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Dietary fibre and health: what is the connection?

Kathy Cowbrough
BSc MPHN
Freelance Dietitian and Public Health Nutritionist
E:kathy.cowbrough@virgin.net

Dietary fibre*, or nonstarch polysaccharides (NSPs), consists of the parts of the plants we eat that the body cannot digest or absorb, mainly cellulose, hemicelluloses and pectin substances.(1,2) They are found in fruits, vegetables, cereals (especially wholegrains) and legumes (or pulses). Dietary fibre is also defined as: "Food carbohydrate that is not digested or absorbed and which contributes to positive physiological functions in the body."(3)
Fibre is not a nutrient - the benefits of fibre derive from the fact that it is not absorbed by the body, but it stills plays a very important part in maintaining good health. It helps prevent many disorders of the digestive system as well as coronary heart disease.
A further problem is the classification of NSPs into soluble or insoluble in terms of physiological effect. This approach makes it difficult to evaluate the effects of fibre provided by a mixed diet, as some fibres are classed in one category but have major benefits attributable to both soluble and insoluble fibre. Thus it has been suggested that these terms be eliminated and replaced by the specific beneficial physiological effects of fibre.(4,5)
Insoluble NSPs consist of cellulose, one of the most abundant organic substances in nature and the principal component of cell walls in higher plants. Insoluble fibre is found mainly in cereals and grains, especially wholegrain.
Soluble NSPs are a diverse group of compounds and include pectin, glucans, gums and mucilages. Soluble fibre is found in all fruits, some cereals (such as oats and barley) and legumes (dried peas and beans and lentils). Vegetable fibre contains equal amounts of insoluble and soluble NSPs.
Insoluble fibre usually resists fermentation in the large bowel, enabling it to hold water like a sponge, making stools bulkier and reducing their transit time. The action of soluble fibre is complex. Soluble fibre can be fermented by colonic bacteria and forms a viscous gel, so is less bulky but tends to blunt glucose and lipid absorption. The fermentation process also produces short-chain fatty acids (SCFAs), carbon dioxide, hydrogen and methane as byproducts.

Health benefits of dietary fibre(6,7)

Fibre, cholesterol and cardiovascular disease
Viscous types of fibre (soluble sources such as fruits and vegetables, legumes, oatmeal) are known from numerous investigations to be responsible for cholesterol lowering. While there is debate about the amount of fibre needed to lower the blood cholesterol, the composition of the diet, and whether it is more effective when cholesterol is higher, there is definitely benefit in recommending an increase in dietary fibre to both prevent and manage cardiovascular disease (CVD). Additional benefits are that higher-fibre diets are usually lower in energy, fat and sugars, which can help in the treatment of obesity and hypertriglyceridaemia associated with CVD. Antioxidants found in these foods may also have health benefits.

Fibre, glucose and diabetes mellitus
Considerable experimental evidence demonstrates that viscous dietary fibres slow the rate of absorption of glucose and benefit long-term glucose control in individuals with diabetes mellitus.

Fibre, faecal weight and constipation
One of the most famous effects of fibre is as a laxative. Fibre contributes to increased faecal weight due to the amount that remains undigested and unfermented, and also through the contribution of bacterial bulk from the fibre substrate in the large colon. These factors in turn affect the water-holding capacity of the colon contents, helping food travel along the digestive tract more quickly, thus relieving constipation. Hence fibre in mixed diets, legumes, and whole- and high-fibre grain products are particularly effective at promoting normal laxation.

Dietary fibre, digestion and satiety
NSPs have some subtler effects:

  • A fibre-rich meal is processed more slowly, and nutrient absorption occurs over a greater period of time.
  • Fibre-rich diets are usually less energy-dense and larger in volume than low-fibre diets, which should limit intake of more food.
  • The larger mass of food takes longer to eat, and its presence in the stomach should bring a feeling of satiety sooner, although this may be only short term.

Fibre and diverticulosis
Adequate intake of dietary fibre prevents the formation of diverticula by providing adequate bulk in the colon so that less forceful contractions are needed to propel it through. A high-fibre diet is recommended for diverticular disease of the colon as the bulk will prevent the formation of additional diverticula, but it will not resolve the existing diverticula.

