A High-Fiber Diet

A High-Fiber Diet

Definition

A high-fiber diet is a diet in which an individual consumes foods that meet or exceed the dietary reference intake (DRI) for dietary fiber set by the U.S. Institute of Medicine (IOM) of the National Academy of Sciences.

Origins

No single person developed the high-fiber diet. Over the years, researchers have compared the rate of various chronic diseases in populations that had high-fiber diets with those that had lower dietary fiber intake. They found, for example, that native Africans who ate a high-fiber, plant-based diet were

rarely bothered by constipation. However, in industrialized countries where a lot of animal products are consumed, constipation is common. Observations like this encouraged researchers to look at other roles that dietary fiber might play in health. From their findings came a consensus that a high-fiber diet is a healthy diet. This is reflected in the U.S. Department of Agriculture's (USDA) Dietary Guidelines for Americans, which encourages people to eat more high-fiber foods such as whole grains.

Description

Dietary fiber refers to a group of indigestible carbohydrate-based compounds found in plants. There are two types relevant to human health: insoluble fiber and soluble fiber. 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 fills 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 lowers the risk for heart disease.

Insoluble fiber is found in whole-grain foods, bran, brown rice, 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 U.S. Institute of Medicine (IOM) of the National Academy of Sciences has set dietary reference intakes (DRIs) for fiber based on research data that applies to American and Canadian populations. DRIs provide nutrition guidance to both health professionals and consumers. The recommendations for fiber are as follows:

• children ages 1–3: 19 grams
• children ages 4–8: 25 grams
• boys ages 9–13: 31 grams
• men ages 14–50: 38 grams
• men 51 and older: 30 grams
• girls ages 9–18: 26 grams
• adult women ages 19–50: 25 grams
• women 51 and older: 21 grams
• pregnant women: 28 grams
• breastfeeding women: 29 grams
•  

Function

The average American consumes only 14 grams of fiber each day, despite extensive research that shows that higher levels of fiber provide increased health benefits. The purpose of a high-fiber diet is to encourage people to eat more fiber in order to receive those health benefits. The high-fiber diet is not designed specifically to be a weight-loss diet, although weight loss may occur as a side effect of the diet.

Benefits

Perhaps the most important health benefit of a high-fiber diet is its 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 lead to the build up of plaque, a hard, waxy substance, on the walls of arteries. This can block blood flow and result in stroke or heart attack. The mechanism for lowering cholesterol appears to be connected to the fact that cholesterol binds with soluble fiber in the intestine and can then be eliminated from the body or bile acids. Soluble fiber in oats and oat products appears to be more effective in lowering cholesterol than soluble fiber from other grains. This finding has been accepted by the American Heart Association, which recommends a high-fiber diet to maintain or improve heart health.

A high-fiber diet can prevent digestive system problems such as constipation, hemorrhoids, and diverticulitis by keeping stool soft and easy to expel. Hemorrhoids are swollen veins around the anus caused by straining to eliminate stool. Diverticulitis is a disease in which sections of the intestine bulge out to form pockets (called diverticuli) that can collect food and become infected. Increased bulk and moisture from dietary fiber helps materials move more easily through the intestine and not become trapped in these pockets.

Claims have been made that a diet high in fiber might reduce the risk of colon cancer. The theory is that fiber speeds up the elimination of waste from the colon, decreasing the time that cells lining the intestinal wall are exposed to potential cancer-causing agents. A study completed in the mid-2000s followed 80,000 nurses for 16 years and found no relationship between dietary fiber and colon cancer. More research remains to be done in this area.

Precautions

Fiber should be increased in the diet 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 should avoid a high-fiber diet.

Risks

Few risks are associated with a high-fiber diet in healthy individuals. However, in people with gastrointestinal disorders such as irritable bowel syndrome or inflammatory bowel disease, a high-fiber diet may irritate the bowel and worsen their 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 intakes of some vitamins that bind with phytates or oxalates in many high-fiber foods.

Research and general acceptance

Many large, well-designed, long-term studies have been done on the health effects of a diet high in fiber. It is almost universally accepted that health benefits result when individuals meet fiber requirements for their age group. This concept is so well accepted that it has become the official position of the National Institutes of Health and other U.S. government agencies charged with improving the health of the nation.

One contested benefit is fiber's role in the prevention of diverticular disease. Since the 1960s, doctors have advocated a high-fiber diet to prevent the formation of pouches in the digestive system known as diverticula. When many pouches form on the wall of the large intestine, the condition is known as diverticulosis. The presence of these pouches could lead to diverticulitis, a condition where a diverticulum (one pouch) or diverticula in the digestive tract become inflamed or infected. Complications of diverticulitis include infection and bleeding.

A 2012 study of more than 2,100 people between the ages of 30 and 80, conducted by the University of North Carolina, indicated that individuals who consumed the lowest amount of fiber were 30% less likely to develop the pouches than people with the greatest fiber intake. Further research is needed, however, and there are still many benefits to including fiber in the

diet.

Resources

Spiller, Gene A. and Monica Spiller. What's with Fiber? Laguna Beach, CA: Basic Health Publications, 2005.

Watson, Brenda and Leonard Smith. The Fiber 35 Diet: Nature's Weight Loss Secret. New York: Free Press, 2007.

Anderson, J.W., et al. “Health Benefits of Dietary Fiber.” Nutrition Reviews 67, no. 4 (2009): 188–205.

Clemens, R., et al. “Filling America's Fiber Intake Gap: Summary of a Roundtable to Probe Realistic Solutions with a Focus on Grain-Based Foods.” Journal of Nutrition 142, no. 7 (2012): 1390S–401S.

Ho, K.S., et al. “Stopping or Reducing Dietary Fiber Intake Reduces Constipation and its Associated Symptoms.” World Journal of Gastroenterology 18, no. 33 (2012): 4593–96.

Slavin, J.L. “Position of the American Dietetic Association: Health Implications of Dietary Fiber.” Journal of the American Dietetic Association 108, no. 10 (2008): 1716–31.

American Academy of Family Physicians “Fiber: How to Increase the Amount in Your Diet.” FamilyDoctor.org. http://familydoctor.org/familydoctor/en/prevention-wellness/food-nutrition/nutrients/fiber-how-to-increase-the-amount-in-your-diet.html (accessed September 27, 2012).