Plant fibres are polysaccharides in plants including cellulose, hemicelluloses, gums, waxes, tannins and lignins. The question as to whether fibre is an essential component of the human diet is controversial. High fibre diets do not appear to be a requirement for health if the starch (carbohydrate) content of the diet is low. However, as the starch content rises, fibre appears to be required for correct insulin function, as the fibre may modulate the digestion rate of the glucose and slow its absorption. In this way fibre has beneficial glycaemic effects, and may become conditionally essential if high starch diets are consumed. Fibre is generally part of the the cell wall component of plant foods, and the unprocessed diet will therefore contain both starch and fibre in the original ratio at which it was present in the plant material. Processing starches to remove the fibre they contain may have detrimental effects on the control of blood sugar and this may be a significant contributory factor in the development of Western disease.
Plant fibres have traditionally been thought as not being digested in humans. However, evidence is accumulating to show that certain soluble fibres such as pectins and gums can be fermented in the colon by colonic microflora to produce short chain fatty acids. These short chain fatty acids can then contribute significantly to energy intake. Insoluble fibre such as cellulose and hemicellulose may also be digested to monosaccharides. Hemicellulose is present in the cell wall of plants and is therefore present in most foods of plant origin. Hemicellulose is composed of a number of monosaccharides including arabinose, rhamnose, xylose, galactose galacturonic acid mannose and glucose. The digestion of hemicelluloses through hydrolysis of the glycosidic bonds would therefore yields a number of monosaccharide sugars that could be measured in the gut. However, as humans do not possess the correct digestive enzymes for this process, bacterial digestion of the insoluble fibre in the gut is thought to explain the phenomenon.
The digestion of hemicelluloses has been assessed in humans. For example, researchers1 have assessed the digestion of hemicellulose in subject who had undergone an ileostomy (removal of the ileum of the small intestine) in comparison to healthy controls. Foods consumed by the subjects included bran, corn flakes, white bread, tomatoes, apples, potatoes, carrots, beans, peas and peaches. An analysis of the diet, gut and faeces of the subject was then performed to assess the presence of the monosaccharides derived from the original hemicellulose material. In the ileostomy patients, 65 and 83 % of the hemicellulose was digested by women and men, respectively. However, in the healthy subjects with intact illiums, 97 and 95 % of the hemicellulose was digested, respectively. Analysis of the monosaccharide content of the diet, gut and faeces of the subjects suggests that it is an arabinoxylan component of hemicellulose that is not digested in the the small bowel and only partially digested in the colon.
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