Women with early-stage breast cancer who eat a healthy diet and exercise regularly may significantly reduce their risk of dying, regardless of their weight, a new study suggests.

While previous studies have looked at the effects of diet or physical activity on breast cancer survival, in the study in the June 10 issue of the Journal of Clinical Oncology, researchers from the University of California, San Diego looked at both.

"The key is that you must do both," said study author John Pierce, director of the university's cancer prevention and control program.

The study looked at 1,490 women with an average age of 50 who were diagnosed and treated for early stage breast cancer between 1991 and 2000.

Participants were followed for between five and 11 years. They were not told what to eat or how much to exercise but they were interviewed about their eating and exercising habits.

"Of particular importance, this halving of risk was seen in women who were not obese as well as in those who were obese," the study's authors wrote.

The estimated 10-year mortality rate among women who ate at least five servings of fruits and vegetables a day and exercised the equivalent of walking briskly for 30 minutes, six days a week was seven per cent — about half that for other study participants.

The 50 per cent risk reduction applied to early-stage breast cancer, when a woman's risk of dying is already low.

About 16 per cent of obese women in the study exercised and ate healthily, compared with 30 per cent of non-obese participants.

During the study, 135 participants died, including 118 from breast cancer, 10 from other cancers and seven from non-cancer causes.

Previous studies have also shown reduced risk of dying from breast cancer among women who exercise.