of 20 to 22.4 (in the normal range).
Women whose BMI at age 18 was 27.5 or higher (considered overweight) had a 43 percent lower risk of developing breast cancer, compared with women whose 18-year-old BMI was between 20 and 22.4.
"We have indeed seen this inverse association between BMI and premenopausal breast cancer, but what's new is that the earlier we go in a woman's life, the stronger that association becomes," Michels said. "BMI earlier in her life is more important than her more immediate past."
The findings held true even after adjusting for menstrual cycle patterns and infertility as a result of ovulatory disorder (usually polycystic ovary syndrome or the use of birth control pills).
"We basically figured that ovulation was not an explanation -- which had been hypothesized before, but nobody had put it to the test," Michels said.
So, the exact explanation for the weight-breast cancer link in young women remains unclear.
"We don't really know what the explanation is," Michels said.
For now, the findings may be more pertinent to researchers than to patients.
"It doesn't necessarily have clinical implications other than to say that we want to be aware of earlier life and of the importance of earlier life," Michels said. "For researchers, it is important and interesting to better understand premenopausal breast cancer, because it is not very well understood, and it's increasing. And, I think, it's particularly tragic when young women get breast cancer."
Calculate your BMI at the U.S. Centers for Disease Control and Prevention.