Your confusion is understandable. The finding that extra fat raises the risk of breast cancer, even in women of normal weight, comes from research at New York’s Memorial Sloan Kettering Cancer Center. Doctors often rely on body mass index (BMI) to determine whether a patient is overweight. Your BMI is a rough measure of body fat based on your height and weight. This method has been around for about 200 years and has long been criticized as inadequate because it doesn’t tell us the body’s ratio of fat to lean muscle mass or bone. This ratio is important, because the more fat you have, the greater your risk of diabetes, heart disease, breast cancer and many other health problems.
The Sloan Kettering researchers looked at breast cancer risk among women participating in the Women’s Health Initiative, an ongoing observational study that tracks the health of postmenopausal women ages 50 to 79. They looked at women with a normal BMI (between 18.5 to 24.9) (who also had dual energy X-ray absorptiometry (DEXA), a whole body scan that assesses body fat levels more accurately than BMI. A study published in 2012 found that if DEXA were substituted for the BMI, the almost 4 in 10 adults then identified as “overweight” would be reclassified as obese.
At the outset of the Sloan Kettering study none of the 3,460 women participating had a history of breast cancer, but over the following 16 years, 182 of them developed the disease. Most of these cases – 146 – were estrogen-receptor-positive, indicating that estrogen had helped fuel the cancer growth. Fat is the primary source of estrogen production in postmenopausal women. The researchers reported that women whose body fat levels were in the top 25 percent according to their DEXA scans were twice as likely to develop breast cancer as women in the bottom 25 percent. They also found that the risk of estrogen-receptor-positive breast cancer rose by 35 percent for each 5-kilogram increase in whole body fat in women with a normal BMI. Study leader Neil Iyengar, M.D., noted that the level of physical activity was lower among women in the study with higher amounts of body fat, suggesting that physical activity may be key to breast cancer prevention.
While the study didn’t prove definitively that the women’s body fat and the estrogen it produces were responsible for their cancers, the risk remained even after taking account of other factors associated with the disease, including family history, use of hormone replacement therapy and exercise and drinking habits. The new findings pertain only to postmenopausal women. The suggestion that physical activity seems to protect against breast cancer fits in with the results of earlier research.
Andrew Weil, M.D.
Neil Iyengar et al, “High body fat levels associated with increased breast cancer risk in women with normal BMI.” Presentation at the American Association for Cancer Research Special Conference: Obesity and Cancer, January 26, 2018, Austin, Texas