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High BMI in pre-menopausal women lowers breast cancer risk
Written by Jeanne Bohm, Ph.D.   
A higher body mass index (BMI) in early adulthood may reduce breast cancer risk before menopause, according to an article in the November 27 issue of Archives of Internal Medicine.

Previous studies have shown an association between higher body mass index and a lower breast cancer risk in premenopausal women. It was believed that the risk reduction was caused by a disruption of ovulation and the ensuing decrease of estradiol and progesterone levels associated with high BMI. A recent study, however, suggests that a high BMI reduces breast cancer risk even with a normal ovulation history.

Karin B. Michels and co-workers at Brigham and Women's Hospital and Harvard Medical School studied 113,130 premenopausal women as part of a larger study of female registered nurses. They found that women with a current BMI of 30 or higher had a 19 percent lower risk of breast cancer than women with a BMI between 20 and 22.4. Women whose BMI was 27.5 or higher at age 18 had a 43 percent lower risk of developing breast cancer than those whose 18-year-old BMI was between 20 and 22.4.

The researchers concluded that a low BMI may be a risk factor for the development of breast cancer before menopause. However, after menopause, women with a high BMI show a greater risk of developing breast cancer.

Surprisingly, the authors also found that anovulation does not seem to be a primary explanation for the reduced risk in heavier women. Adjusting breast cancer rates for menstrual cycle patterns, infertility due to ovulatory disorder, probable polycystic ovary syndrome and use of oral contraceptives did not lower the association with BMI. The association was true among women with no history of infertility due to ovulatory problems, another indication that anovulation was not the primary mechanism.


AMA Media Release

Longitudinal Study on the Role of Body Size in Premenopausal Breast Cancer. Arch Intern Med. 2006;166:2395-2402.

About the Author

Jeanne Bohm, Ph.D. is a cancer biologist by training, a medical writer and an experienced science educator.

The author has no financial relationship to any of the companies listed in the article.

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