Originally thought of as providing roughage for the transit of food through the gastrointestinal tract, dietary fibre may now be far more important for human health than was once considered. Although dietary fibre does evidently improve food transit rates during digestion, more recently a plethora of new evidence is supporting the role of dietary fibre in a wider range of physiological functions. For example, the role of dietary fibre in maintenance of the correct balance of microorganisms in the colon, the ability of fibre to provide energy in the form of short chain fatty acids through bacterial degradation and the role of fibre in cholesterol metabolism have been extensively investigated. Even more so, evidence from studies reporting on the link between dietary fibre and disease suggest that dietary fibre, or at least certain forms of dietary fibre, should be re-classified as essential nutrients in man. In particular, a deficiency of fibre in the diet may be a major cause of loss of glycaemic control, metabolic syndrome, type 2 diabetes and cardiovascular disease.
Fibre is defined as the part of the plant material that is resistant to digestion, and this includes mostly non-starch polysaccharides and lignins. Plant cell walls are the major source of dietary fibre, with intakes coming from mainly cereal grains, fruits and vegetables. Refined cereal grains do not provide the same level or quality of fibre as whole grain cereals, and this issue has been explored extensively with regard possible causes of disease. The high intakes of such refined grains in Western countries means that the average intake of fibre in developed nations is only around 20 grams per day, with about one third of this intake coming from cereal grains. Switching to whole grain cereals from refined cereals is therefore is a proven solution to increasing fibre intakes. There is no recommended intake for dietary fibre, but intakes of higher that 100 grams per day are known to occur in developing nations such as in rural African diets, and this level appears to be particularly protective of certain diseases.
The main types of fibre from plant materials are cellulose, hemicellulose (e.g. xyloglucans, arabinoxylans, glucuronoarabinoxylans), pectins, lignins, cutins and waxes and some glycoproteins from the parenchymal cells, lignified tissues and cutinised tissues of fruits and vegetables, cereals and seeds. Fibres from food additives include gums such as gum arabic, alginates, carrageenan, guar gum, carboxymethylcellulose and modified starches. The exact composition of most plant material differs between species and varieties, but some generalisations exist. The traditional classification of fibre splits types into water soluble fibre that can form a viscous gel on contact with water, and the non-water soluble fibres. Pectins and gums are water soluble and found in high concentrations in some fruits as well as legumes. These fibres for gel-like layers in the unstirred layer of the small intestine brush border region and act as a physical barrier to glucose absorption. Soluble fibre may therefore hold special nutritional properties with regard glycaemic control.
Parenchymal cells have thin primary cell walls, whereas the lignified tissues of plants have ceased growth and undergone secondary thickening, altering the degree of fibrous tissue contained within. Dicotyledonous plants deposit pectin as calcium salts on their cell plate to form a middle lamella that holds cells together (a form of cement), and this acts as the primary cell wall. Hemicelluloses, pectin and glycoproteins are then deposited along with highly packed microfibrils (mainly cellulose) as an amorphous matrix. Specialisation of cells to form xylem and phloem requires lignification as the plant organ matures. Such lignification begins in the primary cells walls and extends outwards to the middle lamella and inwards to the secondary cell wall. Such lignified cell walls also contain microfibrils (celluloses) and hemicelluloses dispersed within the lignin. Parenchymal cells of dicotyledons therefore are composed of pectins, hemicelluloses (xyloglucans) and cellulose, whereas the lignified tissue is composed of lignin, hemicelluloses and celluloses.
In contrast to the parenchymal cells in dicotyledons, cereal grains (monocotyledons) contain little or no pectin. Hence cereal grains are a poor source of dietary pectin. Cereals instead have primary cell walls composed of microfibrils (cellulose) with glucomannan embedded in an amorphous matrix of hemicellulose (arabinoxylans and β-D-glucans). Rice cell walls are halfway between those of dicotyledonous plants and cereal grains because they contain about 10 % pectin. In the case of the cereal grains, removal of the germ and bran layers in the refining of the grain removes most of the dietary fibre contained within, leaving just the starchy non-fibrous endosperm layer. This alters the starch to fibre ratio of cereal grains and this may be a cause of disease. Making juice from dicotyledonous fruits and vegetables has the same effect as refining cereal grains as it causes an unbalance in the fibre to sugar ratio. Therefore fruit juice absent its fibre may be no better than soft drinks in terms of health.
RdB
