Dietary Fiber 101

Confused by fiber? Well, you're not the only one. Basically, the term fiber (aka roughage) refers to ingested compounds that are resistant to digestion and cannot be absorbed, passing through the human digestive system largely unchanged. Fiber is not a nutrient, but it is an important part of a healthy diet and adequate intake is associated with a number of beneficial health effects. The bulk, softness, hydration and laxation of feces are dependent upon the fiber content in the diet.

Natural fiber is found mainly in the outer layers of plants and is present in all plants that are eaten for food. The components of dietary fiber include many substances: celluloses, hemicelluloses, polysaccharides (primarily non-starch), oligosaccharides, lignin, chitin, chitosan, gums, inulin, oligofructose, fructooligosaccharides, dextrins, mucilages, pectins, resistant-starches and synthetic (polydextrose and polyols) compounds.

Dietary fiber has no generally accepted exact definition. But not all fiber is the same, and it is categorized in a number of ways. For example: 1) by its source or origin (e.g., from grains, etc.); 2) by its type (cellulose, hemicelluloses, pectin, gums, lignins, mucilages, resistant starches, etc.); and 3) by how easily it dissolves in water (soluble fiber partially dissolves in water and insoluble fiber does not dissolve in water). However, most nutritional and medical sources generally categorize dietary fiber in two general categories based upon water solubility.

Soluble fiber is primarily made up of polysaccharides (complex carbohydrates that contain three or more molecules of simple carbohydrates) and dissolves in water. Soluble fiber is found naturally in oats, peas, beans, apples, citrus fruits, carrots, barley and psyllium, and more concentrated in substances such as gums, pectins, mucillages, galactomannans, arbinogalactans, beta-glucans, etc.

Soluble fiber absorbs water to become a viscous gelatinous mass. Ingestion helps to promote regular bowel movements and ease defecation. This type of fiber has several positive effects on body chemistry: 1) it tends to bind to bile acids in the small intestine, which can help lower cholesterol levels; 2) it slows the absorption of sugar (slowing sugar response after eating); and 3) it can help normalize blood lipid (triglyceride) levels.

Soluble fiber is fermented by anaerobic bacteria in the colon of the digestive tract. Thisproduces an increase in gases (such as CO2, hydrogen and methane) and stimulates intestinal production of short-chain fatty acids (SCFAs), which can help balance intestinal pH levels.

Insoluble fiber is mainly made up of polysaccharides from plant cell walls (i.e., celluloses, hemicelluloses, pectins, lignins, and mucilages) and cannot be dissolved in water. Insoluble fiber is found naturally in whole grain cereals, wheat flour, wheat bran, nuts and many vegetables.

Insoluble fiber is metabolically inert (it does not get digested or ferment), helps to control and lower the pH in the intestines, absorbs water throughout the digestive system and eases defecation. This type of fiber promotes the movement of material through the digestive system and increases stool bulk, so it can be of benefit to those who have constipation issues or irregular stools. Insoluble fiber is associated with reduced diabetes risk (but the mechanism by which this occurs is unknown).

Beneficial Effects of Fiber Intake

The daily ingestion of dietary fibers is normally a combination of both typesdivided approximately as two-thirds insoluble and one-third soluble.

Soluble fiber supplements may be beneficial for alleviating symptoms of diarrhea, constipation, abdominal discomfort and irritable bowel syndrome (IBS). Prebiotic soluble fiber products (like those containing inulin or oligosaccharides) may contribute to relief from inflammatory bowel disease (IBD), as in Crohn's disease, ulcerative colitis, and Clostridium difficile, due in part to the SCFAs produced with subsequent anti-inflammatory actions upon the bowel. They have the ability to swell with water, expanding up to about 20 times its original size (depending upon the fiber), which creates volume and weight in the stool as it is formed.  Stools with good volume and weight stimulate muscle contractions in the colon, which move the stool through the colon. The addition of water helps to prevent stool from becoming dry, making it easier to pass.

Insoluble fibers do not dissolve in water, so they pass through the gastrointestinal (GI) tract relatively intact, and speed up the passage of food and waste through the gut. They have a laxative effect and add bulk to the diet, helping prevent constipation. These fibers have been characterized like a broom that helps push out intestinal contents.

One potential benefit of increasing dietary fiber intake may be an increase in satiety and a reduction in caloric intake, effects that could help weight loss.

How Much?

Current recommendations (from the U.S. National Academy of Sciences, Institute of Medicine) suggest adults should consume between 20 to 35 grams of dietary fiber per day.  Average intake of fiber is low in the United States, ranging between 5 to 18 grams of fiber daily.

No recommended dietary allowance (RDA) has been established for fiber, but the American Dietetic Association (ADA) recommends eating 20 to 35 grams of dietary fiber from a variety of food sources every day. The ADA's recommendation for children is that intake should equal age in years plus 5 g/d (e.g., a four-year-old should consume 9 g/day). No guidelines have yet been established for the elderly or very ill. The new Dietary Reference Intake (DRI)/Adequate Intakes (AI) for fiber are 25 g/d for women and 38 g/d for men.

While fiber intake is good for health, excessive fiber intake can cause diarrhea, intestinal gas, abdominal bloating, abdominal cramps and, in rare cases, intestinal obstruction or impacted colon. It is best to increase fiber in the diet gradually over a period of a few weeks. This allows the natural bacteria in the digestive system to adjust to the change. Drink plenty of water, since fiber works best when it absorbs water, making stool soft and bulky. Without the added water, you could become constipated.

Plant Sources of Fiber

Soluble fiber includes:

Insoluble fiber includes:

Fiber Supplements

Note that bulk fibers being specifically sold as laxatives are considered by FDA to be over-the-counter (OTC) drug products. Here are a few examples of fibers that are being sold as bulk laxatives (OTC drug item), supplements or food additives.

These may be marketed to consumers for nutritional purposes, treatment of various GI disorders, and for such possible health benefits as lowering cholesterol levels, reducing risk of colon cancer and losing weight.

Fiber and Calories

Fibers are considered to be carbohydrates. For nutritional labeling in the United States, soluble fiber must be counted as four calories per gram, but insoluble fiber may be (and usually is) treated as zero calories per gram.  Why the difference? Because soluble fiber is fermented during digestion in the colon, so it can provide calories to the body. However, insoluble fibers do not change when inside the body, so no calorie absorption occurs.

Dieticians still disagree on how much energy is actually absorbed from soluble fiber, but many approximate it is only around two calories per gram, not four. Note that there is difference in labeling requirements between international countries on how to state fiber content.

Robin Koon, executive vice-president at Best Formulations, has more than 25 years of pharmaceutical experience in clinical pharmacy as a retail drug chain executive overseeing operations and in managed-care.