Fibre in the Diet

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Lack of fibre or a decrease in dietary fibre has been the leading theory in the aetiology of diverticular disease since 1971. Studies undertaken by Painter and Burkitt (1971), using geographical and historical research, have continually received support for this hypothesis. Studies undertaken with case-controls by Brodribb and Humphreys (1976) suggested that patients with diverticular disease had low dietary intakes of crude fibre, but this is not an indicator of overall dietary fibre intake. Gear et al. (1979) and Berry et al. (1984) undertook studies on rats and found that they develop more and more diverticula when they are given less and less bran in their diet. Yet vegetarians have been shown to have less than half the expected prevalence of asymptomatic diverticulosis, which results only partly from their higher fibre intake (Gear et al., 1979).

Although diverticular disease of the colon is viewed as a disease of western civilization and ascribed to inadequate dietary fibre intake, there is a strong clinical impression by the medical profession that increased dietary fibre intake will relieve the symptoms of diverticular disease (Ornstein et al., 1981). In a controlled clinical study, 58 patients with uncomplicated diverticular disease of the colon ingested a bran crispbread, an ispaghula drink (Psyllium) and a placebo for 4 months each in a randomized, cross-over, double-masked, controlled trial. Subjective assessments were made monthly using a self-administered questionnaire. Objective studies were made by examination of a 7-day stool collection from each patient at the end of each treatment period. Using a pain and lower bowel symptom score, which included incomplete sensation of emptying the bowel, straining, stool consistency, aperients taken and nausea, and a total symptom score, which included nausea, belching, dyspepsia, vomiting and abdominal distension, it appeared that supplementation with fibre was of no benefit. Benefit from fibre was found for those with constipation while undertaking this regimen. Both regimens of fibre produced the expected changes in stool consistency, weight and frequency of defecation. This trial tested the usual therapeutic dose of bran and ispaghula supplements, which is equivalent to two tablespoons of natural bran, and increased the patients' intake by 50-70%, as opposed to the large quantities of bran used in studies by Painter and Burkitt (1971), Plumley and Francis (1973), Taylor and Duthie (1976) and Brodribb and Humphreys (1976). The conclusion of the above trial showed no difference among the three regimens, although it confirmed that dietary fibre has a well-known and considerable effect on constipation, with ispaghula being better than bran. It was felt that the placebo supplement of 2-3 g dietary fibre in the controlled clinical trial could relieve symptoms while producing fewer objective changes than either bran or ispaghula. The conclusion reached was that, unless the patient's symptoms were those of constipation, dietary fibre supplements are unnecessary in the long-term management of uncomplicated diverticular disease of the colon (Ornstein et al., 1981).

However, the most well-known substantiated theory on the aetiology of diverticular disease is the lack of dietary cereal fibre - a hypothesis put forward by Painter and Burkitt (1975). They had observed that diverticular disease was rarely seen in African countries where dietary fibre intake was high, yet in western countries, where there was a higher incidence of the disease and a lower intake of dietary fibre, they felt that the refining of flour and cereals was the prime cause of diverticulosis. Although this can be confirmed by epidemiological studies, animal experiments and fibre replacement trials (Garry, 1971; Gear et al., 1979; Manousos et al., 1985), few studies are available to show how two other dietary intakes may be relevant to diverticular disease. These are the lack of fresh fruit and vegetables and excess red meat (Aldoori et al., 1998), which can increase the risk of symptomatic disease.

In a cohort study of 48 000 male American health professionals (see Chapter 10) it was found that beef consumption doubled the risk of symptomatic disease and lamb consumption almost quadrupled the disease. Therefore, is red meat a contributory factor to the symptomatic increase of diverticular disease? Although this is a difficult question to answer, this hypothesis considers whether the aromatic heterocyclic amines produced by cooking red meat are the chief offenders. It is known that these compounds can induce neoplasia in animal trials for colonic cancer, but whether they produce diverticular inflammation is not yet known.

The limited intake of fruit and fibre in most people's diets may be a contributing factor to diverticular disease. There is a great deal of publicity recently in the media and shops to encourage people to eat at least five portions of fruit and vegetables each day. Much of this publicity is concerned with the findings that this helps to prevent bowel cancer, but it appears that fibre from fruit and vegetables is as important in the aetiology of diverticular disease as the lack of cereal fibre (Manousos et al., 1985).

Many of the clinical studies have concentrated on treating patients with cereal fibre such as bran or isphagula supplements (Plumley and Francis, 1973; Brodribb and Humphreys, 1976) and the physiological evidence shows that bran can reduce the bowel intraluminal pressure. However, as beneficial as this may be, bran does not abolish the patient's symptoms.

Previously, treatment of diverticular disease was with a low-residue diet and this had been accepted without any proof of its therapeutic value, although medical opinion now favours a high-fibre diet as previously discussed. In the study by Brodribb and Humphreys (1976) in the UK, 40 patients who presented over 12 months with symptoms and underwent a barium enema for diverticular disease were studied. After the initial assessment patients were instructed to take three heaped tablespoons of wheat bran daily and to keep to their normal diet. After 6 months the patients were reassessed and a further barium enema carried out. The original films and the 6-month films were compared and the number of diverticula counted.

All the patients tolerated the bran and said that their symptoms had improved. Of the patients, 60% stated that their symptoms were abolished and 28% that their symptoms were relieved. The study concluded that treatment with cereal fibre can provide good symptomatic relief in patients with uncomplicated diverticular disease and can improve colonic function by increasing stool weight. Changes in barium enema appearances after treatment have a limited clinical significance. Although no decrease of diverticula was seen on radiographs it was noted that there had been no increase either.

Fibre, more than any other dietary component, affects human large bowel function, causing an increase in stool output, dilution of colonic contents, a faster transit rate, and changes in the colonic metabolism of minerals, nitrogen and bile acid (Stephen and Cummings, 1980). It is supposed that these changes are caused by the amount of water that undigested fibre holds within its cellular structure (McConnell et al., 1974; Eastwood and Mitchell, 1976). In this study, two fibres were used to demonstrate that the main component of human faeces is bacteria: cabbage fibre, which is extensively broken down and stimulates microbial growth, and wheat fibre, which remains largely undigested and retains water in the gut lumen. Wheat fibre was seen to survive digestion in the bowel and to alter colonic function by holding water and increasing the bulk of the colonic content. Transit time is decreased and less water is absorbed from the lumen. With cabbage fibre, the faeces were better hydrated than with the wheat, perhaps because the increased bacterial mass stimulated faster transit time and there was less water absorbed from the luminal contents by the bowel mucosa. Cabbage fibre influences colonic function through its stimulation of microbial growth, whereas with wheat fibre there is a smaller increase in the bowel bacteria (Cummings and Stephen, 1980). It has been suggested by Burkitt et al. (1972) that colonic disease is more common in people who have small stool outputs and slow transit time, so the control of colonic microflora is important in determining disease susceptibility in individuals. Therefore, the type, amount and digestibility of fibre in the diet will make a considerable contribution to the microflora.

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