Dietary fibre is considered to be an important ingredient of a healthy diet. The term “dietary fibre” was coined by Hipsley (1953) to denote indigestible plant cell wall material (CWM). In the olden days dietary fibre (DF) was known as 'roughage or bulk'. Now dietary fibre is referred to as non-starch polysaccharide (NSP). It is resistant to digestion and absorption in the human small intestine. It undergoes complete or partial fermentation in the large intestine. Fibre is thought to exert significant effect on faecal volume, laxation, intestinal transit time, flatus production, bacterial metabolism and output of short chain (or volatile) fatty acids(VFA). Fibres help us clean our digestive tracts("Your Body's Broom"). Plant cell walls are the major source of dietary fiber. The major fibres of foods are cellulose, hemicelluloses, β- glucans and pectins. Gums and lignin are minor fibres of foods. The recommended dietary allowance for fibre is 25 grams.
“Dietary fibre is the edible parts of plants or analogous carbohydrates that are resistant to digestion and absorption in the human small intestine with complete or partial fermentation in the large intestine. Dietary fibre includes polysaccharides, oligosaccharides, lignin and associated substances (Anon 2001, adopted by American Association of Cereal Chemists, AACC).
The dietary fibre hypothesis
Dietary fibre hypothesis was based upon the pioneering observations of physicians and epidemiologists (Burkitt and Trowell 1975, Burkitt 1983).It is postulated that a high intake of fibre – containing foods is directly related to or is associated with a low incidence of many disorders and diseases common with a western lifestyle (e.g., chronic bowel diseases, diabetes, coronary heart disease, obesity and colon cancer). Dietary fibre intake is inversely associated with the risk of both cancer and adenomas.
Analytical fibre fractions
Total fibre – it is the aggregate amount of fibre in a food product. In other words total fibre is the sum of dietary fibre and functional fibre.
Functional fibre – it consists of isolated or purified nondigestible carbohydrates that have beneficial physiological effects in humans. It is also referred to as “isolated” or “novel” fibre mostly from animal sources. Cellulose, chitin, beta glucans, gums, inulin, oligofructose, fructoligosaccharides, lignin, pectins, psyllium, and resistant starches are forms of functional fiber when added to foods.
Crude fibre (CF) - It is the residue of plant food left after extraction with solvent, dilute acid and dilute alkali (Williams and Olmstead, 1935). Crude fibre is only 1/7 to ½ of total dietary fibre.
Total dietary fibre (TDF) - refers to total amount of nondigestible (unavailable) material naturally occurring in foods and mainly of plant origin and it includes fiber from foods such as whole legumes, vegetables, fruits, seeds and nuts, undigested products, and undigested biosynthetic polysaccharides, whereas crude fiber is the material that in chemical analysis remains after vigorous treatment with acids and alkalies (Mehta and Kaur 1992).
Kinds of dietary fibres
Dietary fibres are usually classified as soluble or insoluble based on their solubility. Plant foods contain both types of fibres in varying amounts, according to the plant’s characteristics.
Soluble fibre (SF) –It partially dissolves in water to form gel like texture. It helps to slow down digestion. It can help lower blood cholesterol and glucose levels. Soluble fibres are found in oats, rye, barley, legumes (peas, soybeans, and beans), fruits (berries, plums, avocados, bananas) vegetables (broccoli, carrots, sweet potatoes, onions) and nuts (almonds).
Insoluble fibre (IF) – It does not dissolve in water and passes through the gastrointestinal tract without being changed. It has positive water-attracting properties that help to increase bulk, soften stools and shorten stool transit time. Whole – wheat flour, wheat bran, corn bran, nuts, seeds, beans and vegetables such as cauliflower, green beans and potatoes are good sources of insoluble fibre. Insoluble fibres are gut –healthy fibres which are metabolically ferment in the large intestine (“the colon’s portion”) and can be prebiotic.
Chemical composition of dietary fibre
Dietary fibre is made up of 3 components. The largest component consists of polysaccharides or plant fibres such as bran, pectins from fruits and vegetables, various gums and beta-glucans from oats and rye. The second – largest component is lignin, which is made up of polyphenylpropane molecules and found in stalks and stems of plant products. The third component includes resistant starches and nondigestible oligosaccharides. The heterogeneity of dietary fibre is the primary reason for the diversity of its physiological effects.
