High-Fiber Diet

A high-fiber diet is one that emphasizes consuming whole, unprocessed, plant-based foods such as fruits, vegetables, and grains that contain dietary fiber. Dietary fiber refers to a group of indigestible carbohydrate-based compounds found in plants. People on a high-fiber diet are advised to consume foods that meet or exceed the dietary reference intake (DRI) for dietary fiber as determined by the U.S. Institute of Medicine (IOM) of the National Academy of Sciences.

Food sources of fiber

Dietary fiber refers to a group of indigestible carbohydrate-based compounds found in plants that give the plants rigidity and structure. Two types of fiber are important to human health, soluble fiber and insoluble fiber. A fundamental difference is that soluble fiber dissolves in water and insoluble fiber does not, a distinction that determines how each type of fiber functions in the body.

Soluble fiber is found in many vegetables and fruits, including carrots, apples, pears, and citrus fruits. Other sources include legumes, barley, oats, and oat bran. Soluble fiber helps to slow digestion so that a person feels full for a longer period of time. This sense of fullness, known as satiety, helps with appetite control and weight loss. Soluble fiber also helps to decrease cholesterol levels, which reduces the risk for heart disease.

Insoluble fiber is found in whole grains such as whole wheat, wheat bran, brown rice, buckwheat, millet, and raw vegetables. This fiber, known as roughage, helps move food waste through the digestive system by adding bulk. The increased bulk causes the walls of the intestine to contract rhythmically (peristalsis) so that waste moves through the large intestine more rapidly. In the colon, most of the water in digested food is reabsorbed into the body, and then the solid waste is eliminated. By passing through the colon more rapidly, less water is reabsorbed from the waste. The stool remains soft and moist and is easy to expel without straining, preventing constipation.

The Food and Nutrition Board at the U.S. Institute of Medicine (IOM) of the National Academies (formerly the National Academy of Sciences) has determined dietary reference intakes (DRIs) for fiber based on research data derived from American and Canadian populations. DRIs are established to provide nutrition guidance for both health professionals and consumers and, in the United States, are reflected in the Dietary Guidelines for Americans introduced by the U.S. Department of Agriculture (USDA). These guidelines encourage people to eat more high-fiber foods such as whole grains. The recommendations for daily fiber intake are as follows:

The purpose of a high-fiber diet is to support the digestive process and help maintain an ideal weight and normal cholesterol levels. People are encouraged to eat enough fiber to receive those health benefits. A high-fiber diet is also reported to improve blood sugar levels through its effects on glucose and insulin response. Although a high-fiber diet is not designed as a weight loss diet, weight loss may occur as a desirable side effect.

One of the most important health benefits of a high-fiber diet is reducing cholesterol levels, which has the potential to protect against heart disease. Multiple large, well-designed studies have shown that soluble fiber can lower blood cholesterol levels. High levels of cholesterol can combine with other components carried in the blood and lead to the buildup of plaque, a hard, fatty substance, on the walls of arteries. Plaque buildup narrows the arteries and can block blood flow, increasing blood pressure and inducing clot formation, which sometimes results in stroke or heart attack. Soluble fiber binds bile acids (cholesterol is a component) in the intestine, allowing them to be eliminated from the body. As a result, more cholesterol in the body is used to make new bile acids. A particular type of soluble fiber known as beta glucans is found in oats and oat products and is noted to be especially effective in lowering cholesterol; this type of fiber is also found in barley and mushrooms. The American Heart Association recommends a high-fiber diet to maintain or improve heart health.

A high-fiber diet has been shown to help prevent digestive system problems such as constipation, hemorrhoids, diverticulosis, and diverticulitis by keeping stools soft and easy to expel. Hemorrhoids are tiny veins just inside the anus that swell and protrude from the anus when people repeatedly strain to eliminate stool. Diverticular disease begins with a condition called diverticulosis in which small pockets (diverticula) form in the walls of the large intestine. When these pouches fill with digestive waste, they may become inflamed, which may lead to diverticulitis, a painful condition that can progress to infection and bleeding. Lack of sufficient fiber in the diet is a contributing factor and research suggests that the increased bulk and moisture from dietary fiber helps waste move more easily through the intestine rather than becoming trapped in the diverticula. Although studies have shown that a high-fiber diet may reduce abdominal symptoms of uncomplicated diverticular disease, researchers suggest that the benefits of dietary or supplemental fiber in treating or preventing diverticulitis still remain to be established.

Research continues on possible associations between a diet high in fiber and reduced risk of colon cancer. One theory is that fiber speeds the elimination of waste from the colon, decreasing the time that cells lining the intestinal wall are exposed to potential cancercausing agents. Because certain anti-inflammatory medications (e.g., aspirin and other nonsteroidal anti-inflammatory drugs) are known to suppress tissue inflammation that may lead to cancer, studies have examined combining anti-inflammatory drugs with a high-fiber diet as a way to prevent colon cancer in high-risk individuals. Early studies suggest that the dietary phytochemicals in a high-fiber diet may work well with anti-inflammatory drugs to protect the colon, but no actual clinical successes have yet been shown.

Dietary fiber should be increased gradually. If fiber intake increases too suddenly, abdominal pain, gas, and diarrhea may result. When eating a high-fiber diet, it is important to drink at least eight glasses (64 oz. or 2 L) of water or other fluids daily. People whose fluid intake must be restricted for medical reasons are advised to avoid a high-fiber diet.

Few risks are associated with a high-fiber diet in healthy individuals. In people with gastrointestinal disorders such as irritable bowel syndrome or inflammatory bowel disease, a high-fiber diet may irritate the bowel and worsen the symptoms. Likewise, people who have had a surgical weight loss procedure may be unable to tolerate a high-fiber diet. Adding bran fiber to foods is not recommended due to the risk of poor intake of certain vitamins that bind with phytates or oxalates in high-fiber, plant-based foods.

See also Constipation ; Digestive diseases ; Diverticular disease diet ; Fiber ; Hemorrhoids ; Inflammatory bowel disease ; Irritable bowel syndrome diet ; Whole grains .