Correspondence to: Peter Gibson, Department of Gastroenterology, Box Hill Hospital, Level 8, Clive Ward Centre, Arnold Street, Box Hill, Vic. 3128, Australia. Email: Peter.Gibson@med.monash.edu.au
The concept of dietary fibre is a complex one that incorporates the physical and physiological functions of fibre and its effects both systemically and local to the gastrointestinal tract. Dietary fibre can be usefully classified according to its solubility and fermentability, which allows rational clinical application. Fibres may act in several ways including by gel-forming effects in the stomach and small intestine, by its fermentation by colonic bacteria, by a ‘mop and sponge’ effect, and via concomitant changes in other aspects of the diet. These actions lead to potentially beneficial effects in the gastrointestinal tract and systemically, such as lowering serum cholesterol and improving glycaemic control. Dietary fibre has been implicated in multiple clinical situations but, although an extensive literature on putative actions and proposed physiological bases is available, high-level evidence of efficacy is limited. Nevertheless, encouraging the intake of a high-fibre diet is likely to have a range of health benefits and physicians are encouraged to follow simple practical guidelines in their everyday practice. (Intern Med J 2003; 33: 291−296)
Dietary fibre is best considered as a dietary concept that integrates its physiological and physical functions, and its diverse effects, both locally to the gastrointestinal tract and systemically. Dietary fibre consists of plant substances that resist hydrolysis by small bowel digestive enzymes and is an extremely complex group of substances. Opposing groups around the world argue about which substances should be classified as fibre but most would agree that dietary fibre includes the indigestible, non-starch polysaccharides, cellulose and hemicellulose, oligosaccharides, pectins, gums and waxes.1 There is ample evidence to include resistant starch (that is starch that escapes digestion in the small intestine) and lignin as dietary fibre. It is important to recognize the vast heterogeneity of fibre. Each different fibre type has unique physical, chemical and physiological characteristics, which poses an enormous and dynamic task of further identification and classification.
The use of dietary fibre as a treatment and preventive strategy is highly attractive for many scientific and social reasons. The general public may see it as natural and healthy and there are sound physiological theories to support its widespread use. The literature, however, does not unequivocally parallel this enthusiasm. In fact, most data are at best lukewarm in support, and occasionally the findings are in the negative. Contributing causes for this apparent lack of benefit may not be within the fibre itself, but also in the difficulty of dealing with the heterogeneity of dietary fibre, its unknown optimal dose and duration of treatment. The majority of studies suffer from a lack of comparability with, for example, the use of different definitions and types of fibre and different dietary intake assessment methodologies.
Clinically useful classification of dietary fibre
Understanding the heterogeneity of fibre is vital to assess the literature accurately but, as clinicians, the intricacies of and controversies surrounding fibre definition and classification only interfere with the optimal application of dietary fibre in patient management. A simple conceptual classification that has proven very useful clinically is the classification of fibre according to its solubility and fermentability by bacteria, which, for practical purposes, are equivalent. In other words, a soluble fibre is readily fermentable by colonic bacteria, while an insoluble fibre is only slowly fermentable. It is not difficult then to understand the potential effects of soluble and insoluble fibres, as outlined in Fig. 1.
Modes of action of fibres
There are five basic ways in which fibres exert their effects.
1. Gel-forming effects in the stomach and small intestine
Soluble fibres rapidly form gels when combined with water. This is believed to be the mechanism by which soluble fibres slow gastric emptying, hasten small intestinal transit and slow absorption of nutrients such as glucose from the small intestine. Indeed, soluble fibres have a major impact on the glycaemic index (GI) of foods.
The GI reflects the ability of a food to raise blood glucose and is a useful way in which carbohydrate-containing foods can be ranked. Low GI foods are digested and absorbed slowly along the small intestine and reduce the post-prandial blood glucose and insulin responses. These foods are useful in the clinical management of glucose intolerance and diabetes. Some examples of foods with a low GI are legumes, pearled barley, and pasta and breads made with unrefined grains. Insoluble fibres themselves tend not to affect the GI, but the physical properties of food containing insoluble fibre may have an impact; for instance, the starch in coarsely ground grains is more slowly digested and absorbed in the small intestine, thereby reducing the GI. It is important to note, however, that the entire food composition must be considered in conjunction with the GI value of a food, because some low GI foods are high in fat and may not represent the best overall choice for patients.
2. Effects of fibre fermentation by colonic bacteria
Effects on the colonic bacteria
The energy generated leads to a marked expansion of bacterial populations and subsequent bulking of the colonic contents.
The increased metabolic and proliferative activity leads to increased utilization of potentially toxic thio-compounds and nitrogenous substances, such as phenols and ammonia, within the lumen. Their luminal levels subsequently are reduced.
Some fibres, especially resistant starch and fructooligosaccharides, have variable capacities for being fermented by colonic bacteria, and this leads to selective expansion of beneficial bacterial groups, the so-called ‘prebiotic’ effect. These probiotic bacteria, such as lactobacilli and bifidobacteria, have multiple effects that are currently under intense investigation but which may include immunomodulation,2 antibacterial,3 and anticarcinogenic4 effects.
Effects of the products of fermentation
The end products of colonic fermentation are short-chain fatty acids (SCFA), carbon dioxide and hydrogen.
SCFA, especially butyrate, are metabolized by the colonic epithelium and are its major energy substrate. Butyrate has multiple health-promoting effects including induction of differentiation of colonic epithelial cells, suppression of epithelial proliferation, improvement of tight junction barrier function and suppression of tumorigenesis. Acetate and proprionate are also produced, and are utilized by the heart and liver, respectively.
