BMI does not tell the whole story

Several studies have demonstrated a link between a normal body mass index, or BMI, and lower cancer risk. But research by Dr. Neil Iyengar, a medical oncologist at Memorial Sloan Kettering Cancer Center, might put a bit of a wrinkle in those findings. The BMI measures body fat in relation to height and weight. It is recognized as a helpful, albeit imperfect, tool. For instance, muscular people often have a higher BMI because muscle weighs more than fat. In addition, a person can be of an acceptable weight but carry too much fat.

It’s the excess body fat that got Iyengar’s attention. At the recent conference Obesity and Cancer: Mechanisms Underlying Etiology and Outcomes hosted by the American Association for Cancer Research, he presented his research finding that postmenopausal women with a normal BMI had an increased risk of invasive breast cancer if they had higher levels of body fat. The investigators evaluated 3,460 participants in the Women’s Health Initiative, an observational study that follows the health of postmenopausal women between the ages of 50 and 79.

The women studied had a normal BMI (between 18.5 and 24.9) and no history of breast cancer. Their body fat was measured by an x-ray technique known as DXA. According to the American Council on Exercise, 25 to 31 percent body fat is an acceptable range for most women. During the roughly 16 years of follow-up, 80 percent of the women who developed invasive breast cancer were diagnosed with a specific type of breast cancer called estrogen receptor-positive, or ER-positive. This diagnosis is significant because many breast cancers are fueled by estrogen, and in postmenopausal women, fat tissue is the main source of estrogen synthesis.

The researchers found that women with the highest body fat were twice as likely to develop ER-positive breast cancer as those in the lowest fat group. Moreover, the team noted that the risk increased by 35 percent for each 5-kilogram (11 pounds) growth in total body fat, even as the BMI remained within normal limits.

“Our findings show that the risk of invasive breast cancer is increased in postmenopausal women with normal BMI and higher levels of body fat, meaning that a large proportion of the population has an unrecognized risk of developing cancer,” Iyengar told the American Association for Cancer Research. He also cautioned that the findings of this study apply to only postmenopausal women and cannot be generalized to other populations or other cancers.

Another significant takeaway of the study is that the level of physical activity was lower in women with higher amounts of fat. This finding underscores the value of healthy eating and exercise even in those who are not overweight or obese.

The BMI remains the standard method to assess the correlation between body weight and the risk for several diseases, including cancer, diabetes, stroke, high blood pressure and heart disease. For postmenopausal women, however, an assessment of body fat may prove to be an additional effective strategy for prevention.