Fibre and cancer
Extensive epidemiological evidence supports the theory that dietary fibre may protect against large bowel cancer. While there have been intervention studies that did not support these protective properties,(8,9) the scientific consensus is that there is enough evidence that dietary fibre protects against colon cancer and that health professionals should promote the increased consumption of dietary fibre.(10)
Low intake of fruits and vegetables are associated with increased risk of colorectal and gastric cancer.
 
What about too much fibre?
Reduced absorption of vitamins, minerals, proteins and calories can occur with a high-fibre diet, although it is unlikely that healthy adults consuming within the recommended amounts will have problems with nutrient absorption. However, high-fibre diets are not appropriate for children or the elderly. Children have high-energy needs and may have small appetites, and high-fibre diets are bulky. Elderly people, particularly the vulnerable elderly, may have reduced energy needs and can have poor appetites, so their food choices need to be nutrient-dense and not too high in fibre. However, constipation can be a problem with the inactive elderly, so fluid intake and other forms of increasing fibre that are not covered in this article may need to be considered.

How much NSPs do we eat - and how much should we eat?
The average intake of NSPs in the UK is 11-13g per day, with some as low as 5g per day. The government recommends an NSP intake for adults of 18g per day from a variety of fibre-rich foods, not from supplements or supplemented foods.
Table 1 lists dietary sources of NSPs related to their fibre content. Below is a list of broad practical suggestions to increase dietary fibre in people's diets. It is very important when recommending an increase in dietary fibre to make sure fluid consumption is also increased to at least 6-8 large glasses of water a day, as well as suggesting a gradual increase in higher-fibre food choices.

[[NIP20_table1_48]]

Increasing fibre intake

  • Eat more wholemeal bread or high-fibre white bread.
  • Eat more wholegrain breakfast cereals or those that contain bran or oats.
  • Eat more brown rice or wholewheat pasta.
  • Eat more fruits and vegetables.
  • Use more pulses (eg, peas, beans, lentils) - add them to casseroles, soups and composite dishes.
  • Use some wholemeal flour in baking, such as a 50:50 mixture of wholemeal and white flours.

These changes should be introduced gradually, as a sudden large increase in fibre consumption can result in distension and flatulence. Additional fluids will also be necessary.

*The term dietary fibre is more commonly used in lay literature as the general public have some concept of what it is, having seen it on food labels. However, it has no universally accepted definition, and scientists prefer the term nonstarch polysaccharides. In this article the two terms are used interchangeably.

References

  1. Trowell HC. Dietary fibre: a ­paradigm. In: Trowell HM, Burkitt D, Heaton KW, editors. Dietary fibre, fibre depleted foods and disease. New York: Academic Press; 1985. p. 1-20.
  2. Champ M, Langkilde A-M, Brouns F, Kettlitz B, Collet YLB. Advances in dietary fibre characterisation. 1. Definition of dietary fibre, ­physiological relevance, health benefits and analytical aspects. Nutr Res Rev 2003;16(1):71-82.
  3. Gordon DT, Stoops D, Ratliff V. Dietary fiber and mineral nutrition.In: Kritchevsky D, Bonfield C, editors. Dietary fiber. Minneapolis (MN): American Association Cereal Chemists; 1994. p. 279-303.
  4. Dietary reference intakes proposed definition of dietary fiber. Washington (DC): National Academy Press; 2001. p. 1-64.
  5. FAO/WHO. Carbohydrates in human nutrition. Report of a joint FAO/WHO consultation. FAO Food and Nutrition Paper 66. Rome: FAO; 1998.
  6. American Dietetic Association. Position Paper of the American Dietetic Association: health ­implications of fiber. J Am Diet Assoc 2002;102:993-1000.
  7. Briony Thomas, editor. Manual of dietetic practice. 3rd ed. London: British Dietetic Association; 2001. p. 162-4.
  8. Schatzkin A, Lanza E, Corle D, et al. Lack of effect of a low-fat, low-fibre diet on the recurrence of colorectal adenomas. N Engl J Med 2003;342:1149-55.
  9. Alberts DS, Marinez ME, Kor DL, et al. Lack of effect of a high fiber cereal supplement on the recurrence of colorectal adenomas. N Engl J Med 2000;324:1156-62.
  10. Department of Health. Report of the Working Group on Diet and Cancer and the Committee on Medical Aspects of Food and Nutrition Policy. Nutritional aspects of the development of cancer. Report on Health and Social Subjects 48. London: The Stationery Office; 1998.