Of all the cancers, colorectal is the one where the role of diet as a cause of the disease has been the most intensively studied. As a result, there has been a plethora of research studies on all aspects of diet. Although not all the findings agree, there's enough evidence to formulate broad dietary strategies that will minimise your risks of developing the disease.
Eat at least 30 g of fibre a day. That was the recommendation of a recent colloquium of experts (American Health Foundation, New York, April 1998). Although the American Health Foundation recommended that most fibre come from wheat, wheat has been found to present many other problems and might well be substituted with a gluten free grain such as rice, millet or buckwheat, or ancient wheat precursors such as kamut or quinoa.One study showed that a daily intake of less than 7.5 g doubles the risk of colon cancer (Int J Cancer, 1989; 44: 1-6).
The generally accepted theory has been that a high fibre diet increases the bulk of the faeces, accelerating their "transit time" in the gut, and so reducing the exposure of the colon wall to possible carcinogens. But high fibre foods also seem to actually limit the number of carcinogens present. Experiments on human volunteers, comparing oats, wheat bran and cellulose, showed a considerable reduction in carcinogens present in the faeces with wheat and cellulose, but not with oats (Cancer Research, 1989; 49: 4629-35). Further animal tests showed that all the chemical constituents of the whole wheat germ had a protective effect against polyps and tumours, but that the largest effect came from the crude fibre itself (Mutat Res, 1996; 350: 185-97).
Such findings have been confirmed in epidemiological studies which show that diets high in cereal fibre reduce the risk of colon cancer, while refined grains increase it (Cancer Causes Control, 1997; 8: 575-90).
Increase your consumption of vegetables to at least five portions a day. Higher intakes of vegetables, but not fruit, also seem to have a protective effect, according to a long term study using data from the Food and Agriculture Organisation, where both cereals and vegetables were found to protect against colon cancer, particularly in later life (Eur J Cancer Prev, 1998; 7: S11-S7).
Limit your intake of red meat to one or two servings a week. Although the data about red meat is confusing, it is prudent to keep consumption to a minimum that will support health.
Increase your consumption of fish and fish oils, and limit corn oils. Studies in South Africa have shown that coastal populations whose staple food is fish have significantly less colorectal cancer (S Afr Med J, 1997; 87:152-8). In the laboratory, experiments with rats with a predisposition to cancer have demonstrated that high amounts of fish oil, which is naturally rich in omega-3 polyunsaturated fatty acids, significantly reduce the number of colon tumours formed. In contrast, tumours proliferate in rats fed a diet high in corn oil (Carcinogenesis, 1998; 19: 985-9 ). Studies on human volunteers have reinforced the fish v corn oil story, showing that corn oil produces carcinogens in the faeces, while fish oil does not (Nutr Cancer, 1996; 25: 71-8).
Take supplements of vitamin E. In one US survey, general multivitamin use was associated with a halving in risk, with vitamin E being particularly protective (Cancer Epidemiol Biomarkers Prev, 1997; 6: 769-74). This finding replicates earlier work demonstrating that people who take high levels of vitamin E have reduced their risk of colon cancer three fold (Cancer Res, 1993; 53: 4230-7).
Supplement your diet with at least 400 mcg of folic acid. Folic acid has been found to reduce the risk of colon cancer developing after inflammatory bowel diseases (Ital J Gastroenterol Hepatol, 1998; 30: 421-5). It also appears to have effects on the progress of the cancer itself. In one long term study, more than 400 mcg of dietary folate daily was shown to nearly halve the incidence of this form of cancer. Supplemental folate also had a progressively protective effect, rising from a 20 per cent reduction of risk after 10 years of use, to a 300 per cent reduction after 15 years' intake (Ann Intern Med, 1998; 129: 517-24). Nevertheless, this benefit may have been associated with general multivitamin intake. In this study, folic acid also seems to have a protective effect against the consequences of excessive alcohol intake (J Natl Cancer Inst, 1995; 87: 265-73).
Selenium supplements may also be beneficial. Recent laboratory work with rats prone to cancer shows that selenium may both inhibit the initiation of colon tumours and hinder them once established (J Natl Cancer Inst, 1997; 89: 506-12).
Take at least 1 g of vitamin C per day. Numerous 25 year old studies inspired by the late Linus Pauling have shown that high doses of vitamin C greatly reduce the mutagenic and carcinogenic substances in the bowel contents (Cancer, 1981; 47: 1121-5).
Launch a rigorous programme of exercise, three to four times per week. In one study comparing three groups, people who had a typically "Western" lifestyle (low fibre, high calorie, high sugar and cholesterol intake with a higher Body Mass Index) had four times the rate of colon cancer, while the moderately health conscious (preferring low fats, white meat and whole grains) had a marginally reduced risk. The people least at risk were those with the smallest body size and the most "prudent" lifestyles, involving vigorous exercise and diets rich in fibre and folate; in this group, colon cancer was halved (Am J Epidemiol, 1998; 148: 4-16).