Dietary fibres are the indigestible portion of plant foods that move food through the digestive system, absorbing water and easing defecation. Dietary fibres are also commonly called “roughage”. Fibre is important for the health of the digestive system and for lowering cholesterol.
Foods containing fibre often are good sources of other essential nutrients. Depending on how they are prepared, these foods can also be low in trans fat, saturated fat and cholesterol.
Food is digested by gastric acid and digestive enzymes in the stomach and small intestine, where the nutrients are released, and then absorbed through the intestinal wall for transport, via the blood, throughout the body. Any food resistant to this process is undigested, as insoluble and soluble fibres are. They pass to the large intestine only affected by their absorption of water (insoluble fibre) or dissolution in water (soluble fibre).
Soluble and insoluble fibre
Sources of dietary fibre are usually divided according to whether they are water-soluble or not. Both types of fibre are present in all plant foods, with varying degrees of each according to a plant’s characteristics.
Insoluble fibre possesses passive water-attracting properties that help to increase bulk, soften stool and shorten transit time through the intestinal tract.
Sources of insoluble fiber include:
- whole grain foods
- nuts and seeds
- vegetables such as green beans, cauliflower, courgette, and celery
- the skins of some fruits, including tomatoes
Functions of Insoluble Fibre
- move bulk through the intestines
- control and balance the pH (acidity) in the intestines
- promote regular bowel movement and prevent constipation
- remove toxic waste through colon in less time help prevent colon cancer by keeping an optimal pH in intestines to prevent microbes from producing cancerous substances
Soluble fibre is the edible part of plants or similar carbohydrates resistant to digestion and absorption in the human small intestine with complete or partial fermentation in the large intestine. It undergoes metabolic processing via fermentation, yielding end-products with broad, significant health effects.
Soluble fibre is found in varying quantities in all plant foods, including:
- legumes (peas, soybeans, and other beans)
- oats, rye, and barley
- some fruits and fruit juices (particularly, plums and berries, oranges, apples)
- certain vegetables such as broccoli, carrots
- root vegetables such as potatoes, sweet potatoes, and onions (skins of these vegetables are sources of insoluble fiber)
Functions of soluble fibre
- bind with fatty acids
- prolong stomach emptying time so that sugar is released and absorbed more slowly
- lower total cholesterol and LDL cholesterol (the Bad cholesterol) therefore reducing the risk of heart disease
- regulate blood sugar for people with diabetes
Diets naturally high in fiber can be considered to bring about five main physiological consequences:
- Improvements in gastrointestinal health-prevents inflammatory bowel disease, diverticulitis and constipation.
- Improvements in glucose tolerance and the insulin response
- Reduction of hyperlipidemia, hypertension and other coronary heart disease risk factors
- Reduction in the risk of developing some cancers-colon cancer
- Increased satiety and hence some degree of weight management. In this case dietary fiber may promote satiety by slowing gastric emptying, leading to an overall decrease in calorie intake.
Therefore, it is not appropriate to state that fiber has a single all encompassing physiological property as these effects are dependent on the type of fiber in the diet. The beneficial effects of high fiber diets are the summation of the effects of the different types of fiber present in the diet and also other components of such diets.
Oats have the highest proportion of soluble fiber of any grain. Insufficient fiber in the diet can complicate defecation. Low-fiber feces are dehydrated and hardened, making them difficult to evacuate and possibly leading to development of hemorrhoids or anal fissures.