Fiber is found in the stems, seeds and leaves of plants. Fiber is made up of long chains of sugar, but as humans, we lack the digestive enzymes to break down these complex polysaccharides (although a small amount of ingested fiber can be partially broken down by enzymes produced by bacteria in the intestines). Most of the fiber, however, is not broken down and exits the body in the feces. “Fiber” is a very broad term. More precise terms are soluble and insoluble fiber. Soluble fiber dissolves in water and can be broken down by bacterial enzymes, while insoluble fiber cannot. The distinction is important because the solubility of the fiber determines its health benefit. Fiber found in food is usually a mixture of both types of fiber, while purified fiber supplements may contain just one type of fiber.
Dietary sources of fiber are plentiful. Fruits, vegetables, seeds and legumes (dried peas and beans such as lentils, split peas, red beans and pinto beans) contain both types of fiber. Barley, oats, oat bran and rye contain predominantly soluble fiber. Wheat bran, brown rice and whole grains (grains that have not been refined) are excellent sources of insoluble fiber.
Supplemental sources of fiber include psyllium, methylcellulose or polycarbophil as well as fiber extracted from fruits, vegetables and grains. Psyllium is a concentrated source of fiber from the husks of the psyllium plant. Methylcellulose and polycarbophil are chemically altered forms of cellulose (the cell wall of many plants). The chemical alterations make them resistant to bacterial breakdown. The United States Food and Drug Administration (FDA) has approved health claims for 2 dietary fibers, beta-glucan (0.75g/serving) and psyllium (1.78g/serving), based on evidence that 4 servings/day can reduce cardiovascular disease risk – reducing total cholesterol by an average of 2% and overall cardiovascular disease risk by about 4% (Jenkins et al. 2002).
General health recommendations call for a daily consumption of 20-40 grams of fiber, but the average American consumes less than 15 grams. Although the amount of soluble and insoluble fiber is not specified, it is assumed that people will receive both types of fiber. In general, claims made for supplemental forms of dietary fiber are based on scientific evidence showing that soluble fiber reduces the risk of heart disease (by lowering total and LDL cholesterol levels) and that insoluble fiber reduces the risk of colon cancer (by reducing the concentrations of fecal bile acids).
There is no question that adequate fiber is necessary for good health. Consumption of 20 to 40 grams of fiber per day is possible from food sources alone – but this entails a high consumption of legumes and whole grains. However, it is recognized that in the U.S., the average fiber intake is less than 15 grams per day. In light of average intake, fiber supplements are often recommended and warranted.
Soluble Fiber and Heart Disease Risk
Hundreds of studies have been conducted to examine the role of soluble fiber in reducing the risk of heart disease by lowering cholesterol levels. Although it is difficult to compare the studies because of differences in the type and amount of soluble fiber, the number of people studied and the initial cholesterol levels of the subjects, some general conclusions can be drawn. It is generally accepted by nutrition scientists that approximately 2-10 grams of soluble fiber daily appears to reduce blood cholesterol and LDL cholesterol (Jenkins et al. 1993, Jenkins et al. 2001, Jenkins et al, 2002). The reduction is small, but makes a substantial contribution to reducing heart disease risk. The source of the fiber (oats, pectin found in fruits or psyllium) seems to make little difference, as all types have been shown to be effective (Jenkins et al. 2002, Lampe et al. 1992). The inclusion of soluble fibers in the diet is both practical and safe as long as the individual is not allergic to the source of the fiber.
Soluble fiber reduces the risk of heart disease by lowering cholesterol and LDL (Jenkins et al. 1993, Jenkins et al. 2001). Although the mechanism is not entirely known, soluble fiber is thought to decrease the absorption of bile. Bile, which contains cholesterol, is necessary for the digestion of fat. It is secreted into the intestine in response to food intake and most is reabsorbed after digestion is complete. When soluble fiber is present in the digestive tract, not as much bile is reabsorbed and more must be made by the liver. Some of the cholesterol that would have circulated in the blood is used to make the bile. In addition, soluble fiber can be partially broken down by intestinal bacteria which produce fatty acids that keep the liver from making cholesterol. It is generally accepted by nutrition scientists that soluble fiber intake can help reduce cholesterol levels. In one study, a high-fiber fruit and vegetable diet was shown to reduce LDL cholesterol levels by 33% within 1 week (Jenkins et al. 2001) – and a number of studies have shown that the drop in total and LDL cholesterol from a diet high in fiber is nearly 50% greater than would be predicted by differences in dietary fat and cholesterol (Haack et al. 1998, Jenkins et al. 1993, Jenkins et al. 1997).
