Trends In Carbohydrate Research, Vol.6, No.1 (2014) 1-19
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Dietary fibre and related antioxidant components from cereal sources and their role in health: A review Aisha Siddique and Jamuna Prakash* Department of Food Science and Nutrition, University of Mysore, Manasagangotri, Mysore 570 006 INDIA
Abstract Cereals are a major source of carbohydrates in diet occurring as soluble and insoluble non-starch polysaccharides, including dietary fibre. Dietary fibre plays an important role in the intestinal health and appears to be significantly associated with lower risk of developing coronary heart disease, stroke, hypertension, diabetes and obesity. They consist of a diverse group of molecules with varying degrees of water solubility, size and structure, which may influence the rheological properties of the gastrointestinal contents, flow of digesta and the digestion, absorption processes. Dietary antioxidants protect against oxidative damage to DNA, proteins, lipids and have a significant impact on the regulation of gene expression. The dietary antioxidants mainly polyphenolics, traverse the small intestine linked to dietary fibre. These antioxidants are released from the fibre matrix in the colon by the action of the bacterial microbiota, producing metabolites and an antioxidant environment. The present review focuses on the role of antioxidants linked to dietary fibre. Key words: Physical properties, Chemical composition, Health benefits, Resistant starch, Non-extractable polyphenols, Short chain fatty acids.
Introduction Cereals have been the most important food source of the Indian population and therefore account for a high carbohydrate consumption of 60-70% of the total food intake. The major cereals include (Maize; Zea mays), sorghum (Jowar; Sorghum vulgare), oats (Jai; Avena sativa), barley (Jow; Hordeum vulgare), pearl millet (Bajra; Pennisetum glaucum) and other minor millets such as finger millet (Ragi; Eleusine coracana), kodo millet (Arikalu; Paspalum setaceum), proso millet (Cheena; Penicum miliaceum), foxtail millet (Kauni; Setaria italic), little millet (Kutki; Panicum sumatrense) and barnyard millet (Sanwa; Echinochloa utilis) 1. All grains have a bark like protective hull, beneath which are the endosperm, bran, and germ. The germ contains the plant embryo. The endosperm supplies food for the growing seedling. Surrounding the germ and the endosperm is the outer covering or bran which protects the grain from its environment, including the weather, insects, molds and bacteria. About 50-75% of the endosperm is starch and it is the major energy supply for the embryo during germination of the kernel. The endosperm also contains storage proteins, typically 8-18% along with cell wall polymers. Relatively few vitamins, minerals, fibre or phytochemicals are located in the endosperm fraction. The germ is a relatively minor contributor to the dry weight of most grains 2. * Corresponding author : Prof. Jamuna Prakash Tel: 0821-2419634 E-mail:
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Cereals and millets are rich in dietary energy, vitamins, several minerals, especially micronutrients such as iron and zinc, insoluble dietary fibre and phytochemicals with antioxidant properties 3. Finger millet is the richest source of calcium (300 to 350 mg/100 g grain). Small millets are a good source of phosphorus and iron. Cereal grains are rich in a wide range of phytochemicals which have been recognized to support overall health through their antioxidant potential. These are thought to act as direct free radical scavengers, co-factors of antioxidants enzymes or as indirect antioxidants 4. Free radical compounds result from normal metabolic activity as well as from the diet and environment but can cause oxidative damage to large biomolecules such as DNA, lipids and protein, resulting in an increased risk of several chronic diseases and contributing to general inflammatory response and tissue damage 5. Reactive oxygen species are involved in a variety of physiological and pathological processes, including cellular signal transduction, cell proliferation, differentiation and apoptosis, as well as ischemia reperfusion, inflammation and many neurodegenerative disorders. In healthy individuals ROS production is continuously balanced by natural antioxidative defense system. A process where the physiological balance between pro-oxidants and antioxidants is disrupted, resulting in potential damage
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of the organism, is called oxidative stress 6. Dietary antioxidants reduce free radical activity in the body, reducing the disease potential. Carbohydrates, in the simplest form, can be divided into two basic groups based upon their action on digestibility in the GI tract, the first group includes starch, simple sugars and fructans, which are easily hydrolyzed by enzymatic reaction and absorbed in the small intestine, these compounds are usually referred to as non-structural carbohydrate, non-fibrous polysaccharides, or simple carbohydrates, the second group includes cellulose, hemicellulose, lignin, pectin and β-glucans, which are resistant to digestion in the small intestine and require bacterial fermentation located in the large intestine, therefore these
