(CBS News) Obese and overweight women with breast cancer are significantly more likely to have the cancer return regardless of treatment, new research suggests.
The study, published online Aug. 27 in the American Cancer Society's journal, Cancer, suggests that excess body fat may cause hormonal changes in the body that fuel cancer to spread and recur.
"We found that obesity at diagnosis of breast cancer is associated with about a 30 percent higher risk of recurrence and a nearly 50 percent higher risk of death despite optimal treatment," study author Dr. Joseph Sparano, a professor of women's health at the Albert Einstein College of Medicine in New York City, said in a press release.
For the study, researchers looked at data from three studies sponsored by the National Cancer Institute that involved nearly 7,000 women. All the women had stage 1 to 3 breast cancer but were otherwise healthy with normal heart, kidney, liver and bone marrow function.
By looking at the participants' body mass indexes (BMI) - a ratio of height over body weight that scientists use to measure obesity - the scientists found that BMI increases significantly increases a women's risk of her breast cancer coming back. These women also were more likely to die of breast cancer than their thinner counterparts in the study.
The effect remained despite optimal treatments including chemotherapy and hormonal therapy. Women who had hormone receptor-positive breast cancer, the most common type of breast cancer that's found in two-thirds of all breast cancer cases worldwide were most likely to experience these negative outcomes if obese or overweight.
"To me the big news is that it is showing obesity is only related to one subtype [of breast cancer]," Dr. Bette Caan, a senior research scientist at the Kaiser Permanente Division of Research in Oakland, Calif. who was not involved in the study, said to HealthDay. "It's the most common subtype."
Sparano told TIME that overweight and obese people produce more estrogen, which may cause hormone receptor-positive cancers to grow. He also said insulin-resistance found in overweight people may contribute to this effect, or a combination of these factors along with general inflammation.
"Treatment strategies aimed at interfering with hormonal changes and inflammation caused by obesity may help reduce the risk of recurrence," Sparano said in the press release.
Obesity is associated with increased risks for cancers of the esophagus, breast, the lining of the uterus, colon and rectum, kidney, pancreas, thyroid and gallbladder, according to the National Cancer Institute.