Insoluble Fiber and Colon Cancer Risk
For nearly 30 years, researchers have been studying the effect of fiber on colon cancer. People who consume diets high in fiber tend to also consume diets that are low in total fat, low in animal fat and high in fruits and vegetables – all of which are factors that might reduce colon cancer risk. Fiber is also a vehicle for other compounds, such as phytic acid, an antioxidant thought to prevent colon cancer. Researchers are trying to determine whether fiber should be considered an independent factor in reducing colon cancer risk or whether the effect of fiber is due to a combination of other factors. Results of human studies have been mixed but a number of recent large studies suggest that fiber is not protective against colon cancer (Hung et al. 2004, McCullough et al. 2003, Michels et al. 2000, Sellers et al. 1998). In particular, McCullough (2003) and Michels (2000) looked at very large populations (cohorts from the 62,609-men and 70,554-women Cancer Prevention Study II Nutrition Cohort, the 88,764-women Nurses Health Study and the 47,325-men Health Professionals Follow-up Study) – finding that higher intakes of plant foods or fiber were not related to lower risk of colon cancer (but that low intakes were related to a higher cancer risk).
There are, however, numerous studies that have suggested that diets high in insoluble fiber reduce the risk of colon cancer (Alberts et al. 1996, Kesaniemi et al. 1990, Lampe et al. 1992) – but not all studies have supported these findings (McCullough et al. 2003, Michels et al. 2000). If, and how, fiber reduces the risk of cancer is not completely known and is undoubtedly complicated. The theory that supports the role of a high fiber diet in the prevention of colon cancer is controversial. It is suggested that insoluble fiber may work by helping to excrete bile from the body. Bile, which is necessary for the digestion of fat, is also thought to promote tumor growth. Insoluble fiber may bind with the bile – thus preventing it from being a promoter. Insoluble fiber also reduces the amount of time that fecal material is in the colon. Exposure to potential cancer-causing compounds is reduced because the fiber binds and removes these compounds quickly. Researchers who question whether or not insoluble fiber actually reduces colon cancer risk do so because they are not convinced that the fiber itself is responsible. They suggest that foods that contain insoluble dietary fiber also contain substances such as antioxidants, folate or dozens of other phytonutrients that protect the body from colon cancer (McCullough et al. 2003, Slattery et al. 1997, Voorrips et al. 2000). They argue that it is these compounds, rather than the dietary fiber, that help to protect against colon cancer.
Constipation is a condition where bowel movements (feces) are hard and dry. The strain of trying to pass the hard, dry feces may result in hemorrhoids (swollen veins in the rectum). Adequate fiber, fluid and exercise help prevent constipation. Both soluble and insoluble fibers help the feces stay moist because fiber attracts water. Insoluble fiber also has a laxative effect and helps speed up the time it takes for feces to move out of the body. This theory is well accepted. Scientific studies support the role of dietary fiber to relieve constipation. Fluid and exercise are also mentioned as important factors. Both dietary fiber and fiber supplements are beneficial.
The intake of dietary fiber or fiber supplements within the recommended doses (see below) is considered safe. To prevent dehydration, adequate fluid must be consumed. Side effects such as excessive gas or bloating may occur. Some individuals may be allergic to the source of the fiber, for example, wheat or psyllium.
Total daily fiber consumption should be within the range of 20-40 grams – with supplemental sources of fiber advised when dietary sources (fruits, vegetables, whole grains, legumes, and nuts) are below recommended levels. Popular fiber supplements (all with 8 ounces of water) may include, psyllium (7 grams – up to 3 times daily), methylcellulose (10 grams – up to 3 times daily), and polycarbophil (1 gram – up to 4 times daily).
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EDITOR'S NOTE: This monograph can be found in The Health Professional's Guide to Dietary Supplements (Lippincott, Williams & Wilkins) by Shawn M. Talbott, PhD and Kerry Hughes, MS.