compounds are referred to as complex carbohydrates, non-starch polysaccharides (NSP) or structural carbohydrate 7. The 'dietary fibre hypothesis', first proposed during 1972-1976 by Trowell, 8 stated that consumption of high levels of dietary fibre is protective against chronic conditions like chronic bowel disease, colon cancer, heart diseases, and diabetes. Over the past few decades this hypothesis has been studied extensively and significant epidemiological evidence has accumulated to support the beneficial effect of dietary fibre on several chronic condition, in particular cardiovascular disease (CVD), certain types of cancer and diabetes 9-10. In 2008, the Codex Commission on Nutrition and Foods for Special Dietary Uses defined dietary fibre as
Table 1. Dietary fibre content in different cereals and their fractions (g/100g) Grain Wheat
Flour and bran from one source
Wheat fractions from single batch Ragi Bajra Ragi Sorghum Barley Rice Rice Rice Rye Maize Oat
Description Wheat bran Wheat germ Wheat bran Whole grain Hard wheat Soft wheat White bread sliced Whole wheat flour Whole wheat bran Bran fraction- Coarse Bran fraction- Medium Bran fraction- Fine Whole wheat flour Refined wheat flour Fine semolina Coarse semolina Wheat bran Whole flour Sieved flour Milled rice Rice bran Raw rice Parboiled rice Whole Bran Whole Bran
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Fibre TDF 36.5-52.4 14.0 44.4-46% 11.6-17.0 4.59±0.21 3.65±0.11 13.80±0.45 11.5 11.3 18.89 ± 0.41 12.90 ± 0.33 21.01±0.41 24.63±0.52 4.58 ±0.06 38.20 ±0.40 17.77±0.53 13.1-19.6 86.7 11.5-37.7 18.1-25.2
IDF 35.0-48.4 12.9 10.2-14.7 2.98±0.01 1.87±0.01 5.45 42.8 49.9 37.05 15.2 12.45 3.39 5.27 5.36 42.47 9.9 9.1 17.55 ± 0.49 11.24 ± 0.43 19.59±0.41 22.07±0.41 0.75 4.32 ±0.03 33.60 ±0.84 4.32 ± 0.03 4.73 ± 0.10 14.07±0.23 11.6-16.0 86.5 8.6-33.9 14.5-20.2
Reference SDF 1.5-4.0 1.1 1.4-2.3 1.61±0.01 1.78±0.01 1.3 2.8 2.8 2.6 1.3 0.51 0.27 0.22 0.39 4.45 1.6 2.2 1.34 ± 0.07 1.66 ± 0.09 1.42±0.01 2.56±0.003 0.19 4.58 ±0.06 38.20 ±0.40 0.227 ± 0.00 0.145 ± 0.00 3.70±0.02 1.5-3.6 0.2 2.9-3.8 3.6-5.0
18 19 20 19 21 22
23
24
19 25 21 20 26 27 21 18
Dietary fibre and related antioxidant components from cereal sources and their role in health: A review carbohydrate polymers with ten or more monomeric units, which are not hydrolyzed by endogenous enzymes in small intestine of human being and belong to the following three categories; a. Edible carbohydrate polymers naturally occurring in food as consumed, b. Carbohydrate polymers, which have been obtained from raw food material by physical, enzymatic or chemical means and which have been shown to have physiological effect of benefit to health, c. Synthetic carbohydrate polymers, which have been shown to have physiological benefit to health 11. Another definition derived by the dietary reference intake deliberations, divide fibre into three categories; a. Dietary fibre (includes wheat and oat bran), b. Functional fibre (includes resistant starches) and c. Total fibre, which is the sum of dietary and functional fibre. However, none of these definition account for specifically defined particulate, because of different physico-chemical properties of fibre, which includes the viscosity, soluble and insoluble fibres12. This is particularly relevant because viscous fibre account for the majority of the clinical benefits observed with dietary fibres. The viscosity of these fibre slows digestion of nutrients by preventing bulk diffusion of foods across the intestinal lumen. This reduction in absorption lowers postprandial glucose and insulin responses, which has significant implication in prevention and management of insulinresistance and type II diabetes. Furthermore, the increased viscosity in the terminal ileum, along with blunted postprandial insulin response, effects on lipids and lipoprotein synthesis, by possibly enhancing colonic propionate synthesis and physical binding of bile acids to the fibre, which are the principle mechanism by which viscous fibre may reduce serum cholesterol 12-14.
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Dietary fibre is composed of non-digestible carbohydrate, lignin and other associated substances of plant origin, fibres of animal origin and modified or synthetic non-digestible carbohydrate polymers. The non-digestible carbohydrates are composed of the following polysaccharides: cellulose, β-glucan, hemicelluloses, gums, mucilage, pectin, inulin, r es is tan t s tar ch ; o lig o s acch ar id es : f r u cto oligosaccharides, oligofructose, polydextrose, galactooligosaccharides; and soybean oligosaccharides, raffinose and stachyose. Chitosan is an example of fibre of animal origin, derived from the chitin contained in the exoskeletons of crustaceans and squid pens; its molecular structure is similar to that of plant cellulose 15. Cereals are the principle sources of cellulose, lignin and hemicelluloses, whereas fruits and vegetables are the primary sources of pectin, gums and mucilage. Each polysaccharide is characterized by its sugar residues and by the nature of the bond between them. The complexity of fibres: their chemical nature, degree of polymerization, presence of oligosaccharide and polysaccharide require various analytic methods for the measurement of dietary fibre, to precisely estimate its composition in food and other food by-products 16. Classification of Dietary Fibre The chemical, physical and functional properties of dietary fibre are used to classify fibre into two types: water insoluble or less fermented fibre and water soluble or well fermented fibre. Water insoluble or less fermented fibre includes cellulose, hemicellulose, lignin 7-17. The insoluble and soluble dietary fiber in different cereals and their fractions as reported by different authors are presented in Table 1. The chemical composition is depicted in Fig.1.
