Several lifestyle changes can improve outcomes after a breast cancer diagnosis, but exercise is far and away the best habit to establish, researchers say.

Women with breast cancer, whether newly diagnosed or at any time in their "survivorship" phase, need to exercise regularly and avoid weight gain, said Dr. Ellen Warner from Odette Cancer Center at Sunnybrook Health Sciences Center in Toronto, who coauthored the research review.

Warner and her colleague Julie Hamer joined forces to review nearly 70 articles that addressed lifestyle modifications that might have an impact on the risk of breast cancer recurrence and survival after breast cancer.

They found that regular physical activity can reduce the risk of dying from breast cancer by 40 percent compared to women who didn't exercise. Unfortunately, less than 13 percent of women with breast cancer achieve the recommended 150 minutes per week of physical activity.

"Exercise has the greatest benefit on lowering risk of recurrence and has many other secondary benefits like helping with weight management (which itself lowers the risk of recurrence) and fewer side effects from chemo, radiation, and hormone therapy," Warner told Reuters Health by email.

Gaining weight during or after breast cancer treatment is risky - it increases the chance of recurrence and decreases survival rates, the review concludes.

Women who are already overweight or obese also have a higher risk of recurrence and death, but it's not clear whether weight loss actually improves those outcomes. Studies are underway to examine this further, the researchers write in the Canadian Medical Association Journal.

Does diet matter? Yes and no. Breast cancer recurrence rates are similar whether women eat a diet high in fruits, vegetables, whole grains and chicken or a diet high in processed grains, processed meats and red meat. But high dietary saturated fat can increase the risk of death from breast cancer. Soy products, however, do not increase the risk of breast cancer recurrence and might even reduce it.

"Women with breast cancer don't need to make extreme diet changes (like cutting out meat, dairy, sugar, soy, etc.)," Warner said. "There is no evidence any of these are effective. They can eat anything in moderation and following Canada's food guide would be helpful if they don't know much about nutrition."

Women with breast cancer - well, everyone, really - should stop smoking. It's strongly associated with the risk of death from breast cancer, and stopping improves overall survival.

What about alcohol intake and vitamin supplementation? The evidence is limited and inconsistent, so further study is needed before making specific recommendations, the team notes.

"There's a large ongoing Canadian study of women age 40 and under newly diagnosed with breast cancer called RUBY, and one of the projects in this study is to look at how various lifestyle factors (diet, exercises, supplements, etc.) affect prognosis for that specific age group," Warner said.

"Adopting a healthy lifestyle is great but should never be seen as a substitute for conventional therapy," she concluded.

In their review, the authors note that very few of the included studies met the highest standards of clinical trials.

Dr. Livia Augustin from St. Michael's Hospital in Toronto and the Fondazione Giovanni Pascale National Cancer Institute in Naples, Italy, has, along with others, designed a clinical trial (DEDiCa) to investigate whether low glycemic index diet, exercise and vitamin D reduces breast cancer recurrence.

"People with breast cancer suffer from several comorbidities, including cardiovascular disease, diabetes, and osteoporosis, and therefore many health complications; therefore, quitting smoking, increasing vitamin D when necessary, increasing physical activity, and improving dietary aspects are crucial therapeutic targets to reduce complications and health care costs as well as help to live longer with a better quality of life," Augustin told Reuters Health by email.