British researchers looked at a data set of 80,000 women across 70 clinical trials and found that being obese raised mortality risk by one third in young women with a certain type of tumor.
The higher mortality was seen in women who had not entered menopause and whose tumors were a type known as estrogen-receptor (ER) positive, said the findings, released ahead of a May 31 presentation at the American Society for Clinical Oncology conference in Chicago.
ER-positive tumors are the most common in breast cancer, making up about 75 percent of cases.
In young women with ER-positive breast cancer, obesity was associated with a 34 percent higher risk of breast cancer-related death than in postmenopausal women, or women with ER-negative cancers.
In contrast, obesity had little effect on breast cancer outcomes among 40,000 post-menopausal women with ER-positive disease, or among the 20,000 who had ER-negative tumors.
"Obesity substantially increases blood estrogen levels only in post-menopausal women, so we were surprised to find that obesity adversely impacted outcomes only in pre-menopausal women," said Hongchao Pan, a researcher at Oxford University.
"This means we don't understand the main biological mechanism by which obesity affects prognosis."
The study was funded by Cancer Research UK and the British Heart Foundation.
"This study is part of the growing body of evidence showing that patients who are obese generally fare worse with cancer -- in this case, younger women with breast cancer," said Clifford Hudis, president of the American Society for Clinical Oncology.