Many Hormone Receptor-Positive Breast Cancers Linked to Excess Body Fat

Much research shows a link between excess weight and breast cancer risk. A Spanish study of white post-menopausal women suggests that 38% of hormone receptor-positive breast cancer cases in this age group may be linked to excess body fat. This rate is much higher than the 10% of cases that have traditionally been associated with extra weight, which was calculated using body mass index (BMI).

“These findings suggest that the significance of excess body fat in post-menopausal hormone receptor-positive breast cancer could be underestimated when assessed using only BMI,” the researchers wrote.

They also pointed out that knowing the percentage of cancer cases linked to obesity is important when creating programs to reduce cancer risk.

While BMI can be a reliable indicator of body fat for many people, it isn’t perfect and has limitations. BMI is calculated by multiplying your weight in pounds by 703 and then dividing that number by your height in inches. If you use the metric system, you can calculate your BMI by dividing your weight in kilograms by your height in meters. There are also many online tables that will calculate your BMI for you after you enter your height and weight.

One of the biggest limitations of BMI is that it doesn’t make a distinction between lean body mass and bones and fat — the calculation simply considers total weight. As a result, BMI may overestimate body fat in people who are tall or have a lot of muscle mass. And BMI may underestimate body fat in people who are older or who have lost muscle mass, but have had no change in weight.

BMI is now seen as so problematic that in June 2023, the American Medical Association (AMA) adopted a policy saying that BMI is an imperfect way to measure body fat.

In the study, the researchers measured body fat using two methods: BMI and a body fat measuring tool they developed, called CUN-BAE (Clínica Universidad de Navarra–Body Adiposity Estimator). The CUN-BAE accounts for age and sex, but not ethnicity because it was developed using information from only white people.

The study included more than 2,100 white post-menopausal women: 1,033 diagnosed with breast cancer and 1,143 who hadn’t been diagnosed. The researchers collected information from the women about other possible risk factors, including lifestyle, diet, and health history.

Among the women in the study, average BMI was just above 26 for women without breast cancer and over 27 for women who had been diagnosed. People with a BMI of 25 to 29.9 are considered to have excess weight. People with a BMI of 30 or higher are considered to have obesity.

The average body fat estimates using the CUN-BAE tool were just under 40% in women without breast cancer and almost 40.5% in women who had been diagnosed.

The researchers’ analysis found that BMI linked 23% of the breast cancer cases to excess weight, while CUN-BAE linked 38%.

A CUN-BAE of 45% or higher body fat was linked to more than double the risk of developing breast cancer.

The study doesn’t prove that excess body fat causes breast cancer in post-menopausal women. But it does add to other research linking excess weight and a higher risk of being diagnosed, as well as higher risk of breast cancer coming back, also called recurrence.

If you’re struggling with excess weight, exercise can help you lose weight and boost your well-being, whether or not you’ve been diagnosed with breast cancer. One of the videos in our series, Exercises to Relieve Side Effects, is specifically designed to build muscle, which boosts metabolism and can help you lose weight.

Watch the Exercises to Help With Weight Management video.