High fiber diet

Related Terms

Bulk, colon cancer, constipation, dietary fiber, diverticulosis, gastrointestinal, high cholesterol, high fiber, insoluble fiber, irregularities, psyllium, roughage, soluble fiber, wheat, wheat flour, whole grain.

Background

The high fiber diet is a diet that incorporates large amounts of dietary fiber. Adding fiber, or bulk, in the diet is thought to help keep the bowels regular and possibly treat or prevent certain diseases. This diet involves specifically choosing a variety of foods that have high fiber content. A high fiber diet usually contains 20-35g of fiber per day.
According to the American Dietetic Association, the average American eats only about 12-17g of fiber per day, far less than the recommend daily intake of 20-35g. Only about 1/4 of this average daily intake is soluble fiber; therefore, the average American is eating only 3-4g of soluble fiber per day, which is well below the recommended amount of 5-10g.
Dietary fiber, also referred to as roughage or bulk, is the part of a plant that cannot be digested or absorbed by the body. Dietary fiber is found in grains, fruits, and vegetables. There is no fiber in animal products such as meat, fish, eggs, or dairy products. Soluble fiber can be found in foods such as oat bran, apples, citrus, pears, peas, beans, potatoes, seeds, oranges, grapefruit and psyllium (a plant product used in common over-the-counter bulk laxative and fiber supplement products such as Metamucil). Soluble fiber is dissolved in water and forms a jelly-like bulk inside the small intestine, which may help to lower cholesterol and reduce blood sugar. Soluble fibers act mostly in the small intestine, since they are destroyed in the large intestine through bacterial action.
Insoluble fibers cannot be dissolved in water and are not destroyed by bacteria in the colon. They are found in wheat bran, corn bran, nuts, cabbage, and root vegetables. Insoluble fibers work mainly in the colon where they add bulk and help retain water, resulting in a softer and larger stool, which may aid in the treatment of constipation.
The treatment of several gastrointestinal conditions is based upon the establishment of increased fiber in one's diet. Such conditions include irritable bowel syndrome, diverticulosis and internal/external hemorrhoids. Some research data also indicates that increasing the amount of fiber in the diet may decrease the incidence of colon cancer. Colon cancer includes cancerous growths in the colon, rectum and appendix. It is the third most common form of cancer and the second leading cause of death among cancers in the Western world. Additional benefits of a high fiber diet may include more favorable cholesterol levels and a lower risk of developing heart disease. The Food and Drug Administration recently authorized food companies to use a health claim for soluble fiber from both psyllium and oats. For example, the new claim for psyllium states, "Soluble fiber from foods with psyllium husk, as part of a diet low in saturated fat and cholesterol, may reduce the risk of heart disease."

Theory / Evidence

Dietary fiber, also referred to as roughage or bulk, is the part of a plant that cannot be digested or absorbed by the body. Soluble fiber is dissolved in water and forms a jelly-like bulk inside the small intestine, which may help to lower cholesterol and reduce blood sugar. Soluble fibers act mostly in the small intestine, since they are destroyed in the large intestine through bacterial action.
Evidence: Current studies have investigated the high fiber diet for high cholesterol, heart disease, type II diabetes, colorectal cancer, constipation and weight loss. Evidence regarding the high fiber diet and the prevention of colorectal cancer is controversial. More high quality, long-term studies are necessary before any firm recommendations can be made.
Daily intake of 3g of soluble fiber from oats or 7g of soluble fiber from psyllium has been shown to lower blood cholesterol levels. A diet high in soluble fiber may reduce total serum cholesterol and LDL ("bad cholesterol") by as much as 15%. By forming a gel, water soluble fibers may stay in the stomach longer and help slow food absorption. The Food and Drug Administration recently authorized food companies to use a health claim for soluble fiber from both psyllium and oats. For example, the new claim for psyllium states, "Soluble fiber from foods with psyllium husk, as part of a diet low in saturated fat and cholesterol, may reduce the risk of heart disease."
A diet high in insoluble fiber is most often used in treatment of constipation resulting from poor dietary habits. Insoluble fibers hold water to produce softer, bulkier stools. By promoting better regularity, a diet high in insoluble fibers may help relieve constipation and control diverticular disease. Thus, people with diverticular disease are often encouraged to eat a high fiber diet with a general avoidance of nuts, seeds, hulls and some skins since these can cause acute inflammation of the diverticula resulting in diverticulitis.
According to the American Cancer Society, increasing the amount of fiber in the diet results in a lower chance of developing colon cancer. In theory, a low fiber diet allows cancer-containing agents (carcinogens) to remain in contact with the colon wall for a longer time and in higher concentrations. A large bulky stool may act to dilute these carcinogens by moving them through the bowel more quickly. Less carcinogenic exposure to the colon may mean fewer colon polyps and less cancer.
However, researchers at the Harvard School of Public Health have determined that high dietary fiber intake may not be associated with a reduced risk of colorectal cancer. Over 725,000 subjects were studied between 6 and 20 years to determine whether a high fiber diet was inversely related to colon cancer. The studies initially related the results from intakes between 10 and 30g of fiber from cereals, fruits, and vegetables and had found an inverse correlation of fiber intake and colorectal cancer risk. After further analysis of these 13 studies, Harvard researchers found that this association may not have been accurate after adjusting for other dietary risk factors.
A randomized crossover study conducted in 2006 attempted to measure the effects of fiber on colonic mucosal growth by evaluating cell proliferation and colonic mucosal protein synthesis in 9 healthy volunteers after they consumed a typical Western diet (<20g fiber per day) or a Western diet supplemented with wheat bran (24g fiber per day). Biopsies taken from the sigmoid colon were used to assess mucosal proliferation by determining proliferating cell nuclear antigen (PCNA) in crypt cells and to assess mucosal protein synthetic. The results of the study demonstrate that increased wheat bran consumption may decrease colonic mucosal proliferation, thus supporting the potential importance of dietary fiber in preventing proliferative diseases of the colon.
In 2006, a Japanese study investigated the association between dietary fiber intake and the subsequent risk of colorectal cancer. Dietary fiber intake was estimated from food-frequency questionnaires comprising 44 or 52 items at the baseline survey and 138 food items at a 5-year follow-up survey. The authors identified 907 cases of colorectal adenocarcinoma diagnosed pathologically in 86,412 subjects during a 10-year follow-up from the baseline survey. After the 5-year follow-up survey, 522 cases were identified in 78,326 subjects during a 5.8-year follow-up. The authors found no statistically significant association between dietary fiber intake and colorectal cancer in analyses using data either from the baseline survey or from the 5-year follow-up survey. The study concluded that although a minor effect cannot be ruled out, the results did not support a hypothesis of a dose-dependent protective effect of dietary fiber intake against colorectal cancer.

