A diet rich in whole-grain foods and other forms of fiber, rather than simple or refined carbohydrates, will help to avoid the obesity that may increase the risk of cancer. Fibers make up the structural parts of plants and therefore are found in all plant-based foods — including fruits, vegetables, grains, breads and cereals. They are not found in meat, milk, cheese or oils. The refining process used to make white flour removes almost all of the fiber from grains.

By definition, fiber is resistant to digestion, so its effects are mainly what it does as it passes through the body. There are many different types of fiber with different effects in the body, but they are generally classified into two groups: soluble and insoluble.

Soluble fibers are those that can dissolve in water. They typically are in highest amounts in fruits, legumes, barley and oats. They generally slow down digestion time, allowing you to feel full longer and allowing your system plenty of time to absorb nutrients from the foods you eat. More significantly, soluble fibers bind with bile acids in your intestines and carry them out of your body. Since bile acids are made from cholesterol, soluble fiber can lower your cholesterol levels.

Insoluble fibers are the type found in vegetables, whole-grain breads and whole-grain cereals. They increase the bulk of stool, help to prevent constipation and aid the removal of bound bile acids from your system. Insoluble fiber also increases transit time, or the speed that food moves through your gastrointestinal system.

Both types of fiber are important for good health and cancer prevention, and you can get them by eating a varied plant-based diet. Ideally, everyone should get at least 25 grams of fiber each day — about twice the amount most Americans currently consume. A good way to achieve that amount is to eat the NCI's recommended five fruits and vegetables each day. You can boost your fiber intake by eating the skins of potatoes and fruits such as apples and pears, since the skins contain a lot of fiber. Switching from refined foods to whole-grain foods also is advisable. That means choosing whole-wheat breads over white and brown rice over white. Other good sources of fiber include legumes, lentils and whole-grain cereals.

If a diet lower in saturated fat keeps weight under control, it may help decrease the risk of developing cancer of the prostate, endometrium and breast.

The three types of dietary fats — saturated, monounsaturated and polyunsaturated fats — differ in their molecular structure: Saturated fats come almost exclusively from animal products, such as meat, milk and cheese. A diet high in saturated fats has been associated with increased risk of both cancer and heart disease. Monounsaturated fats, found in olive oil and canola oil, and polyunsaturated fats, found in vegetable oils, are now considered healthy fats as long as they are used in moderation so that total calories consumed does not increase body weight.

Trans fatty acids are especially unhealthy. Trans fats do not occur naturally. They are produced by a chemical process called partial hydrogenation that changes the consistency of vegetable oil from liquid to solid or semi-solid. Trans fatty acids are abundant in stick margarine. They are used in most restaurants for deep frying foods such as French fries. Although no firm association with cancer has been found, recent studies suggest that trans fats are even worse for the heart than saturated fats.

The usual recommendation is that you should get no more than 10% of your daily calories from saturated fats. The remainder of calories should come primarily from whole-grain foods, vegetables, fruits, nuts and healthy protein sources like fish, beans and soy.