Why is exercise protective against cancer?
Many studies show that people who are physically active are less likely to cancer. Such associations don't prove that exercise prevents cancer. But they are a hint. And if there's a biological explanation for a protective effect, the case gets that much stronger.
Here are several biological explanations mentioned in the Physical Activity Guidelines Advisory Committee (PAGAC) report, which laid the scientific groundwork for the new set of exercise guidelines due out in October 2008.
There's still quite a bit of conjecture involved, but you can see why exercise might keep some people out of harms way of cancer.