The non-digestible fructo-oligosaccharides (FOS) fibres help keep the digestive tract healthy by nourishing and promoting friendly bacteria (Bifidobacteria, Lactobacilli). These microbes use some of the "prebiotic fibres,” in the food as fuel for their own growth, and through their own metabolism produce molecules called short-chain fatty acids (SCFA). SCFA production by these microbes has been associated with a decrease in cancerous colonic cells, reduction of serum cholesterol, and maintenance of healthy blood sugar levels and healthy intestinal tract cell walls. Natural food sources of FOS include onions, garlic and asparagus. FOS helps to heal irritable bowel syndrome by exerting a regulatory action on bowel movement.
Recommended dietary intake
The American Dietetic Association (ADA) recommends an intake of 20 to 30 g fibre/day for adults. The World Health Organization (WHO) recommends an upper limit of 40 grams of dietary fibres a day. Experts hypothesize that primitive diets contained between 80 and 250 grams of dietary fibre/ day (Sobolik 1994, Dreher 2001), whereas the mean dietary fibre intakes of the present are only 13 – 18 g/day (Institute of Medicine, 2002). The average daily intake of fibre in the United States is about 12g/day. The average daily intake of dietary fibre in the United Kingdom is around 12g/day in both men and women. In the developing tropical countries, the dietary fibre intake of people eating rice as a staple food (India, China, South America) are similar to Western intakes. Dietary fibre intake in the Asia – Pacific region and in most industrialized nations in Europe are also far below the recommended levels (Galvin et al 2001). The adequate intake (AI) of total dietary fibre for children, adolescents and adults was set to 14 g dietary fibre / 1000 kcal by the Institute of medicine, National Academy of Sciences, USA. The National Cancer Institute recommends an intake of 20 – 30 g fibre/day because it helps to reduce the risk certain types of cancer.
Food sources of dietary fibre
The fibre content of different foods varies greatly, e.g., cereal products 2.0% (white rice) to 42.0% (wheat bran); dried vegetables, 2.0% (chickpeas) to 25.5% (beans); dried fruits and nuts 5% (walnuts)to 18.3 (figs); fresh fruits, 0.5% (most fruits) to 3.0% (pears); and green vegetables, 1.4% (most vegetables) to 5.3% (garden peas) (Thebaudin et al., 1995).All plant – based foods contain mixtures of soluble and insoluble fibre. Legumes, whole grains and nuts are generally more concentrated sources of fibre that fruits and vegetables. Eating fresh fruit is better than fruit juice because most of the fibre in fruit is damaged when it is squeezed to make juice. It is better to get fibre from natural foods rather than from fibre tablets, power or other supplements. There is no fibre in fish, meat, shrimps, eggs and milk.
Foods high in soluble fibres
Oats, barley, oat bran, psyllium husk; Legumes –peas, beans lentils; fruits-apple, orange, passion fruit, pear; carrots, broccoli.
Foods with insoluble fibresBrown rice, rice bran, whole wheat, wheat bran; seeds and nuts; green leafy vegetables, cabbage, tomato; fruits – cherries, grapes, melons, prunes, berries.
Dietary fibre intake provides many health benefits. Because dietary fibre retains its ‘bulk’ as it moves through the digestive system, it creates a sense of fullness and satiety. The promotion of satiation, lower calorie intake, and more feelings of fullness play a positive role in preventing obesity. Dietary fibres even control the rate of digestion. Dietary fibre decreases the absorption of macronutrients and minerals.
Being indigestible and hydrophilic, dietary fibres add to the bulk of stool and soften it. It improves large bowel function and elimination (“Nature’s laxative). High fibre intakes promote bowel health by preventing constipation and diverticular disease. Dietary fibres positively modulate the colonic microflora and increase colonic fermentation. The production of volatile short chain fatty acids is used by the microflora to derive energy or for inhibiting pathogens. Viscous fibres such as those found in oats and legumes can lower serum LDL cholesterol levels and normalize blood glucose and insulin responses.
Dietary fibres bind to bile salts and they are valuable in the treatment of recurrent gall stones. The binding of dietary fibres with bile acids is integral to cholesterol homeostasis and fat absorption. Dietary fibre tends to absorb the intestinal toxins or carcinogens that build up in the large intestine and carries them out of the body.
High fibre intake is recommended (20-30g/day) in irritable bowel syndrome, haemorrhoids, diabetes mellitus and hypercholesterolemia. High fibre intake has a 40% lower risk of heart disease. Some researchers found that increasing fibre intake decreases the body’s need for insulin.
Some disadvantages of high fibre intake
Some people experience abdominal cramping, bloating or gas, when they suddenly increase their dietary fibre intake. Too much of fibre intake may lead to malnutrition and decreased absorption of minerals. Another disadvantage of foods rich in fibres is that they contain a large amount of phytic acid. Phytic acid hinders the absorption of calcium, zinc and iron.