New research shows a link between obesity and an aggressive type of breast cancer.

Previous studies have linked obesity to estrogen-fueled breast cancer. But now scientists say that being overweight also increases the risk of triple-negative breast cancer.

Triple negative is not fueled by estrogen and is a rare and more deadly cancer type. It makes up between 10 and 20 percent of all cancers of the breast.

The finding that obesity also appears to raise the risk for triple-negative tumors, which are not fueled by estrogen, was unexpected, study researcher Amanda I. Phipps, PhD, a postdoctoral fellow at the Fred Hutchinson Cancer Research Center, told WebMD.

"Breast cancer is not just one disease. It is a complex combination of many diseases," she explained. "The fact that we found an association with triple-negative breast cancer is unique because, biologically, this subtype is very different from other breast cancers."

Women who are overweight have a 35 percent higher risk of developing triple negative breast cancer.

Researchers evaluated data from 155,723 women enrolled in the Women's Health Initiative, a large-scale study of postmenopausal women begun in 1993. The study's findings were published in Cancer Epidemiology, Biomarkers & Prevention, a journal of the American Association for Cancer Research.