(HealthDay News) — In estrogen receptor (ER)+ breast cancer, obesity is positively associated with breast cancer mortality only among pre-/peri-menopausal women, according to new research. These findings have been released in advance of presentation at the annual meeting of the American Society of Clinical Oncology, to be held from May 30 to June 3 in Chicago.

Hongchao Pan, Ph.D., from the University of Oxford in the United Kingdom, and colleagues used data from the Early Breast Cancer Trialists’ Collaborative Group study to analyze the independent effects of body mass index (BMI) on outcome. Data were included for 80,000 patients in 70 trials, with mean follow-up of eight women-years.

The researchers observed little correlation between BMI and breast cancer mortality for 20,000 women with ER+ disease, and the association no longer persisted after adjustment for tumor diameter and nodal status. BMI correlated positively with breast cancer mortality in 60,000 women with ER+ disease, including women who were pre-, peri-, and post-menopause. After adjustment for tumor characteristics, the correlation persisted only for the 20,000 pre-/peri-menopausal women with ER+ disease (breast cancer mortality rate ratio for BMI ≥30 versus 20 to 25 kg/m², 1.34; P < 0.00001). For the 40,000 post-menopausal women with ER+ disease, little association persisted (rate ratio, 1.6; P = 0.12).

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“This means we don’t understand the main biological mechanisms by which obesity affects prognosis,” Pan said in a statement.

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