BMI Not Best Measure of Body Fat When Estimating Breast Cancer Risk in Postmenopausal Women

BMI Not Best Measure of Body Fat When Estimating Breast Cancer Risk in Postmenopausal Women

A study suggests more detailed measures of body fat, such as a DEXA scan, are better than BMI when estimating breast cancer risk in postmenopausal women.
Jan 2, 2019.

A study suggests more detailed measures of body fat, such as a dual-energy X-ray absorptiometry (DEXA) scan, are better than body mass index (BMI) when estimating breast cancer risk in postmenopausal women.

The research was published online on Dec. 6, 2018, by the journal JAMA Oncology. Read the abstract of“Association of Body Fat and Risk of Breast Cancer in Postmenopausal Women With Normal Body Mass Index: A Secondary Analysis of a Randomized Clinical Trial and Observational Study.


Extra weight and breast cancer risk

Women who are overweight or obese — defined as having a BMI higher than 25 — have a higher risk of being diagnosed with breast cancer compared to women who maintain a healthy weight, especially after menopause. Being overweight also can increase the risk of breast cancer coming back (recurrence) in women who’ve been diagnosed with the disease.

This higher risk is partially because fat cells make estrogen — extra fat cells mean more estrogen in the body, and estrogen can make hormone-receptor-positive breast cancers develop and grow. Scientists also have found that extra fat cells can trigger long-term, low-grade inflammation in the body. Chronic inflammation has been linked to a higher risk of breast cancer recurrence. The proteins secreted by the immune system seem to stimulate breast cancer cells to grow, especially estrogen-receptor-positive breast cancer in postmenopausal women.

Statistics show that more than 66% of U.S. women are overweight or obese, which puts them at higher risk for breast cancer.


BMI isn’t perfect

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.


Do women with a normal BMI but more body fat have a higher risk of breast cancer?

In this study researchers wanted to know if women who had a BMI of 18.5 to 24.9 — which would make them neither overweight nor obese — but higher levels of overall body fat, had a higher-than-average risk of breast cancer.

The analysis is part of the very large Women’s Health Initiative Observational Study, commonly called the WHI. Overall, the study includes information from more than 161,608 postmenopausal women who were ages 50 to 79 when they joined from 1993 to 1998. None of the women had been diagnosed with breast cancer when they joined the study. The WHI wants to find any links between health, diet, and lifestyle factors and health problems such as cancer.

This analysis included 3,460 postmenopausal women with a BMI ranging from 18.5 to 24.9. The women’s body fat was measured with a DEXA scan, which is commonly used to measure bone density and diagnose osteoporosis. A DEXA scan can differentiate between lean body mass, bones, and body fat. The women had DEXA scan body fat measurements when they joined the study, and again 1, 3, 6, and 9 years after joining the study.

Follow-up time ranged from 9 to 20 years.


The results

During follow-up, 182 breast cancers were diagnosed; 146 of the breast cancers were hormone-receptor-positive.

Overall, the researchers found that the more body fat the women had as measured by the DEXA scan, the higher their risk of breast cancer. Compared to women with the lowest amount of fat measured by DEXA scan, women with the highest amount of body fat had an 89% higher risk of breast cancer.

This link was even stronger for hormone-receptor-positive breast cancer. Women with the highest levels of body fat had more than double the risk of hormone-receptor-positive breast cancer compared to women with the lowest amounts of body fat.

“To our knowledge, this is the first study of body fat and risk of invasive breast cancer in a cohort of women with exclusively normal BMI,” the researchers wrote. “Normal BMI categorization may be an inadequate proxy for the risk of breast cancer in postmenopausal women. Postmenopausal women with higher body fat levels are at elevated risk for breast cancer despite having a normal BMI.

“When considering an individual's health, physicians generally assess BMI by absolute categorical levels (i.e., normal, overweight, or obese),” the researchers continued. “As such, increased adiposity in an individual categorized as having normal BMI is likely to remain clinically unrecognized. Indeed, nearly half of individuals in this study who had the highest amounts of trunk fat had BMIs within the lower quartiles.”

“These findings will probably be surprising to many doctors and patients alike, as BMI is the current standard method to assess the risks for diseases related to body weight,” said Andrew Dannenberg, M.D., one of the study’s authors and associate director of cancer prevention at the Sandra and Edward Meyer Cancer Center at Weill Cornell Medicine. “We hope that our findings will alert women to the possibility of increased breast cancer risk related to body fat, even if they have a healthy weight.”


What does this mean for you?

While the results of this study are concerning, they underscore the importance of exercise and being active throughout your entire life. Exercise is one of the best ways to maintain your muscle mass and a healthy weight, as well as keep your bones healthy and strong. Exercise also can help boost your self-esteem, help you sleep better, and help you handle any stress in your life.

If you’re busy with work, household chores, and family matters, finding time to start an exercise program can be hard.

It can help to break up your exercise into 20- or 30-minute sessions that add up to about 5 or more hours per week. Walking is a great way to start. Maybe you walk 30 minutes in the morning and 30 minutes after lunch. You can add a few more minutes by parking farther away from stores when you shop or taking mass transit. Or you can make plans to walk with a friend — you’re more likely to stick with an exercise regimen if someone else is counting on you. Plus, you can socialize at the same time.

If you’re a postmenopausal woman and have a normal BMI but feel a little weaker than you used to be, or if you feel you have lost muscle mass for any reason, you may want to talk to your doctor about assessing your body fat mass with a DEXA scan. Many breast imaging centers provide this service at a reasonable cost. Your doctor can order this test to assess your bone health — and your body composition can be checked at the same time. Once you have an accurate assessment of your body fat, you can talk to your doctor about how it may affect your personal risk of breast cancer.

For more information on how you can keep your risk of breast cancer as low as it can be, visit the Breast Cancer Risk Factors pages.

Written by: Jamie DePolo, senior editor

— Last updated on September 26, 2022, 5:09 PM

Reviewed by 1 medical adviser
Brian Wojciechowski, MD
Crozer Health System, Philadelphia area, PA
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