How Fiber Helps Protect Against Cancer

The Physicians Committee

How Fiber Helps Protect Against Cancer

You don't usually see it or taste it, but fiber works wonders for your body. Dietary fiber, or roughage is a known cancer fighter found only in the cell walls of plant foods.1 For years, studies have pointed to the fact that increased fiber intake decreases the risk of colorectal cancer. This protective effect may be due to fiber's tendency to add bulk to your digestive system, shortening the amount of time that wastes travel through the colon.2As this waste often contains carcinogens, it is best if it is removed as quickly as possible; so, increased fiber decreases chances for intestinal cells to be affected. In addition, when bacteria in the lower intestine break down fiber, a substance called butyrate is produced which may inhibit the growth of tumors of the colon and rectum.3

The U.S. Polyp Prevention Trial (1991–1998) on colorectal adenoma recurrence examined the effect of strict adherence to a low-fat, high-fiber, high-fruit and -vegetable intervention over four years among almost 2,000 participants. This study, published in the American Journal of Epidemiology in 2009, shows a wide range of individual variation in the level of compliance among intervention participants. Researchers observed a 35 percent reduced odds of adenoma recurrence among “super compliers,” the most adherent participants, compared with controls. Findings suggest that high compliance with a low-fat, high-fiber diet is associated with reduced risk of colorectal adenoma recurrence.4

Fiber may also help protect against breast cancer, an effect noted especially with consumption of whole grains and wheat bran.5,6 The fact that high-fiber diets are often lower in fat may partially explain this benefit of eating fiber-rich grains, beans, vegetables, and fruits, since dietary fat is believed to increase the risk of breast cancer. Additionally, studies suggest that high amounts of fiber may also prevent breast cancer by binding to estrogen. High amounts of estrogen can be potentially cancer causing. Your liver filters these estrogens out of the blood by passing them into the digestive tract where fiber escorts them away.7 As such, increased fiber in the diet expedites the removal of potentially harmful excess estrogens.

Fiber may also have a protective effect against mouth, throat, and esophageal cancers.8 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.9 (To learn more about antioxidants, follow these links: carotenoids, vitamin E, and lycopene.)

Food Sources

If you're like most North Americans, you take in only 10 to 15 grams of fiber per day. However, most studies have shown that optimal intake for cancer prevention is at least 30 to 35 grams per day.10 Recent 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. On the other hand, when we replace high-fat, animal products such as chicken, fish, cheese, and eggs with plant foods, we easily boost fiber to levels where real protection is possible.

There are two types of dietary fiber—soluble and insoluble.1 Soluble fiber dissolves in water and is found in a variety of fruits, vegetables, legumes, and grains. It cuts cholesterol, adds to your 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.1

As you may have guessed, 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.1

It is best to choose fiber-rich foods over fiber supplements in order to get the full range of cancer-fighting phytochemicals ("phyto" means plant so phytochemicals are simply plant-compounds) that fruits, vegetables, legumes, and grains contain. The following table shows the fiber content of some common foods. It is not a complete list; fiber is found in all vegetables, fruits, grains, and legumes.

Food Dietary Fiber Soluble Insoluble

Apple, 1 medium

2.9 0.9 2.0
Banana, 1 medium 2.0 0.6 1.4
Orange, 1 medium 2.0 1.3 0.7

Broccoli, 1 stalk

2.7 1.3 1.4
Carrots, 1 large 2.9 1.3 1.6
Corn, 2/3 cup 1.6 0.2 1.4
Potato, 1 medium 1.8 1.0 0.8
Tomato, 1 small 0.8 0.1 0.7

All-Bran, 1/2 cup

9.0 1.4 7.6
Cornflakes, 1 cup 0.5 0 0.5
Macaroni, 1 cup cooked 0.8 0.5 0.3
Oat Bran, 1/2 cup 4.4 2.2 2.2
Rolled oats, 3/4 cup cooked 3.0 1.3 1.7
White bread, 1 slice 0.4 0.3 0.1
Whole-wheat bread, 1 slice 1.4 0.3 1.1

Green peas, 2/3 cup cooked

3.9 0.6 3.3
Kidney beans, 1/2 cup cooked 6.5 1.6 4.9
Lentils, 2/3 cup cooked 4.5 0.6 3.9
Pinto beans, 1/2 cup cooked 5.9 1.2 4.7
Sources: Anderson JW, Bridges SR. Dietary fiber content of selected foods. Am J Clin Nutr 1988;47:440-7; Pennington JAT. Bowes and Church's Food Values of Portions Commonly Used. 17th ed. New York: Harper & Row, 1998.

Tips for Increasing Fiber in Your Diet

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

1.  Tran M. High-Fiber Diet. Gale Encyclopedia of Alternative Medicine 2001. Retrieved on May 24, 2001, from research database
2. Fuchs CS, Giovannucci EL, Colditz GA, et al. Dietary fiber and the risk of colorectal cancer and adenoma in women. N Engl J Med. 1999;340:169-176.
3. Avivi-Green C, Polak-Charcon S, Madar Z, et al. Apoptosis cascade proteins are regulated in vivo by high intracolonic butyrate concentration: correlation with colon cancer inhibition. Oncol Res. 2000;12:83-95.

4. Sansbury L, Wanke K, Albert P, et al. The Effect of Strick Adherence to a High-Fiber, High-Fruit and -Vegetable, and Low-Fat Eating Pattern on Adenoma Recurrence. Am J Epidemiol. 2009;170:576-584.
5. Ferguson LR, Harris PJ. Protection against cancer by wheat bran: role of dietary fiber and phytochemicals. Eur J Cancer Prev. 1999;8:17-25.
6. Slavin JL, Martini MC, Jacobs DR Jr, et al. Plausible mechanisms for the protectiveness of whole grains. Am J Clin Nutr. 1999;70:459S-463S.
7. Bagga D, Ashley JM, Geffrey SP, et al. Effects of a very low fat, high fiber diet on serum hormones and menstrual function. Implications for breast cancer prevention. Cancer. 1999;76:2491-2496.
8. Soler M, Bosetti C, Franceschi S, et al. Fiber intake and the risk of oral, pharyngeal and esophageal cancer. Int J Cancer. 2001;91:283-287.
9. de la Taille A, Katz A, Vacherot F, et al. [Cancer of the prostate: influence of nutritional factors. A new nutritional approach]. [Article in French] Presse Med. 2001;30:561-564.
10. American Dietetic Association. Colorectal cancer (preventative effects of dietary fiber). J Am Diet Assoc. 2001 (Jan). Retrieved May 25, 2001, from the research database