Research on the link between cancer and diet is still ongoing, but diet is thought to cause 30% - 40% of all cancers. Here's what you can do. 1. Keep your body weight in a healthy range. BMI is not a perfect measurement, especially for people who are more muscular, but it's a good average estimate for a healthy body weight range. 2. Eat more than 5 cups of vegetables and fruits per day. 3. Get less than 20% of your daily calories from fat. This may be hard to calculate, but it is an extremely small amount of calories. The average American gets 30-35% of all calories from fat. Start by eliminating foods with the highest fat content, such as fried foods, margarine, and desserts, and slowly removing other sources of fat from the diet. 4. Eat foods with lots of omega-3 fatty acids. 5. Avoid trans fats, red meats, smoked, or charred foods.