Over the years, many terms have been used to describe the indigestible residue that remains in the digestive tract after all the available nutrients have been absorbed. In fact, when research on the subject began in earnest, one of the first problems the researchers faced was agreement on terminology and definition. It was decided that the terms 'crude fibre', 'roughage' and 'fibre' were inappropriate because some components of dietary fibre are water soluble (see below) and not fibrous or 'rough' at all. The term 'dietary fibre' is however firmly established in the history of nutrition and in the public mind and it was agreed that this should continue to be used.
Hugh Trowel and Denis Burkitt, two British doctors working in East Africa and pioneers in dietary fibre research, recognised early on that fibre comprises a group of substances, all either polysaccharides or lignin. Polysaccharides consist of a multiple grouping of glucose units (as in starch) which are however impossible for humans to digest, while lignin is a coarse, 'woody' compound found in the gritty part of pears and vegetable stalks. Unfortunately, the method they used to determine the quantity of this material also measures a variety of other substances (including some starch) which resist digestion in the small intestine and are only handled by the large intestine later.
Researchers from the Dunn Clinical Nutrition Centre in Cambridge England, developed a new and more appropiate method (the Eglyst method) of analysis, in which starch is completely removed and dietary fibre measured as the non-starch polysaccharides. This has led to a redefinition of dietary fibre as NSP (Non-Starch Polysaccharides). Lignin, although not a polysaccharide, is also regarded as dietary fibre; it is bound to the cell wall and probably contributes to the effects of NSP on body functions. It is however very difficult to measure and amounts present in edible foods are believed to be small.
Soluble vs insoluble fibre
A more practical classification, based on solubility and differences in effects in the body, is that of water soluble and insoluble dietary fibre. Insoluble fibre binds water and has a 'bulking' effect, whereas soluble components are inclined to form gels in water and are therefore also referred to as 'gel-forming' dietary fibre. Interestingly, recent research has shown that water soluble fibre (found in oats, oat bran, dried legumes and some fruit) has a significant cholesterol-lowering effect.
Extraordinary oats
The well-known quip that oats is 'a grain which in England is generally given to horses but in Scotland supports the people' gives no clue as to the diverse nutritional benefits of this cereal product. Firstly, the protein present in oats is of good quality and, coupled as it is with fibre and carbohydrate, it provides a breakfast with a high satiety value at a low price. Foods which - like oats - banish hunger for lengthy periods are particularly useful for schoolchildren, who won't feel the need to dash to the tuckshop at first break!
But this does not mean that oats is strictly for the kids, since the type of fibre which oats contains is of particular significance. Dietary fibre, or roughage, is made up of insoluble fibre (which forms bulk in the digestive tract and plays an important role in promoting peristalsis and preventing constipation), and soluble or gel-forming fibre. The fibre in oats is predominantly of the latter type, which has been shown to play a useful role in lowering blood cholesterol levels. It does this by binding with cholesterol in the digestive tract and preventing its absorption into the bloodstream. In addition, it slows down digestion and, at the same time, the release of glucose into the bloodstream, which is of special benefit to diabetics. Soluble fibre also promotes bacterial action in the digestive tract, helping to prevent constipation, and may well play a role in reducing the risk of bowel cancer.
