The cornerstones of colorectal prevention are screening and detection of polyps. We can also try to modify our risk with lifestyle, dietary changes, physical activity and medication.
Moderate physical activity (such as brisk walking 3-4 hours per week) can reduce your risk of colorectal cancer by 20-30%. The reason for this is not clear, but could be because exercising reduces insulin levels, abdominal fat and systemic inflammation.
Fruit, vegetables and fibre
For about 40 years it was believed that a diet high in fibre, especially from fruit and vegetables, lowered the risk of colorectal cancer. However, recent large case-controlled studies showed a nonexistent or weak association between fibre intake and colon cancer. Furthermore, no association was found between specific subtypes of fibre (including fibre from cereal, fruit or vegetables) and cancer location or stage.
However, in a study that included more than 700 000 people, it was concluded people who naturally eat a high-fibre diet, also follow a generally healthy lifestyle and have other protective factors that reduce their cancer risk. They consume other nutrients with anti-cancer effects, such as folate. In these studies, ispaghula husk (a type of fibre used in laxatives) was unexpectedly associated with an increased risk of polyp formation.
Finally, it was thought that resistant starch (starches that take a long time to digest, as in lentils, beans, etc.) might also have anti-cancer properties, but doses of 30 g per day failed to reduce the risk in studies done over four years. This dose was also three times higher than typical intake, making it an impractical diet to follow.
Red meat, fat and carbohydrates
In the past, it was believed that there was an association between red and processed meat and colorectal cancer. Men who ate pork, beef and lamb as a main meal more than five times a week, had a threefold increase in risk of colorectal cancer. But today we know that the culprit is not the meat, but the cooking process.
Several studies have found that meat eaters who consume meat with a heavily browned surface or meat that has been prepared at prolonged high temperatures, are those with the increased risk. It also seems as if gender and sex may play a role in this regard. Some women can, because of genetic factors, metabolise (acetylate) these cancerous substances rapidly and have high risk of cancer, in contrast with those who have a slow acetylation with a low risk.
Other sources of animal protein (dairy products, fish and poultry) are associated with a reduced risk of colorectal cancer and polyps.
Calcium and vitamin D
Calcium intake can reduce the risk of developing polyps and colorectal cancer, but the intake should be 700-800 mg per day. This seems to be a threshold level, and higher doses are not beneficial.
The protective effect of calcium appears to be limited to people with higher vitamin D levels. The vitamin D level (25(OH)-D) in the blood should be above 30 nmol/L. This is achieved by a daily intake of 1 00-2 000 IU per day.
Calcium and vitamin D are found in cod liver oil, milk and milk products as well as in cereals, and sun exposure helps to form vitamin D in the skin.
Vitamins B6 (pyridoxine) and B9 (folic acid is the synthetic form, folate is the natural vitamin) seem to play important roles in colorectal cancer. Dietary folate is much better than the synthetic folic acid found in supplements. People who benefit most from extra folate are those that have low baseline levels. Here folate can prevent the formation of polyps and cancer.
However, if you already have polyps or cancer, extra folate may aggravate the growth. Finally, it requires more that 10 years of extra folate intake before it becomes beneficial.
Although a higher intake of vitamin B6 showed a 30-40% decrease in colorectal cancer, especially in those who drink a lot of alcohol, some data suggest too much vitamin B6 could increase the risk for rectal cancer. Further studies are in the pipeline to clear up this controversy.
Antioxidants and other micronutrients
Various antioxidants have been studied to determine their role in cancer prevention, especially selenium. Beta-carotene (vitamin A) raised concerns about an increased risk for lung cancer and ischaemic heart disease in smokers. In randomised trials specifically designed to examine antioxidant supplements, no reduction in risk for colorectal cancer was found.
It seems that a Western diet consisting of large amounts of red meat and highly refined carbohydrates is associated with an increased risk of colorectal cancer compared with a more prudent diet which includes small amounts of red meat and refined carbohydrates. The potential anticancer benefit of such dietary patterns may be related to the levels of insulin or inflammation (see later).
Studies that included more than 500 000 participants showed that high levels of alcohol intake are associated with an increased risk for rectal more than colon cancer. The mechanism by which alcohol promotes cancer, is unknown, but it is possibly because of alcohol’s ability to reduce folate levels. The type of beverage (beer or wine) does not play a role.
Smoking increases the risk for colorectal cancer. The cancer forms about 30 to 40 years after the first exposure, and the risk disappears only after 31 years of cessation. The risk increases especially if you smoke more than 40 cigarettes per day, if you started smoking early in life or if you’ve been smoking for a long time.
Body mass and fat distribution
In a large study that involved more than 7 million people including about 94 000 with colorectal cancer, it was found that a body mass index (BMI, weight in kg divided by the square of height in meters) plays a definitive role in colon cancer. The increased risk was:
• BMI 23-25: 14%
• BMI 25-27: 19%
• BMI 27,5-29,9: 24%
• BMI 30 and higher: 41%
Men have a higher risk than women if their waist circumference increases.
The mechanism of how obesity increases risk for colon cancer is not well established, but it high insulin levels may be the reason. Insulin increases growth factors in the body that may promote the formation of cancer.
Aspirin, non-steroidal anti-inflammatory drugs (NSAIDs – painkillers that don’t contain steroids or paracetamol) and cox-2 selective inhibitors (painkillers that are designed to cause fewer stomach ulcers) can all reduce the risk of colon cancer. However, aspirin-related medications can cause serious bleeding from the stomach and small bowel, especially in older people, and the cox-2 inhibitors can cause a stroke or heart attack. Therefore it is not routinely used to prevent colorectal cancer.
Patients that had their colorectal cancer successfully removed, may benefit from using these medications because it reduces their mortality by 21%. It also decreases the number of polyps in patients with familial andenomatous polyposis.
Postmenopausal hormone therapy appears to be associated with a lower risk for colorectal cancer, but it is not yet clear which preparation is optimal. If hormone replacement is not started soon after the menopause commences, it may increase one’s risk for breast cancer, stroke and a heart attack.
In summary, as many as 70% of colorectal cancers can be prevented through moderate changes in lifestyle and diet.
Reviewed by Dr C R Jacobs, Clinical Oncologist, August 2008.
Reviewed by Dr Estelle Wilken, Senior Specialist, Internal Medicine and Gastroenterology, University of Stellenbosch and Tygerberg Hospital, August 2010