Dietary Fiber

Figure 1. Sources of dietary fiber.

Dietary fiber, fibre, or roughage is a type of carbohydrate that human bodies cannot break down completely. Dietary fibers are found in plants and they are diverse in chemical composition. Dietary fibers are grouped generally by their solubility, viscosity, and fermentability. The combination of these characteristics determine how the fiber is processed within the body and subsequently their role in maintaining good health.

For example:

• Bulking fibers--such as cellulose and hemicellulose (including psyllium)--absorb and hold water, promoting regularity.
• Viscous fibers--such as beta-glucan and psyllium--thicken the fecal mass.
• Fermentable fibers--such as resistant starch, xanthan gum, and inulin--feed the bacteria and microbiota of the large intestine which are critical to gastrointestinal health.

Dietary fiber has two main components: soluble fiber and insoluble fiber. Plants contain both types of fiber in varying amounts, depending on the fiber's viscosity and fermentability.

Soluble fiber (or sometimes known as fermentable fiber, prebiotic fiber) dissolve in water and is generally fermented in the colon into gasses and physiologically active by-products such as short-chain fatty acids. Beta-glucans are soluble fibers. Psyllium is a soluble, viscous, non-fermentable bulking fiber that retains water as it moves through the digestive system, easing defecation and promoting regularity.  

Insoluble fiber does not dissolve in water and is inert to digestive enzymes in the upper gastrointestinal tract. Examples are wheat bran, cellulose, and lignin. Coarsely ground insoluble fibers trigger the secretion of mucus in the large intestine, providing bulking. Finely ground insoluble fibers do not have this effect and can actually lead to constipation.

Food Sources

Some good sources of soluble fiber include:

• Vegetables such as broccoli, carrots, and artichokes
• Skin of fruits such as apples, pears, quinces
• Fruits such as avocados, plums, prunes, berries, and ripe bananas
• Root tubers and vegetables such as sweet potatoes and onions
• Legumes such as peas, soybeans, lupins, and beans
  
• Oats, rye, chia, and barley
  
• Psyllium seed husks and flax seeds
• Nuts such as almonds

Some good sources of insoluble fiber include:

• Whole grains
• Wheat and Corn Bran
• Legumes such as Beans and Peas
• Nuts and Seeds
• Potato Skins
  
• Vegetables such as green beans, cauliflower, zucchini, celery, and nopal (prickly pear)
• Fruits such as avocado, unripe bananas
• Skins of fruits such as kiwis, grapes, and tomatoes

Taken as a Dietary Supplement

Dietary fibers are essential for good health and may lower the risk of developing various non-communicable diseases such as heart disease, diabetes, diverticular disease (i.e. affecting the large intestine or bowel), and constipation.

Dietary fiber's role in the gut microbiome may produce anti-inflammatory effects that can help alleviate chronic inflammation or diseases such as irritable bowel syndrome and inflammatory bowel disease.

Children and adults need at least 25g-35g of dietary fiber per day to maintain good health.