Women who are overweight when diagnosed with breast cancer or who become overweight after learning of their condition are more likely to die or have the disease come back, U.S. researchers reported.

The effect is particularly strong among nonsmokers, the team at Boston's Brigham and Women's Hospital and Harvard Medical School found.

They found women who had never smoked and who were overweight were nearly twice as likely to die of breast cancer than nonsmokers who were normal weight.

And breast cancer patients who gained more than an average of 17 pounds (8 kg) were 1.5 times more likely to have a cancer recurrence or to die, the researchers found.

Although other studies have linked fat mass with breast cancer risk, this one teased out more and stronger detail by separating smokers, study leader Dr. Candyce Kroenke said.

"Combining smokers and nonsmokers in analyzes may mask the true relationship between weight and survival after a breast cancer diagnosis, since smoking is generally related to both lower levels of weight and a higher risk of death overall," she said in a statement.

"Researchers have also speculated that obesity acts on cancer by raising the body's levels of sex hormones such as estrogen, particularly in post-menopausal women," she added.

Writing in the Journal of Clinical Oncology, Kroenke and colleagues said they studied 5,204 breast cancer patients over 24 years who were taking part in the larger Nurses' Health Study.

They used body mass index or BMI -- the ratio of a person's height in meters to their weight in kilograms -- to classify women as overweight. A BMI of 25 or above is considered overweight and a BMI of 30 marks a person as obese.

"Women recently diagnosed with breast cancer or at high risk for the disease should take steps to maintain a healthy weight to reduce the risk of recurrence and death," Kroenke said.