A new study recently presented at the annual American Association for Cancer Research (AACR) Meeting in Washington, D.C. has reported that an increase in body mass index (BMI) between age 20 and age 50 is a significant risk factor for postmenopausal breast cancer. High mid-life BMI has consistently been found to be associated with higher risk of postmenopausal breast cancer, however above-normal BMI appears to offer protection against breast cancer at younger ages. Few studies have examined the impact of the timing of adult weight gain on breast cancer risk.

The study included 72,007 women who were 55 to 74 years old at baseline when they enrolled in the Prostate, Lung, Colorectal and Ovarian (PLCO) Cancer Screening Trial. Between 1993 and 2007, 3,677 postmenopausal breast cancer cases were diagnosed in the study group. Nearly 57 percent of the women experienced a BMI increase of at least 5 kg/m2 between age 20 and study entry. For a woman of height 5'4" an increase of 5 kg/m2 is equivalent to gaining approximately 30 pounds.

Since the positive association between BMI at study baseline and breast cancer risk appeared to be restricted to women who had never used hormone replacement therapy (HRT), the authors chose to focus on the results among women who never used HRT. Postmenopausal breast cancer risk was found to increase steadily across categories of increasing BMI at study baseline irrespective of BMI at age 20. In addition, having been overweight or obese at age 20 as well as at study baseline (when the women were past menopause) was found to reduce the risk of postmenopausal breast cancer compared to having been of normal weight at age 20 and subsequently having become overweight or obese. In other words, gaining weight over time increased the risk of breast cancer compared to maintaining a high, but relatively stable, BMI. Women who gained more than 5 kg/m2 between age 20 and enrollment in the trial had almost twice the postmenopausal breast cancer risk of women with no change or a reduction in BMI. The authors conclude that, among women who had never used HRT, an increase in BMI before and after age 50 was associated with increased postmenopausal breast cancer risk. The study results suggest that, with respect to postmenopausal breast cancer, healthy weight maintenance throughout adulthood is important. An increase in BMI between age 20 to age 50 may particularly heighten postmenopausal breast cancer risk.