Heavier Young Women at Lower Breast Cancer Risk

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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."

More information

Calculate your BMI at the U.S. Centers for Disease Control and Prevention (www.cdc.gov ).