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 Tuesday in the Journal of Epidemiology & Community Health. Advertisement 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