What is dietary fibre?

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.

Where do you find dietary fibre?

By definition, only plant foods contain dietary fibre. Published values of fibre in foods vary greatly because of different analysis methods.

Wheat bran (commonly sold under the name of 'digestive bran') has the highest fibre content of all the items mentioned. Because of this, it should be regarded as a food supplement rather than as a food. (If you eat it as a food on its own - for example as a breakfast cereal - your fibre intake could in fact be too high!)

However, as shown above, an important result of the years of research into fibre is the realization that it can be used to lower the risk of some diseases and to treat them.

Can fibre help you lose weight?

Although there is as yet no firm evidence that fibre can be used to control weight problems, it can indirectly be of help if you're trying to cut down on kilojoule intake.

One of the effects of dietary fibre is that it 'dilutes' nutrients and therefore also the energy or kilojoule values of a food. In addition, foods which are rich in it need a good deal of chewing, which not only stimulates secretion of saliva but also means that it takes longer to eat them. In the stomach, insoluble fibre binds water and has a 'bulking' effect, while soluble fibre tends to form gels which slow down the emptying of the stomach contents into the small intestine. The result of these combined effects is that fibre-rich foods make you feel fuller for longer and less likely to head for the refrigerator half an hour after a meal! For this reason, and also to ensure that you get enough vitamins and minerals and won't suffer from constipation, any slimming diet should include ample fibre in the form of unrefined foods.

Effects of fibre in the small intestine

The small intestine is divided into the duodenum, jejenum and ileum and dietary fibre alters a number of functions throughout the area. Whereas refined foods are quickly digested and absorbed in the upper part or duodenum, fibre ensures that the food moves down into the jejenum and ileum, thus changing the site of digestion and absorption. The result is a slower and more prolonged or sustained absorption of several nutrients, which gives the body time to 'process' these nutrients.

In addition to water-binding and bulking effects, some fibre components have the ability to temporarily bind nutrients such as glucose and fats, as well as digestive enzymes, bile acids and possibly some toxic substances. This results in slower digestion of several nutrients and an increased excretion of bile acids - and perhaps also of poisonous substances.

How to increase your fibre intake

Having established that fibre has definite benefits to offer, how can you make sure you get enough of it on a regular basis? Basically, there are three ways of upping your fibre intake, namely:through following a natural high-fibre diet (lots of fruit and vegetables, wholewheat bread, unrefined rice, etc)by eating fibre-enriched foodsby taking fibre extracts or supplements in the form of bran, pills, tablets, powders or granules.

It probably goes without saying that the first option is the healthiest. A natural high-fibre diet in which more unrefined plant foods and less foods of animal origin are eaten, is preferable not only because it ensures adequate intakes of vitamins and minerals, but because it also includes less animal fat - whereas the pill-popping method will merely increase the intake of indigestible fibre.

Eating fibre-enriched products - such as some breakfa^st cereals and home-baked muffins to which bran has been added - will certainly increase fibre intake, but will not necessarily rectify dietary imbalances. This is because most South African recipes for wholewheat home-baked products contain too much fat and sugar, which may improve the flavour but also markedly increases the kilojoules. However, it is possible to cut both the fat and sugar content to healthier levels when baking at home.

Fibre extracts are useful in the treatment of disease. They should only be used for medical reasons and not as foods, and must be taken under the guidance of a physician or dietitian. Insoluble extracts such as bran should always be used in combination with adequate water intake to treat constipation and related diseases. Soluble extracts, such as konjac-glucomannan, pectin or guar gum, are used to lower blood cholesterol levels and to control glucose levels in diabetic patients.

How much is enough?

Recommendations for fibre intake are based on those of developed populations in the past (before the escalation of the diseases of affluence) and on intakes of population groups at low risk for these diseases - such as rural, less developed populations and vegetarians.

On average, affluent Western populations presently consume 10 to 20g total dietary fibre per day; it has been recommended that this should be increased to between 30 and 40g per day. Our own research has shown that white South Africans on a typically Western diet eat approximately 2g total dietary fibre per 1000 kilojoules (kJ) per day - a figure which should be doubled. Based on this recommendation, a fairly active adult woman with a daily energy need of between 8000 and 9000 kJ should take in 32 to 36g dietary fibre per day, while a moderately active adult man with an energy need of 9000 to 11000 kJ should consume 36 to 44g fibre.

The table below shows that you can obtain approximately 40g of total dietary fibre by eating six small to medium portions of wholegrain cereals (which include wholewheat bread, brown rice and unrefined breakfast cereals), one medium portion of legumes (beans, peas, lentils), three medium portions of vegetables and two medium portions of fruit per day.

Foods (and portion sizes) that will provide approximately 40 g total dietary fibre per day

Food	Portion size	Total amount (g)	Dietary fibre (g) -
4 medium slices"wholewheat bread	1 slice: 95 X 90 X 10 mm = 35 g	140	12,5
1 portion All
Bran Flakes	125 ml	25	7,5
1 portion cooked brown rice	125 ml	70	0,9
1 portion cooked dried beans	125 ml	100	7,4
1 portion cooked broccoli	125 ml	75	3,1
1 portion grated raw carrots	125 ml	95	2,4
1 portion mixed raw salad	125 ml	50	0,8
1 medium apple	52 X 66 mm	150	2,6
1 medium orange	65 X 70 mm	180	3,6
TOTAL			40,8