Worldwide there is a 20-fold variance in the incidence rates of colorectal cancer. In general, this variance has been attributed to dietary and environmental differences. Fibre has long been known to have beneficial effects on transit and bowel movement regulation.

In addition Burkitt, in a 1971 landmark study [1] , concluded that the low incidence of colorectal cancer in Africa was due to high dietary fibre intake. Since then, the association between fibre and colorectal neoplasia has been intensively investigated around the world, in a bid to verify Burkitt’s findings.

Possible mechanisms for the beneficial effect of fibre have been proposed. When entering the large bowel, fibre increases stool weight, reduces transit time, dilutes colonic contents, and stimulates bacterial anaerobic fermentation. This process is supposed to reduce contact between the intestinal contents and colonic mucosa.

Fermentation leads to the production of short-chain fatty acids viz acetate, propionate, and butyrate, which reduce pH and the conversion of primary to secondary bile acids. Butyrate is not only a major source of energy for the distal colon, but it also putatively reduces cell proliferation and induces apoptosis, factors that are associated with inhibition of the transformation of the colonic epithelium to carcinoma.

Unfortunately, the issue of dietary fibre contributing to cancer is not simple to analyse. In the broadest sense, it is the undigested food matter in the diet. In its original concept , dietary fibre consisted of poorly fermentable fibres like cellulose, hemicelluloses, lignins, wheat bran and roughage but now includes soluble fibres like pectins, gums, oat bran and mucilages.

Different forms of fibres vary enormously in structure and composition and have different properties that are likely to have diverse effects on cancer. Thus, contrasting preferences in food in different population groups may lead to apparently similar intakes of dietary fibre but these intakes could be of very different types of fibre, with different implications for cancer.

Most highly processed forms of fibre may not protect against colon cancer, but less processed, more slowly fermented sources such as wheat bran and psyllium have shown benefit in certain studies.

In addition, the retrospective design of many studies may also have introduced recall and selection biases. All these explain the continuous controversy about fibre and colorectal tumours.

Some experimental studies in animal models have demonstrated a protective effect of fibre against tumour induction compared with controls. However, epidemiologic studies investigating a possible link between dietary fibre and colorectal cancer have been inconclusive.

The US Nurses Health Study [2] found no protective effect of dietary fibre on the development of either colorectal adenomas or carcinomas. On the other hand, the European Prospective Investigation into Cancer and Nutrition (EPIC) study [3] and the Prostate, Lung, Colorectal, and Ovarian Cancer Screening project team (PLCO) [4] found fibre to be beneficial.

The EPIC study involved more than 500,000 individuals from 10 countries and found that total dietary fibre consumption was inversely proportional to colorectal cancer risk. This association was stronger for colon cancer, particularly left sided, rather than for rectal cancer.

However, the EPIC study only examined fibre in foods. It could thus be difficult to extrapolate these findings to fibre supplements or additives containing fibre alone, as foods may also contain other substances like phenolic components such as flavonoids, lignans and anthocyanins that could account for the protective effects seen. The PLCO study also found fibre, particularly from grains, cereals and fruits, to be beneficial and associated with decreased risk of colonic adenoma.

A possible reason for the difference in findings between the US Nurses Health Study and the EPIC and PLCO studies, is that the EPIC and PLCO studies spanned several very different population groups, with higher amounts and more varied sources of dietary fibre being preferentially consumed in the latter 2 studies.

Alberts et al [5] and Schatzkin et al [6] , in two well known randomised intervention trials, concluded that high fibre intake did not decrease recurrence of colorectal adenomas, the precursors of colorectal carcinoma. A meta-analysis of all studies performed from 1988 to 2000 relating to fibre and colorectal neoplasia concluded that there was little evidence to support the use of dietary fiber supplements to reduce the risk of colorectal neoplasia. [7] Almendigen et al [8] , however, concluded that low dietary consumption of vegetables, cereals, iron, vitamin C and fibre was associated with an increased risk of adenoma formation.

In the light of all these controversies, it is prudent to promote a moderation approach to fibre intake. Indeed, in individuals with sensitive bowel (eg. some cases of irritable bowel syndrome), increase in fibre intake can lead to excessive bloating, flatulence, colic and diarrhoea. Eating a healthy balanced diet in the form of fruit, vegetables, and whole grain cereals, probably remains the best option for reducing the risk of colon cancer, and for more general health protection.

Dr Tan Wah Siew, Registrar

Dr Tang Choong Leong,
Senior Consultant Department of Colorectal Surgery
Singapore General Hospital

References

1. Burkitt DP. Epidemiology of cancer of the colon and rectum. Cancer 1971;28(1): 3-13.
2. Fuchs CS, et al. Dietary fiber and the risk of colorectal cancer and adenoma in women. N Engl J Med 1999;340(3): 169-76.
3. Bingham SA, et al. Dietary fibre in food and protection against colorectal cancer in the European Prospective Investigation into Cancer and Nutrition (EPIC): an observational study. Lancet 2003; 361(9368): 1496-501.
4. Peters U, et al. Dietary fibre and colorectal adenoma in a colorectal cancer early detection programme. Lancet 2003;361(9368): 1491-5.
5. Alberts DS, et al. Lack of effect of a high-fiber cereal supplement on the recurrence of colorectal adenomas. Phoenix Colon Cancer Prevention Physicians' Network. N Engl J Med 2000; 342(16): 1156-62.
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