A high body mass index in adult women seems to reduce the risks of breast cancer in these women. It is not exactly known how and why things work this way but the statistics show it.

It was previously thought that it was due to ovarian problems obese women tend to develop. Ovarian problems mean that there are reduced levels of estrogen and progesterone, female hormones, whose high levels have been linked to increased breast cancer risk.

Unfortunately there has not been sufficient number of studies to confirm this theory.
Health experts have decided to study 113,130 premenopausal women nurses who have been watched since 1989. The women provided information about their teen weight, adult height, current weight, fertility problems, family history of breast cancer and the characteristics of their menstrual cycle. Follow up information have been added every two years until the year 2003 and included information about breast cancer and benign breast disease, childbirth, alcohol consumption, oral contraceptive use and physical activity.

For the period of 14 years, 1,398 cases of invasive breast cancer have occurred among the women. The statistics show that BMI of 30 or higher had a 19% lower risk of breast cancer in comparison to BMI between 20 and 22.4, with family history, personal characteristics, lifestyle habits and menstrual variables being adjusted. Women who were overweight and obese at the age of 18, with BMI of 27.5 and higher, had a 43% lower risk of developing breast cancer than those whose BMI’s were between 20 and 22.4.

The researchers still suspect that there are other reasons for these reduced risks than anovulation. They think it could be hormonal or the obese women may be less likely to get screened for breast cancer.