Obesity is a more powerful driver of breast cancer than previously thought, a new study suggests.

About 40% of hormone-positive breast cancers in postmenopausal women might be linked to excess body fat, researchers reported Oct. 15 in the Journal of Epidemiology & Community Health.

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That’s significantly more than previous estimates that linked excess weight to 1 in 10 breast cancer cases, based on measures of women’s body-mass index (BMI), researchers said.

The real-world impact of obesity on breast cancer risk likely has been underestimated because BMI isn’t a very accurate measure of body fat, the researchers argued.

“The findings of this study highlight the importance of considering more accurate measures of body fat than BMI to estimate the cancer burden attributable to obesity in postmenopausal breast cancer,” concluded the research team led by Veronica Davila-Batista, an associate professor of epidemiology with the University of Las Palmas de Gran Canaria in Spain.

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For this study, researchers compared BMI with a different measure of body fat called the CUN-BAE, an equation which accounts for gender and age in BMI measurements.

The two measures were used to weigh 1,022 older Spanish women with breast cancer and another 1,143 matched women who didn’t have cancer.

About 23% of breast cancer cases were linked to excess body weight as measured by BMI, researchers found.

However, about 38% of breast cancers were linked to excess fat as measured by the CUN-BAE, results show.

These differences were strongest for hormone-positive cancers, which are those driven by female hormones like estrogen, researchers said.

Twice as much risk for hormone-positive breast cancers could be linked to excess fat when using the CUN-BAE equation — 42%, compared with 20% when using BMI.

“The results of our study indicate that excess body fat is a significant risk factor for hormone receptor-positive breast cancer in postmenopausal women,” the researchers wrote in a journal news release.

Body fat can increase estrogen levels in women because fat tissue secretes the female hormone, according to the Cleveland Clinic.

“Our findings suggest that the population impact could be underestimated when using traditional BMI estimates, and that more accurate measures of body fat, such as CUN-BAE, should be considered when estimating the cancer burden attributable to obesity in postmenopausal breast cancer,” the research team concluded.