When you learn that doctors don't know why certain cancers start growing, and risk factors look like a mystery, it's easy to give up on the idea of cancer prevention. But diet and digestion have more to do with cancer prevention than you may realize: Up to 30 percent of all cancers are linked to poor diet. The good news? The foods you eat every day can build a strong defense against many types of cancer. Find out what to eat and what to avoid to cut your risk.