Dietary fibre, dietary fiber or roughage is the indigestible portion of plant foods having two main components:
- soluble (prebiotic, viscous) fibre that is readily fermented in the colon into gases and physiologically active byproducts, and
- insoluble fibre that is metabolically inert, absorbing water throughout the digestive system and easing defecation.
It acts by changing the nature of the contents of the gastrointestinal tract, and by changing how other nutrients and chemicals are absorbed. Soluble fibre absorbs water to become a gelatinous, viscous substance and is fermented by bacteria in the digestive tract.
Sources of fibre
Dietary fibre is found in plants. While all plants contain some fibre, plants with high fibre concentrations are generally the most practical source.
Some plants contain significant amounts of soluble and insoluble fibre. For example plums or prunes have a thick skin covering a juicy pulp. The plum's skin is an example of an insoluble fibre source, whereas soluble fibre sources are inside the pulp.
Soluble fiber is found in varying quantities in all plant foods, including:
- legumes (peas, soybeans, and other beans)
- oats, rye, chia, and barley
- some fruits and fruit juices (including prune juice, plums, berries, bananas, and the insides of apples and pears)
- certain vegetables such as broccoli, carrots, and Jerusalem artichokes
- root tubers and root vegetables such as sweet potatoes and onions (skins of these are sources of insoluble fiber)
Sources of insoluble fibre include:
- whole grain foods
- wheat and corn bran
- nuts and seeds
- potato skins
- flax seed
- vegetables such as green beans, cauliflower, courgette, (zucchini), celery, and nopal
- some fruits including avocado, and bananas
- the skins of some fruits, including tomatoes
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