Table of Contents
Fiber is found only in foods of plant origin. It occurs in the skins, seeds, leaves and roots of fruits and vegetables, and in the germ and bran layers of grains. Pectins, lignans, cellulose, gums and mucilages are all different forms of fiber found in these foods. Because humans lack the digestive enzymes to break down fiber, it passes through the digestive tract largely unchanged.
Depending on the type, fiber may either slow down or speed up the passage of food through the digestive tract. It contributes to stool bulk and stimulates the colon walls to contract. Foods rich in soluble fiber are often recommended to help improve blood glucose and cholesterol levels, while diets containing high amounts of insoluble fiber are known to contribute to bowel regularity and the prevention of diverticular disease. Since high-fiber diets tend to be satisfying but relatively low in calories, they are often promoted for weight management.
Dietary fiber belongs to one of two types, depending on whether or not it is able to dissolve in water Fiber that dissolves in water is called soluble, while fiber that cannot be dissolved in water is known as insoluble Upon ingestion, soluble fiber dissolves in the fluids secreted by the digestive tract, forming a gel. This gel moves slowly through the digestive tract, thus slowing the rate of digestion and absorption. Diets containing large amounts of soluble fiber have been shown to stabilize blood sugar levels in people with diabetes, and have been shown to reduce blood levels of unhealthy (LDL) cholesterol. Foods high in soluble fiber include beans, lentils, oats, psyllium, citrus fruits, barley and apples. In contrast, insoluble fiber acts as roughage. It contributes to stool bulk and promotes regularity. Foods rich in insoluble fiber include wheat bran, whole grains, dried beans, nuts, seeds, and those fruits and vegetables with an edible outer skin or seeds.
In 2001, the Food and Nutrition Board of the Institute of Medicine established its first recommendations for fiber intake. The recommendations are based on the findings of numerous studies showing a reduced risk of heart disease and type 2 diabetes with a daily fiber intake of approximately 14 grams for every 1,000 calories consumed. For adults who are 50 years
(Illustration by GGS Information Services/Thomson Gale.)
of age and younger, the recommended fiber intake is 38 g/day for men and 25 g/day for women. For adults over 50 years of age, the recommendation is 30 g/day for men and 21 g/day for women.
On average, North Americans consume less than 50% of the dietary fiber recommended for good health.
- A high fiber intake promotes bowel health by preventing constipation and diverticular disease.
- High-fiber diets may assist with weight management because they tend to be satisfying without being calorie-dense.
- Soluble fiber has been shown to help lower blood cholesterol by binding to cholesterol molecules in the digestive tract, thus encouraging their elimination from the body.
- High consumption of fiber-rich whole grains is associated with a lower risk of developing type 2 diabetes.
- Soluble fiber slows the emptying of food from the stomach to the small intestine, thus causing a gradual release of glucose into the bloodstream after a meal. For this reason, a diet high in soluble fiber may promote better blood sugar management in those with diabetes.
- A low-fat, high-fiber diet combined with daily exercise appears to be associated with a reduced risk of developing breast cancer.
- Studies investigating whether a high-fiber diet is protective against colon cancer are inconclusive. Those that support the protective effect of fiber suggest that fiber encourages the movement of food waste through the bowel, possibly reducing the body’s exposure to carcinogens in the waste products.
- High-fiber foods tend to be rich in phytochemicals that have been linked to cancer protection.
- High-fiber diets that are comprised of large amounts of fruits, vegetables and whole grains are associated with better blood pressure control.
Fiber supplements such as psyllium may reduce the absorption of certain medications when taken at the same time. In general, medications should be taken at least one hour before or two hours after fiber supplements.
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Marie Fortin, M.Ed., RD