You won’t find fiber in cheeseburgers, grilled chicken, cheese, or any other animal products. But a diet based on whole grains, beans, fruits, and vegetables can provide plenty of this essential nutrient.

Dietary fiber, or roughage, is a known cancer fighter found only in the cell walls of plant foods. Studies have shown that increased fiber intake decreases the risk of cancer, particularly colorectal cancer.

Fiber adds bulk to the digestive system, shortening the amount of time that waste travels through the colon. This waste often contains carcinogens and potential cancer-causing hormones that need to be removed from the body as quickly as possible. Fiber decreases the chances for intestinal cells to be affected by carcinogens. When bacteria in the lower intestine break down fiber, a substance called butyrate is produced. Butyrate may inhibit the growth of tumors of the colon and rectum.

Fiber may also help protect against breast cancer, especially when the fiber comes from grains and wheat bran. High-fiber diets are often lower in fat, and dietary fat is believed to increase the risk of breast cancer because fat can increase hormones in the body and speed tumor growth.

Increased fiber also expedites the removal of potentially harmful excess estrogen. The liver filters estrogens out of the blood by passing them into the digestive tract where fiber helps remove them from the body.

Fiber may also have a protective effect against mouth, throat, and esophageal cancers. And fiber may be part of the reason that vegetarian diets have been shown to result in low risk of prostate cancer. Of course, vegetarian diets are also rich in cancer-protective antioxidants.

Most Americans consume only 10 to 15 grams of fiber per day. But studies have shown that optimal intake for cancer prevention is at least 30 grams to 35 grams of fiber per day.

Studies suggest that small increases in fiber, such as adding vegetables to a chicken stir-fry or having a hamburger on a whole wheat bun, do not offer much protection. Replacing high-fat animal products such as chicken, fish, cheese, and eggs with plant foods helps boost fiber to levels where real protection is possible.

There are two types of dietary fiber—soluble and insoluble. Soluble fiber dissolves in water and is found in a variety of fruits, vegetables, legumes, and grains. It cuts cholesterol, adds to a feeling of fullness, and slows the release of sugars from food into the blood. These actions reduce your risk for health problems including heart disease, obesity, and diabetes. Good sources of soluble fiber are oats, oat bran, oatmeal, apples, citrus fruits, strawberries, dried beans, barley, rye flour, potatoes, raw cabbage, and pasta.

Insoluble fiber does not dissolve in water and is found in grain brans, fruit pulp, and vegetable peels and skins. It is the type of fiber most strongly linked to cancer protection and improved waste removal. Good sources of insoluble fiber are wheat bran, whole wheat products, cereals made from bran or shredded wheat, crunchy vegetables, barley, grains, whole wheat pasta, and rye flour.

It is best to choose fiber-rich foods over fiber supplements to get the full range of cancer-fighting phytochemicals that fruits, vegetables, legumes, and grains contain.

Adding more fiber to the diet is easy. Just follow these steps:

1. Choose products that are minimally processed, like whole wheat bread instead of white bread and brown rice instead of white rice.
2. Do not remove the fiber-rich peels and skins of fruits and vegetables. Just be sure to wash them thoroughly before eating.
3. Plan each of your meals to include whole grains, fruits, vegetables, and legumes.
4. To avoid intestinal discomfort when increasing fiber intake, it is best to increase gradually and drink plenty of water.
5. Snack on baby carrots, apples, strawberries, oranges, and other fiber-rich fruits and vegetables.
6. Top your breakfast cereals with dried fruits like raisins or dates, or fresh fruits like strawberries or peaches.
7. Sprinkle garbanzo beans or peas on your salad.
8. Add a handful of grated carrots to spaghetti sauce.

Beans present a bit of indigestion or gas for some people. Here are some ideas that will help solve this problem:

1. Start with modest servings.
2. Some people notice that smaller beans are easier to digest, so try black beans, black–eyed peas, and lentils, and work your way up to pinto, kidney, and fava beans.
3. After soaking dried beans, drain them, and then cook them in fresh water. It may also help to add a pinch of baking soda to the soaking water.
4. Always make sure beans are thoroughly cooked. Even some brands of canned beans need more cooking before they are thoroughly cooked.

By Jennifer K. Reilly, R.D.
The Cancer Project