Cellulose Galactoglucomannans β-glucans Xylans Arabinoxylans Hemicellulose Mannans Pectin Glucomannans Inulin Galactomannans Lignin Xyloglucans Oligofructose Polydextrose Resistant maltodextrin Gum Carageenan Chitosan Alignate
Figure. 1. Chemical composition of dietary fibre www.trendscarbo.com
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Cellulose forms the main structural component of plant cell wall insoluble in concentrated alkali, soluble in concentrated acid, and hemicellulose are the cell wall polysaccharides, which contain backbone of β-1,4 glucosidic linkages, soluble in dilute alkali. Lignin is a non-carbohydrate cell wall component, containing a complex cross linked phenyl propane polymer due to strong intra molecular bonding, which includes carbon to carbon linkages. Lignin is very inert and forms greater resistance than any other naturally occurring polymer. Water soluble or well fermented fibre includes pectin, gums and mucilages. Pectins are the complex group of components of primary cell wall with Dgalacturonic acid as principal components. Gums are secreted at site of plant injury by specialized cells. Mucilages are synthesized by plant, where they prevent desiccation or excessive dehydration of seed endosperm 28. Soluble fibre dissolves in water forming viscous gels. They bypass the digestion of the small intestine and are easily fermented by the microflora of the large intestine. They consist of pectins, gums, inulin type fructans and some hemicellulose. In the human GI tract, insoluble fibres are not water soluble. They do not form gels due to their water insolubility and limited fermentation 29.
almost completely metabolized by the microflora. When fermented, inulin favors propionate production which in turn, decreases the acetate to propionate ratio leading to decreased total serum cholesterol and low density lipoproteins (LDL), which are important risk factors for coronary heart diseases (CHD) 31. Inulin has also been found to be beneficial for the health of the human large intestine as a prebiotic. It was demonstrated that inulin stimulated the growth of bifidobacteria, while restricting the growth to potential pathogenic bacteria such as E.coli, salmonella, and listeria. This could be beneficial in such disorder as biological colitis and C.difficile infections 32. Inulin may also provide a way to prevent and treat obesity. Cani et al., 33 demonstrated that oligofructose, a sub-group of inulin, increased satiety in adults which led to decrease in total energy intake. This is thought to be due to short chain fatty acids and their ability to increase appetite suppressing hormones such as glucagon-like peptide-1 (GLP-1). β-glucan
Arabinoxylan, a constituent of hemicellulose, is comprised of a xylose backbone with arabinose side chains. It is a major component of dietary fibre in whole grains having considerable inclusions in both the endosperm and bran. In wheat, arabinoxylan account for around 64-69% of the NSP in the bran and around 88% in the endosperm. During normal wheat flour processing, a majority of the arabinoxylan is removed as a by-product. In the GI tract, arabinoxylan acts much like a soluble fibre being rapidly fermented by the microflora of the colon 30.
β-glucan is linear polysaccharide of glucose monomers with β(1→4) and β(1→ 3) linkages found in the endosperm of cereal grains, primarily barley and oats. β-glucan is water soluble and highly viscous at low concentrations 34. β-glucan's viscosity in the GI tract is the most probable mechanism in which it decreases serum cholesterol levels as well as improves postprandial glucose metabolism. This gelation property may decrease bile acid absorption by increasing viscosity and increased bile acid excretion. This subsequently results in a higher hepatic cholesterol synthesis because of the higher need for bile acid synthesis 35. The same viscosity may also delay glucose absorption into the blood thus lowering postprandial glucose and insulin levels. Nazare et al.,36 observed that 5 g of oat β-glucan added to an oat concentrate cereal significantly delayed, but did not reduce, total glucose absorption. The production of short chain fatty acids from β-glucan may also be a probable mechanism behind its observed metabolic effects. Fermentation of oat β-glucan has been shown to yield larger amounts of propionate 7. Propionate has been shown to significantly inhibit cholesterol synthesis in humans and is thought to be due to the inhibition of the rate limiting enzyme HMG CoA reductase 37.
Inulin
Pectin
Inulin is polymer of fructose monomers and it is used as a functional food ingredient due to its nutritional properties. Enzymatic hydrolysis of inulin in small intestine is minimal (
