Undigested plant residues that represent the outer coating or plant wall material of vegetables, grains, seeds and fruits. Fiber is considered essential to maintain a healthy digestive tract. A low-fiber diet has been associated with constipation, colon cancer, spastic colon, hiatus hernia, varicose veins, hemorrhoids, heart disease, hypertension, gallstones, diabetes, obesity, colitis, and Crohn’s disease.

Fiber can help curb appetite because the stomach feels full. Fiber-rich foods require longer chewing, which stimulates digestive juices and helps digestion. Fiber displaces fat calories and slows fat digestion and absorption by binding bile salts and, therefore, it may help control obesity.

Fiber also helps fight heart disease. In a study of more than 40,000 U.S. middle-aged men, researchers have found that heart attacks were 41 percent less common among men who consumed at least 28 g fiber daily compared with men who ate less than 13 g daily as typical of the usual U.S. diet. It is estimated that fiber-deprived men reduced their risk of heart attack by 20 percent to 44 percent for each 10 g of additional fiber, without lowering their fat intake. The largest reduction was among men who ate wheat bran, which represents mainly insoluble fiber.

Recent studies have confirmed that a high-fiber diet does not reduce risk of colon cancer because it seems to have no effect on the growth of precancerous colon polyps. In one study researchers at the National Cancer Institute (NCI) put one group of people on a high-fiber diet and told another group to simply eat what they usually eat. All the participants had had at least one precancerous polyp removed from their colon in the six months before the study. Four years later the researchers found that the risk of developing another polyp was the same in both groups.

In the other study researchers at the Arizona Cancer Center asked one group of people to eat half an ounce of wheat fiber daily and gave another group a tenth of an ounce. Again, three years later the risk of developing a precancerous polyp was the same in both groups. Bacterial degradation of dietary fiber releases organic acids, such as butyrate and acetate, which promote a healthy colon. A high intake of whole grains, vegetables and fruits, and derived fiber seems to reduce the risk of cancers of the upper digestive tract and ovarian cancer. In women, increased fiber intake is associated with a reduced risk of cardiovascular disease and heart attacks, according to 2002 research.

It is estimated that Americans consume only one-third to one-half the optimal amount of fiber daily. The National Cancer Institute recommends doubling or tripling fiber intake from 12 g to 20 to 35 g per day. At this level, there is minimal interference with nutrient absorption, while assuring normal maintenance of intestinal function. However, there is as yet no formal recommended dietary allowance for fiber because the exact amount required for health has not been established.

“Dietary fiber” refers to the fiber content in plant foods that resists digestion by enzymes of the gastrointestinal tract, and dietary fiber is the usual designation for fiber content in foods. Lab methods for measuring dietary fiber are more gentle than those for crude fiber, which measure only material that resists strong acid treatment. Values for dietary fiber can be up to four times higher than for crude fiber contents.

Food labels are required to list the amount of dietary fiber as a percentage of daily value, based upon the needs of a 2,000 calories per day diet.

Listing the amount of soluble and insoluble fiber is voluntary.

Major Types of Fiber

Fiber consists of a mixture of very different non-starch complex carbohydrate and noncarbohydrate materials. The major classes of dietary fiber are cellulose, hemicelluloses, pectin, mucilage, gums, algal materials, and lignin.

• Cellulose is a linear chain of glucose units.

• Hemicelluloses are highly branched structures found in plant cell walls. They are a diverse group with varying sugar compositions, including the simple sugars mannose, glucose, galactose, xylose, arabinose, and an acid derived from glucose called glucouronic acid. Some hemicelluloses are water-soluble.

• Pectins function as a glue that holds plant tissues together, and they contain an acidic sugar derived from galactose, galacturonic acid, in the primary chains, with side chains or branches containing less common sugars: arabinose, fucose, and xylose.

• Gums are plant secretions and contain acidic sugars.

• Algal polysaccharides are products of edible seaweed.

• Lignin is a noncarbohydrate insoluble material and a principal structural material of wood.

Insoluble Fiber

It is convenient to break down fibers according to whether they are insoluble or water-soluble. Insoluble fiber includes cellulose and lignin. This type of fiber swells in water, increases stool weight and stool frequency, and helps prevent constipation, colonic inflammation (diverticulitis) and hemorrhoids, by softening stools and speeding up the movement of waste through the intestine. Cellulose is not digested, although colon bacteria break down 40 percent to 80 percent of cellulose. Lignin is not degraded and passes through the digestive tract unchanged. Lignin softens stools, increases regularity and may lower blood cholesterol.

