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METABOLICAL AND PHYSICAL ASPECT OF DIETARY FIBRE

Obiettivo

Dietary fibre is principally the cell-wall material of plant foods in our diet. It is important for health for two reasons. Firstly the physical structure that plant cell-walls give to foods affects the rate of digestion of starch and sugars and may also reduce cholesterol absorption. Secondly, fibre is not digested in the upper gut and so reaches the large intestine where it is broken down by bacteria and has many beneficial effects including those on metabolism and laxation.
The study of dietary fibre has been very challenging scientifically because of the difficulties in its chemical analysis and the wide variety of effects in the intestine. For the food industry it has presented unique opportunities for the design of healthier foods.
Since its inception in 1990 COST 92 has tried to help meet these challenges by attempting to co-ordinate activities in Europe and thus stimulate research and the exchange of ideas through meetings and workshops.
The main objectives of the Project, as defined, were :
to increase the knowledge of metabolic and other physiological effects of various kinds of dietary fibre in relation to physical and chemical properties of the fibre and the mechanisms of such effects;
to investigate how various industrial processes alter the different physiological effects of fibre, and which physical and chemical modifications of the fibre are responsible for such alterations;
to investigate physiological effects of indigestible carbohydrates not usually included in the dietary fibre concept, and the effects of processing on the formation of such carbohydrates;
to investigate physiological effects of non-carbohydrate substances associated with dietary fibre, e.g. phytic acid, tannins, saponins and minerals, and the effect of processing on these substances;
to investigate the effect of gross and cellular structure of fibre-rich foods on physiological properties usually attributed to dietary fibre per se;
to develop in vitro and in vivo methods for measuring chemical and physical properties of dietary fibre that are predictive of physiological effects.


Current status
Most of the work of COST 92 was completed before 1995. The initial activity of the Management Committee was to compile a database of dietary fibre research in Europe in order to identify the strengths and weaknesses overall of European work in this field. It was evident from the survey that there was a major lack of information on dietary fibre intakes in COST countries and so the first Workshop of COST 92 was convened in Rome (I), in May 1992 to discuss dietary fibre intakes. Subsequently the Management Committee undertook a survey of dietary fibre intakes and this was eventually published.
The activity then divided into three work groups :
Subgroup I Physico-chemical properties of dietary fibre and effect of processing on micro-nutrient availability;
Group Leader : Dr Frølich
Subgroup II Metabolic effects of dietary fibre, particularly the mechanism for lipid lowering and carbohydrate metabolism;
Group Leader : Dr Lairon
Subgroup III Fermentation, cellular metabolism and proliferation.
Group Leader : Professor Malkki.
The subgroups have provided a forum for scientific discussion, leaving the Management Committee to decide overall strategy and co-ordination.
Workshops followed on :
Physico-chemical properties
Lipid metabolism
Mechanisms of action of dietary fibre
Dietary fibre measurement,

and finally in June 1995 on 'Dietary fibre and fermentation in the colon'.
Over the years, a great number of experiments on dietary fibre have been done and it is clear that its effects on the human large intestine, particularly through fermentation, are the best established and most important of its various properties. Through fermentation, fibre is known to affect the growth and differentiation of epithelial cells in the gut, hepatic metabolism of carbohydrate and lipid, genotoxic events within the large bowel, stimulation of bacterial growth and a host of other events.
Dietary fibre's principal role in fermentation is to provide a substrate and possibly a surface for bacteria in the large intestine. However, dietary fibre is not the only carbohydrate available to the flora. Others include resistant starches, the oligosaccharides such as raffinose, stachyose and fructo-oligosaccharides and even on occasions some sugars. Each of these probably has distinct effects on fermentation. The non-starch polysaccharides have a physical structure which affects bowel function and ultimately bowel habit, whilst the starches are a rich source of butyrate for the epithelium and the oligosaccharides have the property of selectively stimulating the growth of particular bacterial species such as bifidobacteria (prebiotic action).
Fermentation leads to the production of a number of products, the principal ones being the short-chain fatty acids. These are absorbed and through this mechanism we are able to salvage energy from carbohydrates not digested in the upper gut, including fibre. Short chain fatty acids also play a role in regulating the growth and differentiation of the colonic epithelial cell. Butyrate has been implicated as the sole mediator of this effect but there is evidence beginning to emerge that propionate may contribute. If confirmation of the role of butyrate as a regulator of gene expression is confirmed, then fermentation will be seen to be an important part of the defence mechanism against cancer in the large bowel.
Fermentation and its consequences have important implications for bowel disease, especially constipation, cancer, irritable bowel, diverticular disease, acute infections of the gut and inflammatory bowel disease. What is perhaps most significant about these diseases is that, despite the fact that we have been working for more than 20 years on dietary fibre, the work's impact on the prevention and treatment of these diseases has been relatively small. The message from this is not that medical research is ineffective, but rather that we should not be unduly focused on one component of the diet or one aspect of fermentation. Dietary fibre must be seen in context; in, for example, the prevention of large bowel cancer one must have in mind the role of dietary protein and fat, the effect of cooking on heterocyclic amine formation and preventive mechanisms such as the whole issue of oxidant stress in the large intestine, DNA damage and its prevention by anti-oxidant mechanisms.
At the end of the action the Management Committee concluded :
The dietary fibre hypothesis is in many ways revolutionary and has stimulated a great deal of research over the years. During this time, knowledge has increased and a number of more definitive hypotheses have emerged. There is now a very good experimental basis to link dietary fibre with many events in the large intestine and it is highly likely that diets which are characterised by having a lot of fibre in them are important in disease prevention. In looking for mechanisms, however, it is very important to set fibre in context, firstly in relation to the other fermentable carbohydrates in the diet such as starches and oligosaccharides and secondly in the context of other substrates which may simultaneously be present in the colon in varying amounts.
Dietary fibre research has yielded many interesting results but, more than anything, it has become clear that it is not the only mediator of any physiological event in the gut, nor is it the single most important cause of any known disease, even constipation. If we are to understand the role of dietary fibre we must see it in its proper context, both in the diet and alongside other events to which it contributes. This is nowhere better shown than in fermentation.
Finally :
There is as yet no internationally agreed definition of dietary fibre nor universally recommended method for its measurement, yet it is clear that the polysaccharides of the plant cell wall have an important role to play in human health.
The physical properties of dietary fibre are critical to its effects in the upper gut, especially chewing, satiety mechanisms and the moderation of glucose, insulin and cholesterol metabolism.
In the large intestine dietary fibre is fermented along with other carbohydrates to a variety of products which are important in maintaining the health of the colonic epithelium, preventing bowel cancer and moderating bowel habit.
The importance of dietary fibre in human health is unquestioned and the critical nature of its physical properties provides a challenge to the food industry.

One of the most important useful activities of the Management Committee has been to select young people for short-term scientific missions. During 1995 and 1996 eight people from six countries visited the laboratories of Committee Members in five COST countries to learn new techniques and study different populations and diets.

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Ultimo aggiornamento: 20 Aprile 1994

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