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According to the results of a study published in the Archives of Internal Medicine, women who have a lower body weight at the age of 18 are more likely than other women to develop premenopausal breast cancer.

In postmenopausal women, several studies have reported that obesity increases the risk of breast cancer. This link may be explained by higher estrogen levels in obese postmenopausal women. After menopause, when ovarian hormone production drops dramatically, some estrogen continues to be produced in tissues outside the ovary, including fat tissue.

In premenopausal women, the link between obesity and breast cancer appears to be reversed. Studies have fairly consistently reported that obese women have a reduced risk of premenopausal breast cancer. Researchers have speculated that this, too, may be related to estrogen exposure. During the premenopausal years, obese women may be more likely to experience menstrual irregularities that result in lower estrogen exposure and a lower risk of breast cancer.

To explore the links among body size, menstrual characteristics, and premenopausal breast cancer, researchers evaluated information from the Nurses’ Health Study II. This study enrolled close to 120,000 female nurses who were between the ages of 25 and 42 when the study began in 1989.

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Body size was classified using the body mass index (BMI). BMI is a comparison of weight to height (weight in kilograms divided by height in meters squared). A BMI between 18.5 and 24.9 is generally considered healthy, a BMI between 25 and 29.9 is considered overweight, and a BMI of 30 or higher is considered obese.

During follow-up 1,398 cases of breast cancer were diagnosed among the 113,130 study participants who were premenopausal.

  • BMI at the age of 18 (as opposed to current BMI) appeared to have the strongest effect on breast cancer risk. Compared to women with a BMI between 20.0 and 22.4 at the age of 18, women with a BMI of 27.5 or higher were roughly 40% less likely to develop premenopausal breast cancer.
  • The link between low BMI and premenopausal breast cancer was not explained by menstrual cycle length, menstrual cycle regularity, or infertility due to an ovulatory disorder.

The researchers conclude that body size during early adulthood appears to influence the likelihood of premenopausal breast cancer-women who are leaner at the age of 18 are more likely than heavier women to develop premenopausal breast cancer. The link between body size and premenopausal breast cancer was not explained by factors such as menstrual regularity.

Reference: Michels KB, Terry KL, Willett WC. Longitudinal Study on the Role of Body Size in Premenopausal Breast Cancer. Archives of Internal Medicine. 2006;166:2395-2402.