Excess body fat is strongly linked to hormone-dependent breast cancer

Although excess body fat is already known to be a risk factor for hormone-positive breast cancer, the rate from the new study – more than 2 out of every 5 cases – is significantly higher than results from other studies.

“Our results suggest that traditional indicators such as body mass index (BMI) to measure excess body fat may underestimate the risk, and only the use of more accurate tools can detect a higher proportion of breast cancer cases associated with excess fat,” said study author Verónica Dávila Batista, a physician and doctoral student at the University of Las Palmas de Gran Canaria (Canary Islands).

Obesity, traditionally defined through body mass index (BMI), is linked to about 10 percent of postmenopausal breast cancer cases, scientists say.

However, BMI may not be the most accurate way to measure excess fat and obesity because it only takes height and weight into account, says Dr. Dávila Batista.

For this study, the researchers wanted to see if a more accurate assessment tool could better predict an increased risk of hormone-positive breast cancer.

Like BMI, this tool uses height and weight, but it also takes into account age and gender, which are very important in calculating body fat. Numerous studies have shown that this tool, Clínica Universidad de Navarra-Body Adiposity Estimator (CUN-BAE), is a better predictor of metabolic and cardiovascular risk, but this is the first study to use it to assess cancer risk.

The study included 1,033 women diagnosed with breast cancer and 1,143 postmenopausal women without breast cancer (control group).

The researchers controlled for several factors that may influence the risk of developing breast cancer, such as diet, alcohol use, smoking, medical and reproductive history.

When the researchers used body mass index to predict breast cancer risk, obesity explained 19.9 percent of postmenopausal breast cancer cases, but when the CUN-BAE tool was used, that number increased to 41.9 percent.

Hormone receptors are proteins located on breast cells. They pick up signals from the hormones estrogen or progesterone that promote the growth of cells, including cancer cells.

Breast cancer cells that have receptors for either hormone are considered hormone-receptor-positive (HR+), or simply hormone-positive, which means these cells have many hormone receptors.

About 70-80% of first diagnosed breast cancers are hormone-receptor-positive.

The researchers found that as the percentage of body fat increased, the associated risk also increased.

While women with 35-39.9% body fat had about a 50% increased risk of developing breast cancer compared to women with a low body fat percentage, women with more than 40% body fat more than doubled their risk.

Why does excess fat increase the risk of breast cancer? Before menopause, the ovaries are the source of estrogen in the body. After menopause, peripheral tissues – primarily fat – become the source of estrogen.

Excess body fat can cause low-level chronic inflammation and lead to metabolic changes in adipose tissue, which increases estrogen production, which can contribute to cancer.

“As a result, elevated levels of body fat create conditions that increase the likelihood of developing breast cancer, especially after menopause,” the researcher explains.

The findings suggest that traditional measures such as body mass index may significantly underestimate the cancer burden associated with obesity, especially in hormone-receptor-positive breast cancer.