Author information

This information has been edited and peer-reviewed by contributors to the Natural Standard Research Collaboration (www.naturalstandard.com).

Bibliography

American Dietetic Association. 19 June 2006.
Otani T, Iwasaki M, Ishihara J, et al. Dietary fiber intake and subsequent risk of colorectal cancer: The Japan Public Health Center-Based Prospective Study. Int J Cancer. 2006 Apr 26;
Park Y, Hunter DJ, Spiegelman D, et al. Dietary fiber intake and risk of colorectal cancer: a pooled analysis of prospective cohort studies. JAMA. 2005 Dec 14;294(22):2849-57.
Weerasooriya V, Rennie MJ, Anant S, et al. Dietary fiber decreases colonic epithelial cell proliferation and protein synthetic rates in human subjects. Am J Physiol Endocrinol Metab. 2006 Jun;290(6):E1104-8.

Diet outline

The goal of the high fiber diet is to increase the amount of fiber in the diet. In order to do this, a person must know which foods are high in fiber, and which have little or no fiber. Most experts recommend that the amount of fiber is increased gradually (2-4g per week) to prevent flatulence (gas), bloating or diarrhea. There are many ways to increase the amount of fiber in a diet, including:
Substituting whole-wheat flour for half or all of the flour in home baked goods.
When buying breads, crackers, and breakfast cereals, make sure the first ingredient listed is whole-wheat flour or another whole grain. Wheat bran and oat bran can be found in a variety of cereals and breads. The label must say whole wheat or whole grain. Plain wheat bread has little fiber. Also be aware that brown bread is not always high in fiber. Sometimes bread is artificially colored to make it look more wholesome.
Using brown rice, whole grain barley, bulgur (cracked wheat), buckwheat, groats (kasha) and millet in soups and salads or as cereals and side dishes.
Replacing regular pastas with whole-wheat pastas.
Sprinkling bran in spaghetti sauce, ground meat mixtures and casseroles, pancakes, and other quick breads and in cooked cereals and fruit crisp toppings.
Eating skins and edible seeds of raw fruits and vegetables.
Including high fiber snacks, such as fresh fruit and vegetables. Whole fresh fruits contain pectin, which is another soluble fiber. Figs, prunes, and raspberries have the highest fiber content. Certain green leafy vegetables, such as spinach, kale, and broccoli are higher in fiber than leafy vegetables like lettuce, which are mostly water and contain little fiber per serving. Root vegetables, including potatoes, turnips, and carrots are high in fiber as well.
Consuming beans, including pinto, navy, lima, kidney, and baked beans, which are high in soluble fiber.
Using whole grain or bran cereals for crunchy toppings on ice cream, yogurt, salads or casseroles. Nuts, toasted soybeans, sunflower kernels, and wheat germ also can add interesting flavors and increase the fiber content of you meal.
Including a variety of soluble and insoluble fiber food sources including whole grain breads and cereal, fruits and vegetables.
Eating Middle Eastern, Oriental, Mexican or other dishes that make liberal use of vegetables, whole grains and dried beans.
Commercial fiber supplements are available ranging from bran tablets to purified cellulose (an insoluble fiber). While it is more beneficial to increase the amount of dietary fiber by eating a variety of high fiber foods sources, persons unable to change their diets might benefit from fiber supplements. These products often contain plant fiber that absorbs water and adds bulk to the stools. The fiber supplement Metamucil, for example, is made from grain (the husk of the psyllium seed) and works similarly to foods that are high in fiber. The powder form of these supplements may be able to be mixed with water or various juices and then taken at mealtime or any other convenient time. Most of these products are available in instant mix packets, so that they can be used when traveling. Some are available in capsules or edible wafers.
Drinking at least 8 cups of fluid every day is recommended while on this diet; water, milk, juice and decaffeinated sodas, teas and coffee are also sources of fluid.