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The Canadian researchers analyzed 67 published articles to see which habits made the most difference in reducing the risk of either breast cancer recurrence or death.
Exercise came out on top, reducing the risk of breast cancer death by about 40 percent, said review author Dr. Ellen Warner, a medical oncologist at Sunnybrook Odette Cancer Centre and a professor at the University of Toronto.
The new review "pulls everything together," said Leslie Bernstein, a professor in the department of population sciences at the City of Hope Comprehensive Cancer Center in Duarte, Calif. She first reported on the link between exercise and reduced breast cancer risk decades ago.
From the new review, Warner and her co-author Julia Hamer made several recommendations on what habits matter to reduce recurrence and death, but the effect of some habits remain inconclusive.
Besides exercise, the review found weight gains of more than 10 percent after diagnosis were linked to a greater risk of death. So, a 120-pound woman whose weight goes up to more than 132 pounds after diagnosis might increase her risk of dying.
No specific diet has been found better than another to reduce the risk of breast cancer returning, the review found. Warner said the advice to avoid soy, which has weak estrogens, was not supported by scientific studies.
Research on smoking cessation and breast cancer recurrence isn't definitive, Warner said, but stopping smoking is crucial for other health-related reasons. Vitamin C supplements may help, and vitamin D can help maintain bone strength, which is reduced with chemotherapy and hormonal therapy.
Besides the information on exercise and weight, the information on diet is valuable, Bernstein said. Many women have avoided soy in their diets for fear of cancer recurrence. However, she said, the estrogens in soy are "so weak" that the evidence does not support avoiding them. "Of course, everything in moderation," she said.
Bernstein agreed that research is inconclusive on many habits, in particular smoking and drinking. Even so, she said, "We have to counsel everyone to stop smoking. It may have no direct effect on breast cancer and risk of breast cancer death, but it is going to affect their risk of dying of something else," she said.
Weight doesn't affect all races equally, Bernstein said. For instance, she said, "the weight at diagnosis does not seem to affect African-American women as strongly as white women, even though African-American women are far more likely to die of breast cancer."
Perhaps another factor is such a strong predictor of outcome, she said, that it overshadows the weight. However, experts still would advise keeping a healthy weight, Bernstein said.
Women who met recommended exercise levels had a stronger risk reduction, Warner said. She recommends at least 30 minutes of moderate-intensity activity at least five days a week, or 75 minutes of vigorous exercise, plus two to three strength-training sessions each week.
However, research on the best types of exercise are not conclusive, Bernstein said. "We don't know what's better, muscle building or cardio," Bernstein said. "And the prescription has to change with age."
Why exercise helps so much is not known, Warner said, but "I think it's probably not pure exercise. People who exercise are more likely to do other healthy things."
Even so, the exercise may modify the side effects from hormone therapy, she said. So, women on hormone therapy who exercise may be more likely to adhere to their treatment as prescribed.
Exercise also has anti-inflammatory effects, and that may help the body better keep cancer cells in check, Warner said. Excess weight can increase inflammation, she added.
Warner tells patients exercise is part of their treatment, and to consider it as crucial as their other therapies.
The findings were published Feb. 21 in the CMAJ (Canadian Medical Association Journal).
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