Obesity has long been recognized as a major risk factor for breast cancer. However, new research suggests its impact may be far greater than previously thought.

A recent study reveals that excess body fat may be responsible for 40% of postmenopausal hormone-positive breast cancer cases, a common type of breast cancer. This starkly contrasts current estimates based on body mass index (BMI), which links only 1 in 10 cases to excess weight. The new study findings suggest the need to rethink the effectiveness of BMI in assessing cancer risk.

Researchers point out that BMI may not be the ideal body fat measurement as it does not consider age, sex, or ethnicity. Hence for the study involving 1033 White postmenopausal women with breast cancer and 1143 without the disease, the researchers compared the BMI of the participants with another validated measure of body fat: CUN-BAE (Clínica Universidad de Navarra–Body Adiposity Estimator) which takes into account the age and sex of the participants. The results were published in the Journal of Epidemiology & Community Health.

The participants were part of the multi-case-control (MCC)-Spain study, which estimates environmental and genetic factors associated with cancers such as bowel, breast, stomach, and prostate. They were aged from 20-85 years.

The researchers evaluated dietary data and other risk factors including socio-demographics, lifestyle, and personal/family medical and reproductive histories.

Analysis based on BMI revealed the breast cancer group had an average BMI of just over 27 while the comparison group was just over 26. Meanwhile,

Average CUN-BAE was just under 40% for control and almost 40.5% for those in the breast cancer group.

The researchers also estimated that 23% of breast cancer cases were attributable to excess body fat while using BMI, but 38% using the CUN-BAE. However, the differences were observed only in hormone-positive breast cancers, which accounted for 680 cases in the study. For these cases, excess body fat was estimated to contribute to 20% of the risk when using BMI, but 42% when using the CUN-BAE measure.

"A CUN-BAE of 45% or above was associated with a more than doubling in the risk of postmenopausal breast cancer compared with a CUN-BAE of below 35%," the news release stated. However, no similar trend was noted with 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. 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 researchers wrote.

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