Bran is the most common source of insoluble fiber, derived from the outer husk of kernels of wheat and other cereal grains. It contains cellulose and other cell wall materials and slows the rise in blood sugar after a meal. Bran can protect against heart disease in middle-aged men; oatmeal is also effective, though more may need to be consumed to get the same effect as wheat bran.

Soluble Fiber

This type of plant fiber swells in water and forms glue-like gels. Soluble fiber is made up of noncellulose carbohydrates, including pectins, gums, algal polysaccharides and some types of hemicellulose. Soluble fiber has important physiologic effects. It becomes viscous, thus softening stools and slowing the rate of stomach emptying. Soluble fiber also slows starch digestion and glucose uptake, in turn lowering the amount of insulin needed to process blood glucose after a meal, and it may help diabetics. Eating oat bran (2 oz.) regularly each day may effectively lower blood sugar levels. Despite the popularity of oat bran as a source of water-soluble fiber, it provides both water-soluble and insoluble fibers.

The water-soluble fibers are completely degraded by intestinal microorganisms to short-chain fatty acids, which are used as fuels to help maintain the intestinal lining.

Soluble fiber also seems to lower blood cholesterol. Regular consumption of oat bran and legumes may lower blood cholesterol. Fiber binds bile salts in the intestine, which could reduce their resorption, thus forcing the liver to remove more cholesterol from the blood to make more bile. Pectin, guar gum, and locust bean gum have been reported as effective in this regard. Because it believes that long-term health benefits are not yet proven, the U.S. FDA does not permit health claims linking fiber consumption with the prevention of either heart disease or certain types of cancer.

Good sources of fiber include dried beans, lentils, lima beans, pinto beans, sweet potatoes, broccoli, brussels sprouts, spinach, almonds, corn, wheat, oat bran, and fruit (blackberries, pears, apples). In general, the less processed a food is, the more fiber it has. Therefore, eating whole, minimally processed foods assures a mixture of soluble fiber and insoluble fiber; both kinds are needed for health.

Sources of insoluble fiber include skins of vegetables and fruits, whole grains (not white flour), high-fiber cereals, dried beans, broccoli, and bulgur wheat. Bran is a common and inexpensive source of insoluble fiber. Cold bran cereals have more insoluble fiber than hot cereals. Bran is lost in preparing white flour and it is not replenished by enrichment.

Good sources of soluble fiber are fruits, cooked dried beans, chickpeas, barley, lentils, navy beans, vegetables such as squash and carrots, plus barley, oat, and rice bran, guar gum, glucomannan, and pectin are common soluble fiber supplements. Fructo oligo saccharides are also considered to be a form of fiber.

When increasing fiber consumption, the recommendation is to begin gradually because excessive fiber intake causes bloating, gas, cramps, nausea, and diarrhea. A month may be required to adapt to a high-fiber diet. Initial steps can be eating whole fruit instead of juices; popcorn instead of potato chips; whole wheat instead of white bread; and a baked potato with its skin instead of mashed potatoes. Patients should consult a physician when planning to take fiber supplements if they have a serious digestive disease, or if they plan a daily consumption over 35 g of fiber. Following the food guide pyramid, which specifies eating six to 11 servings of grain-based foods and two to four servings of fruits and three to five servings of vegetables daily, will satisfy the recommended fiber intake.

Fiber Supplements

Supplements are a popular way of increasing dietary fiber intake, although the National Cancer Institute recommends increasing fiber intake through whole, fiber-rich foods. Fiber supplements often contain bran, guar gum, pectin, or psyllium. Since they swell in water and help create a feeling of being full, fiber supplements have been used in weight-reduction programs to control appetite, ft is not clear that they help with permanent weight loss, however. Fiber from psyllium seeds is the primary ingredient of several popular brands of non-chemical laxatives.

Patients with serious intestinal disorders such as diverticulitis, ulcerative colitis, or Crohn’s disease should avoid taking fiber supplements without medical supervision. Patients should consult a physician before consuming more than 35 g of fiber supplements a day; high levels of fiber can block the uptake of iron, calcium, zinc, copper, and other minerals and cause calcium losses. Patients should also avoid fiber supplements that contain appetite suppressants like phenylpropanolamine, which can cause side effects.

Ornish, D. et al. “High Fiber Diet and Colorectal Adenomas,” New England Journal of Medicine 343 (September 7, 2001): 736-738.

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