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BMI Is Only Part of the Story

Several studies have suggested a link between a normal body mass index, or BMI, and lower cancer risk. But research by Neil Iyengar, a medical oncologist at Memorial Sloan Kettering Cancer Center, might call those findings into question. The BMI measures body fat in relation to height and weight and is recognized as a helpful, though not perfect, tool. For example, muscular people often have a higher BMI because muscle weighs more than fat. In addition, a person show an acceptable weight on the scale, but carry a high level of fat.

It’s this excess body fat that Iyengar is studying. At a recent conference, he presented his research 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 looked at 3,460 participants in the Women’s Health Initiative, an observational study that follows postmenopausal women between 50 and 79 years of age.

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

The researchers found women with the highest body fat were twice as likely to develop ER-positive breast cancer as those in the lowest fat group. The team noted the risk increased by 35 percent for each 11 pounds in total body fat, even when study participants’ BMI remained with the acceptable range.

“Our findings show the risk of invasive breast cancer is increased in postmenopausal women with normal BMI and higher levels of body fat, meaning a large proportion of the population has an unrecognized risk of developing cancer,” Iyengar told the American Association for Cancer Research. He cautioned that the findings of this study apply to postmenopausal women only.

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

The BMI is still the standard method to assess the correlation between body weight and the risk for a number of illnesses, including cancer, diabetes, heart disease, high blood pressure and stroke. For postmenopausal women a body fat assessment may prove to be an additional prevention strategy.


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