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(CNN) -- The American Cancer Society said Thursday that keeping weight under control is the most important thing non-smokers can do to prevent the disease.
In revised guidelines, the society put top priority on diet and fitness, saying that eating a nutritious diet, staying active and limiting alcohol are the top things that people who don't smoke should do to fight cancer.
The guidelines are published Thursday in CA: A Cancer Journal for Clinicians. (Watch how fitness can cut cancer risk -- 1:53 )
One-third of the more than 500,000 cancer deaths each year are attributable to poor diet and inadequate physical activity, including overweight and obesity, the cancer society said. That's about the same number of cancer cases caused by smoking.
Avoiding tobacco products continues to be the most important avoidable risk factor in reducing cancer risk. But about 80 percent of Americans don't smoke, and for those people, diet and exercise are paramount.
"No doubt about it - if you smoke, the most important thing for you to do is quit. If you don't use tobacco products, your best shot at reducing your cancer risk is to watch your weight, be more active and eat well," said Colleen Doyle, director of nutrition and physical activity for the American Cancer Society and co-author of the report.
Research has long shown that colon, rectal, stomach, breast , prostate and pancreas cancers are related to diet. New studies indicate that for most nonsmokers, weight control can cut other cancer risks.
"There is evidence that losing weight can reduce the risk for postmenopausal breast cancer, and because of hormonal changes that occur with weight loss, there's reason to believe it's beneficial for other cancers as well," said Ms. Doyle.
The guidelines also say that the standard recommendation -- 30 minutes of moderate exercise five days a week -- is still good, but it works better with with more strenuous activities such as biking, running or power walking.
Doyle said it's time people understand they have a say in the future of their health. "Unfortunately, there's no guarantee. You can do all those things and still get cancer. But the good news is that a lot of people think they don't have any control over their risk of cancer and we're here to tell people that absolutely you do have some control."