Dietary Fibre

BACKGROUND

Adequate dietary fibre is essential for proper functioning of the gut and has also been related to risk reduction for a number of chronic diseases including heart disease, certain cancers and diabetes (see 'Chronic disease' section for further discussion).

There is no single definition of dietary fibre, which is a component of all plant materials. What can be said with certainty is that most of the components of dietary fibre are carbohydrate in nature, lignin being an exception. Hipsley first used the term 'dietary fibre' in 1953 to describe plant cell walls in the diet, which were thought to protect against toxaemia of pregnancy. This term, later taken up by Trowell (1972), encompassed only components of the plant cell wall that resisted digestion by secretions of the human alimentary tract, namely cellulose, hemicelluloses, pectin and lignin.

Trowell described dietary fibre as either 'the skeletal remains of cell walls' or as 'remnants of the plant cell wall' (Trowell 1972, 1975). As it is difficult to determine whether indigestible materials from plants came from the cell wall or other parts, the definition was expanded to include all indigestible components of plant origin (Trowell et al 1976). In 1987, the Life Sciences Research Office of the Federation of American Societies for Experimental Biology (1987) adopted a definition of dietary fibre as 'the endogenous components of plant materials in the diet which are resistant to digestion by enzymes produced by humans'. This definition can be considered to include some components of what is now known as resistant starch (RS). As pointed out by Southgate (1991), this definition is virtually identical to that for 'unavailable carbohydrates' as originally defined in McCance & Lawrence (1929).

One difficulty with the word endogenous in this definition is that it excludes, for example, those forms of RS that arise as a consequence of cooking and processing techniques. It also excludes substances which are intimately associated with the major components of dietary fibre and which are capable of having important nutritional and/or physiologic effects such as phytates, lectins, saponins, nonpolymeric polyphenols, and inorganic constituents. Recent data have indicated that while non-starch polysaccharides (NSP) are important for human health, RS may be as significant if not more so for many health conditions (Topping & Clifton, 2001).

Food Standards Australia New Zealand (FSANZ) defines Dietary Fibre as follows: 'Dietary fibre means that fraction of the edible parts of plants or their extracts, or synthetic analogues, that are resistant to the digestion and absorption in the small intestine, usually with complete or partial fermentation in the large intestine. Dietary fibre includes polysaccharides, oligosaccharides (degree of polymerisation >2) and lignins, and promotes one or more of the following beneficial physiological effects:

(i) laxation
(ii) reduction in blood cholesterol
(iii) modulation of blood glucose'.

This definition was gazetted in Standard 1.2.8 of the ANZ Food Standards Code in August 2001. The code also prescribes a number of acceptable Association of Official Analytical Chemists (AOAC) methods of analysis for total dietary fibre or its components that led to the inclusion of inulin, fructooligosaccharides and polydextrose in the category of dietary fibre. At the time of publication of the current document, FSANZ has not assessed a method for assaying RS.

In Australia, the National Nutrition Survey of 1995 indicated that 45% of dietary fibre comes from breads and other cereal foods, 10% from fruit and 30% from vegetables (NNS 1998). The distribution is similar in New Zealand, with 44% from breads and cereals, 13% from fruit and 28% from vegetables (MOH 1999). However, it is worth noting that the food data bases for dietary fibre used for these surveys do not equate precisely to the FSANZ definition as the analytical methods used (AOAC in Australia and Englyst in New Zealand) measure a different set of components. Nevertheless, the differences have been assumed to be relatively small.

Resistant starch comes within the FSANZ definition but is only partially assessed using currently approved methods that account for only about 40% of RS. Baghurst et al (1996) estimated intakes of RS in Australia and New Zealand based on national nutrition surveys in the mid 1980s for Australia and early 1990s for New Zealand. This analysis showed an average figure of 4.0 g RS/100 g starch for men, 4.7 g RS/100 g starch for women and 4.5 g RS/100 g starch for children.

It has been postulated that diets high in fibre have a lower energy density and may therefore help in moderating obesity. The exact mechanisms by which these apparent health benefits may arise have not been determined. In almost every instance, there exists the possibility that the observed associations are indirect as a consequence of chemoprotective effects of non-nutrients closely associated with the fibre components of fruits, vegetables and cereal foods. Further discussion of the potential role of fibre in relation to chronic disease is given in the 'Chronic disease' section.

Only in the case of laxation is there evidence of both protective (Sanjoaquin et al 2004) and therapeutic actions (Topping & Clifton 2001). This laxative effect accounts for the role of dietary fibre in conditions such as hiatus hernia, diverticular disease and haemorrhoids. These latter conditions may also be affected by adequacy of fluid ingestion. Regional differences in the occurrence of these diseases generated the original hypothesis of Burkitt & Trowell (1975). However, there are few studies that have looked at the role of dietary fibre in the aetiology, rather than treatment, of these diseases. Dietary fibre is the most effective treatment for all forms of constipation due to its influence on faecal bulk and consistency.

Assessment of dietary fibre needs is complex as the endpoints are ill defined. There is no biochemical marker that can be used to determine dietary fibre needs, so appearance or disappearance of clinical endpoints needs to be considered. In keeping with the concept of setting EARs and RDIs or AIs for prevention of deficiency states, the endpoints chosen in the estimation of requirements were adequate gastrointestinal function and adequate laxation rather than reduction of risk for chronic disease.

From a meta analysis of about 100 studies of changes in stool weight with various forms of fibre, the increase in faecal weight due to ingestion of fibre has been estimated (Cummings 1993). An increase of 1 g in faecal bulk can be achieved with an additional 3 g of isolated cellulose, 5.4 g of wheat bran, 1.3 g of isolated pectin and 4.9 g of fruit and vegetables (Hillman 1983). Resistant starch has very limited effect (Behall & Howe 1996, Cummings et al 1996, Heijnen et al 1998, Jenkins et al 1998). However, increased faecal weight does not necessarily equate to enhanced laxation as other factors such as water can affect laxation directly or be a necessary adjunct to increased fibre intakes (Anti et al 1998).