Dietary fibre is described as the residue of the edible part of plants and other similar carbohydrates that are neither digested nor absorbed in the small intestine in humans but tend to be completely or partially fermented in the large intestine. Dietary fibres include polysaccharides, oligosaccharides, lignin and associated plant substances. Organic acids (butyric acid) and polyols (sorbitol) are also considered as part of fibre. Animal foods do not contain any fibre. Dietary fibre exhibits a tendency to increase regularity in bowel movement, lower blood cholesterol and also or blood glucose levels.
Digestion of Carbohydrates
Starch is a major component of digestible carbohydrates. Starch is formed of α-glucosidic chain and is found in cereals, potatoes, legumes/pulses and other vegetables. Polysaccharides like starch and cellulose that are available in plant foods and glycogen available from foods of animal origin are the most abundant carbohydrates that human beings eat. Starch and glycogen are broken down completely by enzyme action and this gives rise to free D-glucose.
The process starts in the mouth when you chew and on which salivary amylase act to break down the carbohydrates into starch and glycogen, leading to a mixture of maltose, glucose and oligosaccharides. This process continues in the small intestine where by the pancreatic amylase secreted from pancreas into duodenum act on carbohydrates. Cellulose cannot be broken down in a number of mammals as they do not have the enzyme to break down the carbohydrate links. This undigested fibre from plant foods forms ‘roughage’ and is essential for the bowel movement.
Types of sources
Dietary fibre is characterized by its
- source like cereal, vegetable and fruits
- solubility in water, partly, fully or insoluble
Both kinds of fibres are essential for health. Digestibility of fibre is determined by the physicochemical and structural properties of the dietary component and the processing used. If the fibre remains for a longer duration in large intestines more fibre is likely to get digested. When exposed to longer duration of degradative conditions in the large intestine, more fibre is digested. It forms the material that is fermented by microbes in the intestines. It is through this mechanism that part of the energy in carbohydrates which is resistant to digestion is made available.
Apart from making energy available, dietary fibre, also promotes interactions between nutrients but changes the pattern of microbes colonizing the colon and thus the metabolic products of such fermentation. Vegetarians may have different digestion pattern than that of non-vegetarians and thus derive different health benefits. The study of the various soluble conditions opened up the knowledge of probiotics and prebiotics.
Nutritional and health significance
Fibre is an important component of Indian diets but its benefits have been overlooked for a long time. Fibre is essential for maintaining
- body weight and composition
- blood levels of sugar
- triglycerides and cholesterol
Soluble fibre is derived from a number of sources and some of the common ones are
- modified maltodextrins which reduce blood glucose, promote growth of healthy bacteria
- inulin from wheat, onions, banana and chicory or synthesized which is also a probiotic or laxative in nature
- oligofructose which is seen as being associated with inulin or is formed as a by-product of bacterial or fungal action on inulin.
Insoluble fibre is composed of structural components of plant cells. Cereals, seeds, beans, many fruits and vegetables, bran and whole grain are food sources of insoluble fibre. These fibrous compounds also help to promote weight loss, reduce risk of colon cancer and heart disease. One Indian study also found decreased risk of cardiovascular disease through better control of lipoprotein lipids in those supplemented with good fibre.
Some foods that contain different fractions of soluble and insoluble fibres cause slow release of sugar into small intestine and its absorption into blood. These types of fibres are known as low glycemic index foods and so help to manage diabetes and control of obesity.
The requirements of fibre
The assessment of requirement of fibre is based on factors like ability to provide normal large bowel function and which produces bulk and material for fermentation. Another factor is the risk of chronic disease when dietary fibre intake is lower than certain limits in diet. So far there have been no studies that have evaluated the dietary fibre requirements in Indians. However, US Agencies recommend that minimum intake of fibre must be 20-35 g because that is conducive for long-term good health. ICMR considered this as a guideline.
The WHO Committee on chronic degenerative diseases recommended a daily intake of 30 g dietary fibre. An intake in excess of 60 g of fibre over a day can reduce the absorption of nutrients and may cause irritation in the bowel and also lead to diarrhoea. The intake of 40 g/ 2000 kcal may be rationalized in different groups based on recommended energy intake. However, with steep increase in consumption of processed and refined foods, the consumption of fibre, at least in the urban high income groups has become critical for the maintenance of good health.