While it’s already known that excess body fat is a risk factor for hormone-positive breast cancer, the new percentage — more than 2 out of every 5 cases — is significantly higher than results from other studies.
“Our findings suggest that traditional measures like body mass index (BMI) to measure excess body fat may underestimate the risk, and using more accurate tools reveals a larger proportion of breast cancer cases attributable to extra fat,” says the corresponding author Verónica Dávila Batista, PhD, a postdoctoral researcher at the University of Las Palmas de Gran Canaria in the Canary Islands.
BMI May Not Be the Best Tool to Measure Fat and Obesity — Especially in Older Women
Obesity, measured traditionally by body mass index (BMI), has been linked to approximately 10 percent of postmenopausal breast cancer cases, according to the authors.
But BMI may not be the most accurate way to measure excess fat and obesity because it only considers height and weight, says Dr. Dávila Batista. “This can particularly be true in older women,” she adds.
For this study, researchers wanted to know if a more precise tool known as the Clínica Universidad de Navarra–Body Adiposity Estimator (CUN-BAE) would change the estimated increased risk of hormone-positive breast cancer.
Like BMI, the CUN-BAE uses height and weight, but it also takes age and sex into account, says Dávila Batista.
New Measurement Revealed That 42 Percent of Hormone Positive Breast Cancers Are Linked to Excess Fat
The study design was a case control (an observational study that compares two groups to examine risk factors of a disease) that included 1,033 women diagnosed with breast cancer and 1,143 postmenopausal women without breast cancer (the control group).
Researchers controlled for several factors that may influence breast cancer risk, such as diet, alcohol use, smoking, and medical and reproductive history.
When researchers used BMI to predict breast cancer risk, 23 percent of postmenopausal breast cancer cases were attributed to obesity, but when the CUN-BAE was used, this figure rose to 38 percent.
When focusing specifically on hormone receptor–positive cases — those more closely linked to obesity — the additional risk was even greater: 19.9 percent using BMI compared with 41.9 percent with CUN-BAE.
BMI is generally a good measure of body fatness in large population-based studies, but has limitations, says Fred K. Tabung, PhD, an assistant professor and cancer researcher in the department of internal medicine at the Ohio State University College of Medicine in Columbus.
By including sex and age, the new tool may be a more accurate way to measure body fat and associated risks, although the tool was developed in white populations and may not be generalizable to other races and ethnicities, says Dr. Tabung, who wasn’t involved in the study.
What Is Hormone Receptor–Positive Breast Cancer?
Hormone receptors are proteins found on breast cells. They pick up the estrogen or progesterone signals that promote cell growth, including cancer cell growth.
Breast cancer cells that have receptors for either hormone are considered hormone receptor–positive (HR+), or just hormone-positive, which means they have a lot of hormone receptors. Breast cancer cells can test positive for hormone receptors regardless of the type of breast cancer or breast cancer stage.
The Link Between Body Fat and Hormone-Positive Breast Cancer
Because the study was observational, the findings don’t prove that excess fat caused the increased risk. But it’s worth noting that investigators found that as body fat percentages increased, the associated risk also increased.
While women with 35 to 39.9 percent body fat had an increased breast cancer risk of about 50 percent compared with women with the lowest percentage of body fat, the risk for women with more than 40 percent body fat more than doubled.
Why would too much fat increase the risk of breast cancer? Before the onset of menopause, the ovaries are the body’s source of estrogen. After menopause, the source of estrogen production changes to peripheral tissues — primarily from fat, explains Tabung.
Excess body fat can cause low-level, chronic inflammation and lead to metabolic changes in fat tissue that increases estrogen production, which may fuel cancer growth, says Dávila Batista.
“As a result, higher levels of body fat create conditions that increase the likelihood of developing breast cancer, particularly after menopause,” she says.
Bottom Line: BMI May Underestimate Cancer Risk
The findings suggest that traditional measures like BMI may significantly underestimate the cancer burden linked to obesity, particularly for hormone receptor–positive breast cancer, the authors wrote.
More accurate measures of body fat such as the CUN-BAE could make targeted preventive efforts and public health strategies more effective, says Dávila Batista.
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