Moderate exercise during the year before a breast cancer diagnosis appears to improve overweight women's chances of surviving, researchers say.

Exercise has been shown to help prevent breast cancer, but researchers haven't studied the role it plays in a patient's prognosis as much as they have studied factors such as tumour size.

Page Abrahamson, a postdoctoral researcher, and her team at the Fred Hutchinson Cancer Research Center in Seattle analyzed data on 1,264 women aged 20 to 54 who were diagnosed with invasive breast cancer between 1990 and 1992.

Breast cancer is considered to be a disease that is influenced by hormone levels.

The risk of developing breast cancer also rises with the amount of adult weight a woman packs on.

It's thought that since exercise burns body fat, it also lowers the levels of hormones such as estrogen and, therefore, the risk of developing breast cancer.

The benefits of exercise were found in women with a body mass index of 25 or higher, the cut-off point for being considered overweight.

Obese and overweight participants who had higher levels of moderate or vigorous physical activity within a year before diagnosis tended to have better five-year survival rates compared those who didn't, the researchers said. 

"Given that obesity is relatively well established as a poor prognostic factor in breast cancer, it is hopeful that activity may provide an opportunity to improve survival in this sub-population," the study's authors concluded in the Oct. 15 issue of the journal Cancer.

Women in the highest 25 per cent for activity level were 21 per cent more likely to survive than those in the bottom quarter.

Women of ideal body weight did not experience survival benefits from exercise, in this study, but a previous study did suggest it may help for this group as well.

Physical activity in adolescence or early adulthood seemed to have no effect on survival, but the researchers did not take a detailed look at lifelong exercise. Participants were only asked about their physical activity at ages 13-20 and the year before diagnosis.