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Ten things you should know about ... dietary fibre

Sarah Schenker
BSc SRD PhD
Nutrition Scientist British Nutrition Foundation
London
E:S.schenker@nutrition.org.uk

"Dietary fibre" has been known by various names and has had various definitions. Older people may talk about roughage, whereas healthcare colleagues may have used the term NSPs (nonstarch polysaccharides). Dietary fibre is now defined as food material, particularly plant material, that is not hydrolysed by enzymes secreted by the human digestive tract but that may be digested by microflora in the gut.(1) Plant components that fall within this definition include NSPs such as celluloses, hemicelluloses, gums and pectins as well as resistant starches and lignin (not included in the NSP definition).
Previously, different methods of analysis have been used in the UK for dietary fibre. The Southgate method was used for many years before it was replaced by the Englyst method, which determined NSPs only, and was accepted by MAFF (Ministry of Agriculture, Fisheries and Food) as the recommended method for defining dietary fibre for nutrition purposes. Current recommendations for intakes and nutrition information on food labels are based on this definition. However, as new evidence emerged that compounds present in plants other than the polysaccharides found in cell walls have a physiological effect and may confer health benefits, in 1999 the Joint Food Safety and Standards Group (precursor to the Food Standards Agency) accepted the role of resistant starch and lignin in dietary fibre, and recommended the adoption of the AOAC (Association of Analytical Communities) method as the official UK method for analysing dietary fibre. As such the UK is now in line with the US and continental Europe in the definition of dietary fibre. However, users of analysis data must be aware that different methods give rise to different results, and the newly recommended AOAC method provides higher values than Englyst values.
 
1. Dietary sources and intakes
Dietary fibre may be divided into soluble and insoluble fractions. Insoluble dietary fibre includes celluloses, some hemicelluloses and lignin; soluble dietary fibre includes b-glucans, pectins, gums, mucilages and some hemicelluloses. The insoluble and soluble fractions apart from lignin are NSPs. Cereals, especially wholegrain foods, are a rich source of NSPs. In wheat, maize and rice NSPs are mainly insoluble, but in oats, barley and rye a significant proportion are soluble. Vegetables, which contain more water than cereals, have a lower total NSP content than wholegrains and a different overall composition. Soluble and insoluble fractions are approximately equal, whereas in fruit the ratio of soluble to insoluble has a wide range.
The dietary and nutrition survey of British adults,(2) and a study by Bingham et al,(3) found intakes of NSPs to be between 11 and 13g/day. Fibre data have also been reported by the National Food Survey since 1987, but very little change in intake has occurred since that time (13g/day in 1987 and 12.6g/day in 2000).(4) The data indicate that the main sources of NSPs are vegetables (38%), fruit (12%) and cereals (45%), particularly bread, which contributes 13%, and breakfast cereals, which contribute 12%. The Dietary Reference Value for NSP is set at 18g/day as an average for the population (individual range is 12-24g/day),(5) which should be achieved by consuming a variety of foods whose constituents contain NSPs as a naturally integrated component. This recommendation is for adults and is not applicable to children. However, it is likely that effects of NSP are related to body size, so it is recommended that children have proportionately lower NSP intakes. Children under the age of two years should not eat high-fibre foods at the expense of more energy-dense foods, which they require for adequate growth.

2. Dietary fibre and coronary heart disease
Evidence from a number of sources suggests that consumption of fibre-rich foods is beneficial to heart health. Soluble dietary fibre has been shown to lower blood lipid levels in a number of studies on patients with mild to moderate hypercholesterolaemia, particularly total and low-density lipoprotein (LDL) cholesterol, when consumed as part of a reduced-fat diet. Human experiments have clearly shown that oat fibre (soluble) tends to lower plasma total and LDL cholesterol (through the sequestration of bile acids in the upper intestine), but wheat fibre (insoluble) does not.(6) Rice, bran and barley may also lower cholesterol, but most people do not eat enough of these to have an effect.
Between 1996 and 2001 an accumulation of five very large cohort studies in the USA, Finland and Norway all reported that subjects consuming relatively large amounts of wholegrain cereals had significantly lower rates of coronary heart disease (CHD).(7) This confirms reports from earlier studies of an inverse relationship between dietary fibre intake and CHD mortality. The protective effect does not seem to be due to cholesterol lowering but due to the low intake of saturates when a high-fibre diet is consumed, and from consumption of other bioactive substances present in the wholegrain.
Other studies have shown a weak inverse association between blood pressure and cereal fibre, but this too is attributed to diets characterised by a high-fibre content rather than the effects of fibre itself. The US Food and Drug Administration allows manufacturers of wholegrain products to make health claims that wholegrain cereal foods and oatmeal or bran may reduce the risk of CHD. In the UK, the Joint Health Claims Initiative Expert Committee agreed a more generic claim: "People with a healthy heart tend to eat more wholegrain foods as part of a healthy lifestyle."

