In the UK, colorectal cancer is the third most common cancer and second most common cause of cancer death after lung cancer.
A systematic review from the World Cancer Research Fund (WCRF)1 has suggested that the number of cases of bowel cancer could be reduced by up to 43% (17,000 cases a year) through diet and exercise.
This review updated the findings of the WCRF/Association for International Cancer Research (AICR) 2007 report.2
Epidemiological research has led to the hypothesis that unhealthy diets cause 10-30% of cancer deaths in developed countries.3
Diet has been found to be an important factor in the development of colorectal cancer and changes to diet and lifestyle could significantly reduce the risk of colorectal cancer.
Studies suggest that eating foods containing dietary fibre may have a protective effect. A major cancer study, the European Prospective Investigation into Cancer (EPIC), covering 10 European countries, has shown that people eating the most fibre had 25-40% lower risk of bowel cancer compared with those who ate the least.4
The WCRF has reassessed its view on the protective role of fibre based on the latest review1 and reported that additional evidence indicating that fibre-rich foods can reduce the risk of bowel cancer is convincing.
Several mechanisms have been suggested for the role of fibre in preventing bowel cancer – gut bacteria produce short-chain fatty acids, including butyrate, which appear to slow proliferation of cancerous cells, and fibre is known to increase the weight of stools and frequency of bowel movements, reducing contact time with the bowel tissue.
Fruit and vegetables
In addition to providing dietary fibre, fruit and vegetables contain a number of potentially protective elements.
Studies have found that those people who have the highest intake of fruit and vegetables could lower their risk of cancer by about a quarter compared with those who have the lowest intake,5 supporting the message of eating five portions a day.
Eating five daily portions of fruit and vegetables can also help to maintain a healthy body weight, avoiding obesity. Excess body fat, especially abdominal fat, remains a convincing cause of bowel cancer.1
There is consistent evidence that calcium, including dietary supplements containing calcium, probably protects against colorectal cancer, although the totality of evidence suggests it is preferable to obtain nutrients from foods rather than supplements for cancer prevention. Milk is also thought to reduce bowel cancer risk, although the overall evidence regarding dairy foods is unclear.1
Fish intake has been inversely associated with risk of colorectal cancer. Some evidence suggests that people who consume an 80g portion of fish per day could reduce colorectal cancer risk by approximately one-third compared with people who eat less than that amount weekly.6 The mechanism is not clear.
Oily fish is especially rich in long-chain polyunsaturated omega-3 fatty acids, but there is no strong evidence that these can specifically reduce the risk of cancer.
Heavy alcohol consumption is a recognised risk for many diseases. There is also convincing evidence that alcohol consumption increases bowel cancer risk in men, although the evidence is not so strong for women.
Mechanisms suggested include lower folate concentrations in alcohol consumers or DNA damage from acetaldehyde produced during alcohol metabolism.1
Red and processed meat
There is strong and convincing evidence that red and processed meat increases the risk of colorectal cancer. The most recent WCRF review1 shows that eating an extra 100g red meat a day (beef, pork, lamb, goat) increases bowel cancer risk by 17%.
Processed meat is preserved by smoking, curing, salting or the addition of preservatives. According to the WCRF review, eating an extra 100g processed meat per day increases bowel cancer risk by 36%. There is no strong evidence that eating white meat, such as chicken, can increase cancer risk.
Red and processed meats contain haem iron. It is suggested that haem could stimulate the gut bacteria to produce carcinogenic N-nitroso compounds (NOCs). Haem could also irritate or damage cells in the bowel and lead to cell proliferation.
There is some evidence that the effects of haem could be countered by chlorophyll, which is found in green vegetables.
NOCs may also be produced from nitrates in processed meats. Cooking meat at high temperatures, such as frying or barbecuing, produces heterocyclic amines, which can also damage DNA and increase the risk of cancer.
A moderate intake of lean meat can play a part in a balanced diet, but evidence from the WCRF suggests intake of red meat should be limited to 500g (cooked weight) per week and processed meats should ideally be avoided.
There is strong evidence that many cases of bowel cancer are not inevitable and the risk can be significantly reduced by modest changes to diet and lifestyle.
Reduction in bowel cancer risk can be achieved by following recommendations to consume a plant-based diet, including foods containing fibre, such as wholegrains, fruits and vegetables and pulses, such as beans, with less red and processed meat and alcohol, and by maintaining a healthy weight with regular physical activity.
- Dr Phillips is a an independent registered dietitian in Devon
|DIETARY FACTORS AND BOWEL CANCER
|Red meat||Foods high in fibre|
|Processed meat||Fruit and vegetables|
1. Norat T, Chan D, Lau R et al. The Associations between Food, Nutrition and Physical Activity and the Risk of Colorectal Cancer. 2010.
2. World Cancer Research Fund/American Institute for Cancer Research. Food, nutrition, physical activity and the prevention of cancer: a global perspective. Washington DC, AICR, 2007.
3. Doll R, Peto R. Epidemiology of Cancer. In: Warrell DA, Cox TM, Firth JD (eds). Oxford Textbook of Medicine. Oxford, OUP, 2003.
4. Bingham SA, Day NE, Luben R 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: 1496-501.
5. Benetou V, Orfanos P, Lagiou P et al. Vegetables and fruits in relation to cancer risk: evidence from the Greek EPIC cohort study. Cancer Epidemiol Biomarkers Prev 2008; 17(2): 387-92.
6. Hall MN, Chavarro JE, Lee IM et al. A 22-year prospective study of fish, n-3 fatty acid intake, and colorectal cancer risk in men. Cancer Epidemiol Biomarkers Prev 2008; 17(5): 1136-43.