WASHINGTON (CNN) -- Doctors have long known that being a couch potato increases your chance of getting heart disease. Now, researchers have strong evidence that exercise can also substantially reduce your chances of developing colon cancer.
At a scientific meeting near Washington Monday, researchers presented a long-term study of more than 100,000 women. In the study, the more women exercised, the less likely they were to get colon cancer.
"We are seeing a 50 percent reduction in the risk of colon cancer when we compare the top 20 percent of the population of women who in mid-life are exercising at the equivalent of an hour of walking a day," said Dr. Graham Colditz of Harvard University.
Everyone reaps benefits of exercise
Only women were involved in the study, but everybody can benefit from working out, not just women, the doctors said. "If every adult American increased their activity by walking the equivalent of half an hour a day, we would ... predict a 17 percent reduction in the risk of colon cancer," Colditz said.
Such a drop would save 18,000 lives a year.
It's not clear why being active inhibits cancer, but it could lower insulin production, which has been linked to the disease.
A tomato a day could keep doctors away
Changes in diet could also help prevent cancer, particularly eating more tomato products, like tomato sauce or even the nutritionally maligned pizza, say researchers. Their study showed that adding tomato products to your diet can slash men's odds of getting prostate cancer.
Tomatoes contain a substance called lycopene, which has been linked to a drop in prostate cancer rates.
"We found that the high consumption of tomato and tomato-based product had reduced the risk of prostate cancer by almost 40 percent," said Dr. Edward Giovannucci of the Harvard School of Public Health.
Lycopene, like Vitamin A, is an antioxidant, a chemical thought to inhibit processes in the body that lead to cancer. "Lycopene is absorbed in the body as we are absorbing oils and fats," Giovannucci said. "To absorb lycopene, we do need to have some fat in the diet."
Since the fat substitute Olestra reduces the amount of lycopene in the body, scientists say it might increase cancer risk. But Proctor and Gamble, which makes Olestra, says any loss of lycopene from its product is insignificant and wouldn't be harmful.