Hydrogen ions lower luminal pH and thereby prevent the growth of pathogenic pH-sensitive bacteria. Studies in vitro show inhibition of clostridia, Escherichia coli and pathogenic streptococci and stapyhlococci with acid-producing bacteria.5
The site in the colon where this fermentation occurs is also an important issue. Highly fermentable fibres such as oat bran, guar gum and resistant starch, when they dominate the diet, are fermented predominantly in the proximal colon. The benefits of fermentation products might not then be observed in the distal colon. However, by combining fermentable with less fermentable fibre, the site of fermentation can be shifted around the colon so that more even exposure of all the colonic epithelium to fermentation products occurs.
3. ‘Mop and sponge’ effect in the colon
Insoluble fibres act like a sponge. They have water-holding properties in the distal colon and bulk the stool in this way. They also bind molecules, such as bile acids and carcinogens. Insoluble fibres also appear, in experimental animals, to have a ‘mop’ effect in which their sheer physical presence in some way exerts a trophic effect on the colonic epithelium, enhances barrier function and suppresses tumorigenesis.6
4. Cholesterol-lowering effects
The soluble fibres, especially oat fibre, lower serum cholesterol in several ways. There is reduced absorption of cholesterol and fat from the small intestine in a diet high in soluble fibres. A decreased absorption of bile acids in the ileum has also been noted. Furthermore, the SCFA produced by the fermentation of dietary fibres can directly inhibit hepatic cholesterol synthesis.7
5. Concomitant changes in other aspects of the diet
By choosing a diet rich in fibre-containing foods, the intake of fresh fruit and vegetables will automatically increase while the fat intake decreases. These changes also potentially lead to improved micronutrient and antioxidant intake, weight loss and improved cholesterol profile. Thus, a positive focus on fibre intake can lead to positive effects on other key elements of a healthy diet.
Beneficial effects of fibre: proven and putative
Dietary fibre has been suggested to be a primary therapy or a useful adjunct in the management of several conditions. The putative benefits, proposed physiological bases and level of evidence are outlined in Table 1. Despite widespread advocacy for the use of dietary fibre, data from double-blind, randomized, controlled trials have only recently been available.
Table 1. Physiological basis for proposed beneficial effects of dietary fibre
Consuming large amounts of purified soluble fibre alone may be harmful. Studies conducted in rats have shown injurious effects in the distal colon and enhancement rather than suppression of tumorigenesis.25 The reason why this occurs has not been determined, but may in part relate to massive fermentation in the proximal colon with relatively poor delivery of health-promoting fermentation products to the distal colon. Fibre-induced expansion of the bacterial populations might lead to utilization of alternative metabolic pathways by these populations in times of relative substrate (fibre) stress.26 These alternative pathways may have more toxic products. In the foods we eat, however, fibre is a balance of soluble and insoluble.
The gaseous products of fermentation are hydrogen and carbon dioxide. Hydrogen is disposed via several routes: via the breath, by acetogenesis, luminal formation of methane, and by the reduction of luminal sulphates to sulphides. Evidence has been presented that sulphides are potentially toxic to colonic epithelium and a pathogenic role in ulcerative colitis has been implicated.27 The main components of flatus are carbon dioxide, nitrogen, hydrogen and methane. The production of excess gases from fermentation, with the bulking effects of fibre, can induce bloating and excess wind. Such symptoms are poorly tolerated by the patient with irritable bowel syndrome, and are problematic especially when a high fibre diet or fibre supplements are first initiated. The colon does adapt to these dietary changes but this requires several weeks to occur, and a gradual introduction is recommended.
Increasing the intake of dietary fibre
Several basic principles should be considered when it seems necessary to increase a patient's dietary intake of fibre. These are outlined in Fig. 2. When fibre is being increased for a specific purpose, more careful choice of fibre type is important. For example, if it is desired to lower cholesterol or to improve glycaemic control, soluble fibre (such as oat bran or psyllium) should be chosen. If bulking or correction of constipation is desired but the patient suffers from flatulence, then highly fermentable fibres should be avoided and insoluble fibre such as sterculia should be used in the first instance. A gradual increase in fibre intake is usually recommended to improve tolerance by minimizing problems of gas and bloating. An adequate intake of water is essential for dietary fibre to have efficient bulking ability, therefore a daily intake of at least 2 L (that is eight glasses) of fluid, principally as water, is recommended.
How much is enough?
It is currently recommended that Australians consume at least 30 g of dietary fibre daily. The average Australian woman currently consumes approximately 20 g of dietary fibre a day while men eat approximately 26 g. This implies that average daily dietary fibre intake should be increased by approximately 10 g. The current recommendations for a healthy diet suggest a minimum daily intake of two servings of fruit, five servings of vegetable and four to five servings of grains and cereals. Early referral to a dietitian is recommended to help in the institution of higher fibre alternatives.
While fibre is not the panacea for all ills as suggested in the past, it is a useful functional food (that is, a food with health benefits) in many situations. Dietary advice is more likely to be heeded if presented in a positive rather than negative way. Because fibre-rich foods are easy to identify, increasing dietary fibre intake is easier to sell than many other dietary manipulations. Furthermore, such an approach has multiple other secondary dietary benefits such as reducing fat and increasing antioxidant intake. An understanding of simple concepts regarding the heterogeneity of fibre is recommended because it enhances the ability to use fibre optimally in clinical practice.