What Is Dietary Fiber?

Dietary fiber, or simply called fiber, refers to plant cell wall components that are not digestible by human or other mammalian digestive enzymes. Dietary fiber can be degraded only by anaerobic bacteria in the large intestine (colon).

Dietary (food) fiber is composed of cellulose, hemicellulose, ß-D-glucans, pectin guar, gums, and lignin. Cellulose, hemicellulose, and pectin are carbohydrates whereas lignin is non-carbohydrate. Dietary fiber is also a complex that contains minerals, antioxidants and other chemicals that are useful for health.

Pectin is 90 -100% degradable (by the bacteria in the colon), hemicellulose is 50-80%, and cellulose is 30-50%. Lignin is completely indigestible. Therefore, depending on the concentration of these components in the fiber, the digestibility and calorie value of fiber food varies.

Dietary fiber is classified into soluble dietary fiber and insoluble dietary fiber.

Soluble Dietary Fiber

Soluble dietary fiber includes pectin, beta-glucans, fructans, oligosaccharides, some hemicelluloses, guar and gums. This type of dietary fiber is soluble in water. Food sources rich in soluble dietary fiber components include legumes (beans, lentils), vegetables (such as brussels sprouts and cabbage), fruits (such as apple and berries), oat bran and pysllium seeds. Soluble dietary fiber, due to its higher degradability rate by the bacteria in the large intestine, has higher calorie content compared to insoluble dietary fiber.

Insoluble Dietary Fiber

Insoluble dietary fiber includes hemicellulose, cellulose and lignin. This class of dietary fiber is insoluble in water. Food stuffs that are rich in insoluble dietary fiber are flaxseed, whole grain breakfast cereals, and vegetables such as celery and carrots. The rate and extent of fermentation of insoluble fiber in the colon is slower than soluble fiber.

Vegetables and fruits (any plant product for that matter) contain both soluble and insoluble fiber, but depending on the vegetable and fruit type or maturity, the soluble to insoluble fiber ratio may vary.

Dietary fiber related links:
Total dietary fiber, soluble fiber, insoluble fiber content of foods
Definitions of dietary fiber food.
List of high fiber food: vegetables and fruits
List of low fiber foods
Dietary fiber: Recommended Daily Reference Intakes (DRIs, RDAs) for fiber