An analysis from the Women's Health Initiative (WHI) suggests not only that postmenopausal women who are overweight or obese have a higher risk of invasive breast cancer than women of normal weight but also that the excess risk increases as a woman's weight rises beyond obesity. The results were published online June 11, 2015, by JAMA Oncology.
A team of investigators from several medical centers studied data on 67,000 postmenopausal women who enrolled in the WHI between 1993 and 1998. They were followed for a median of 13 years. During that time, 3,388 invasive breast cancers were detected. The researchers analyzed the distribution of breast cancer among weight classes and calculated the risks for women who were overweight (body mass index, or BMI, of 25 to 30), obese (BMI 30 to 35), or very obese (BMI over 35) compared with women of normal weight (BMI 25 or less). They found that the increased risk of developing breast cancer ranged from 17% in women who were overweight to 59% in those with a BMI over 35. Among women who began the study at a normal weight, those who gained at least 5% of their original weight had a 12% higher risk of developing breast cancer than those who maintained their original weight. Neither losing weight nor using hormone therapy had a significant effect on risk for women of any weight.
You can calculate your personal risk at the National Cancer Institute website, www.cancer.gov/bcrisktool. The WHI analysis can give you a better sense of how being overweight or obese might influence your risk.