3. Dietary fibre and cancer
Dietary fibre is recommended for the maintenance of gut health, and intake has been associated with a decreased risk of colon cancer.(8) In the colon, dietary fibre tends to increase faecal bulking due to increased water retention, and insoluble dietary fibre has been shown to reduce transit time. This is important since the conversion of sterols to carcinogenic polycyclic aromatic hydrocarbons is known to occur with time.
There is some epidemiological evidence to suggest that low faecal weights are associated with increased risk of colon cancer. Dietary fibre may also bind toxins, bile acids and carcinogens in the colon so they are excreted in the faeces, limiting their potential damage. The World Cancer Research Fund has reviewed the evidence from studies looking at the link between fibre intake and cancer and has concluded that diets high in NSP/dietary fibre possibly decrease the risk of colon cancer and cancer at a number of sites, including the pancreas, rectum and breast. The COMA (Committee on the Medical Aspects of Food) working group concluded that there is moderately consistent evidence that higher intakes of fibre are associated with a lower risk of colon cancer, possibly through effects on colonic fermentation and increasing stool weight, although this is not sufficient to amount to evidence of a protective effect.(9)

4. Dietary fibre and weight loss
High-fibre foods may aid weight loss (as part of a reduced-energy diet) because their consistency encourages mastication and stimulates the secretion of digestive juices. The soluble components cause an increase in the viscosity of the stomach contents, retarding gastric emptying. This in turn affects the rate of digestion and the uptake of nutrients and creates a feeling of satiety.
Fibre supplements may also be useful for weight loss; in a recent study 53 moderately overweight females on reduced energy intake were treated for 24 weeks with a fibre supplement on a randomised double-blind, placebo-controlled basis.(10) The fibre was administered as an initial dose of 6g and a maintenance dose of 4g. After treatment, mean weight loss in the fibre group was 8.0kg versus 5.8kg in the placebo group. The results suggest that a dietary fibre supplement in combination with a hypocaloric diet is of value as an adjunct in the management of the overweight.

5. Dietary fibre and diabetes
Recent epidemiological data indicate that diets rich in wholegrains are associated with lower risk of type 2 diabetes.(11) These data are consistent with results from recent metabolic experiments suggesting favourable lipid profiles and glycaemic control associated with higher intake of wholegrains, although the precise contribution of wholegrain foods is unclear. Cereal foods with a low glycaemic index, such as pasta and oats, are encouraged in the diets of people with diabetes. Diabetes UK recommends that people with diabetes choose high-fibre starchy foods where possible.

6. Dietary fibre and the immune system
There is increasing evidence that fermentable dietary fibres and the newly described prebiotics can modulate various properties of the immune system, including those of the gut-associated lymphoid tissues (GALT).(12) Changes in the intestinal microflora that occur with the consumption of prebiotic fibres may potentially mediate immune changes via:

  • The direct contact of lactic acid bacteria or ­bacterial products (the cell wall or cytoplasmic components) with immune cells in the intestine.
  • The production of short-chain fatty acids from fibre fermentation.
  • Changes in mucin production.

Although further work is needed to better define these changes, the mechanisms for immunomodulation and the ultimate impact on immune health, convincing preliminary data suggest that the consumption of prebiotics can modulate immune parameters in GALT, secondary lymphoid tissues and peripheral circulation.
 
7. Dietary fibre and constipation
It is recommended that dietary intervention should be the primary form of treatment for constipation (over use of laxatives or suppositories). Simple constipation can usually be remedied by increasing dietary intake of fibre from cereals, fruit and vegetables, and by increasing fluid intake, both with and between meals. However, where it may be relatively easy to increase the fibre intake of a motivated adult, it can be difficult in those with conservative eating habits, particularly the very young and the very old. Older people, especially those with dentures, may find fibre-rich foods difficult to chew. In both groups, changes should be introduced gradually, using high-fibre white breads, cooked and puréed fruits, and pulses and vegetables in stews and other dishes.

8. Dietary fibre and irritable bowel syndrome
Previous dietary treatment of irritable bowel syndrome (IBS) patients has centred around wheat bran supplementation or manipulating the fibre content of the diet. However, there are currently no evidence-based guidelines for this approach. In a critical review of clinical trials that examined the effect of wheat bran fibre supplementation on symptoms in IBS patients, it was shown that six out of eight investigations detected no significant difference between fibre and placebo treatment.(13) More benefit has been observed with the use of soluble NSPs as they have a greater water-holding capacity than insoluble NSP and a more pronounced effect on faecal bulking. Delayed gastric emptying and lubrication of the stools are also cited as beneficial properties. Fruits with high soluble fibre contents such as apricots, bananas, figs and prunes have been encouraged in the diets of patients with constipation-type IBS, but more research is needed in this area.
 
9. Adverse effects
In-vitro studies have shown that NSPs can bind divalent cations such as calcium, iron, copper and zinc, probably through the action of phytic acid (phytate), often present in legumes and seeds.(14) Other uronic acids present in NSPs have also been implicated in this binding action. The antinutritional effects of phytic acid primarily relate to the strong chelating associated with its six reactive phosphate groups. Its ability to complex with proteins and particularly with minerals has been a subject of investigation; however, in-vivo studies in humans have shown little effect on mineral balance, except where the levels of phytate are also high.(15) It is likely that any direct adverse effect of a high NSP intake would occur only in people whose diet has a relatively low mineral content.
 
10. Practical ways to increase dietary fibre
Encourage clients to choose wholegrain cereal products, such as wholemeal bread and high-fibre breakfast cereals. Other high-fibre cereal products include white bread with added fibre, granary breads and rolls, wholemeal muffins and scones, multiseed bagels and rye bread, which can make good alternatives to white bread or be eaten as snacks between meals. Fruit and vegetables are high in fibre, and at least five portions should be eaten per day, including fresh, frozen, canned and dried varieties. Beans and pulses provide fibre and can be mixed into stews and casseroles or count towards one portion of vegetables.

References

  1. Codex Alimentarius. Food labelling. Rome: FAO/WHO; 1998.
  2. Gregory J, Foster K, Tyler H, Wiseman M. The dietary and nutritional survey of British adults. London: HMSO; 1990.
  3. Bingham SA, Pett S, Day KC. Non-starch polysaccharide intake of a representative sample of British adults.J Hum Nutr Dietet 1990;3:333-7.
  4. Department for Environment, Food and Rural Affairs. National food survey 2000. London: Stationery Office; 2001.
  5. Department of Health. Dietary ­reference values for food energy and nutrients for the United Kingdom. London: HMSO;1991.
  6. Davy BM, Davy KP, Ho RC, Beske SD, Davrath LR, Melby CL. High fibre oat cereal compared with wheat cereal consumption favourably alters LDL-cholesterol subclass and particles in middle-aged and older men. Am J Clin Nutr 2002;76:351-8.
  7. Truswell AS. Cereal grains and ­coronary heart disease. Eur J Clin Nutr 2002;56:1-14.
  8. Kritchevsky D. Dietary fibre and cancer. Eur J Cancer Prev 1997;6:435-41.
  9. Department of Health. Nutritional aspects of the development of cancer. London: HMSO; 1998.
  10. Birketvedt GS, Aaseth J, Florholmen JR, Ryttig K. Long-term effect of fibre supplements and reduced energy intake on body weight and blood lipids in overweight subjects. Acta Medica 2000;43:129-32.
  11. Liu S. Intake of refined ­carbohydrates and whole grain foods in relation to risk of type 2 diabetes ­mellitus and coronary heart disease.J Am Coll Nutr 2002;21:298-306.
  12. Schley PD, Field CJ. The immune enhancing effects of dietary fibre and prebiotics. Br J Nutr 2002;87:S221-30.
  13. Burden S. Dietary treatment of ­irritable bowel syndrome: current evidence and guidelines for future ­practice. J Hum Nutr Dietet 2001;14:231-41.
  14. Kelsay JL. A review of research effects of fibre intake in man. Am J Clin Nutr 1978;31:142-59.
  15. Southgate DAT. The role of dietary fibre in the diet. J R Soc Health 1990;110:174-8.