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Risk factors for colorectal cancer
A risk factor is something that increases the risk of developing cancer. It could be a behaviour, substance or condition. Most cancers are the result of many risk factors. But sometimes colorectal cancer develops in people who don’t have any of the risk factors described below.
Most people diagnosed with colorectal cancer are over the age of 50, and the risk increases with age. More men than women develop colorectal cancer. The number of cases of colorectal cancer is highest in North America, northwestern Europe and Australia. The less industrialized countries in Asia, Africa and South America have lower rates of colorectal cancer.
Risk factors are generally listed in order from most to least important. But in most cases, it is impossible to rank them with absolute certainty.
*You may wonder about laxatives. There is significant evidence showing that there is no association between this factor and colorectal cancer.
Known risk factors
There is convincing evidence that the following factors increase your risk for colorectal cancer.
A family history of colorectal cancer means that one or more close blood relatives have or had colorectal cancer. Some families have more cases of colorectal cancer than would be expected by chance. Sometimes it is not clear whether the family’s pattern of cancer is due to chance, shared lifestyle factors, an inherited (hereditary) factor that has been passed from parents to children through genes or a combination of these factors.
In families with colorectal cancer, the risk is increased if:
- A first-degree relative (such as a parent, sibling or child) has had colorectal cancer. The risk is greater if the first-degree relative had colon cancer than if the first-degree relative had rectal cancer.
- More than one first-degree relative has had colorectal cancer.
- Colorectal cancer was diagnosed in a family member under the age of 50.
Personal history of colorectal cancer
People who have had colorectal cancer have an increased risk of developing a second colorectal cancer.
FAP is a rare, inherited condition usually caused by a mutation of the adenomatous polyposis coli (APC) gene.
FAP causes hundreds to thousands of adenomatous polyps to develop on the inner lining, or mucosa, of the colon and rectum. These polyps can develop as early as adolescence.
If not treated, the majority of people with FAP will develop colorectal cancer by the age of 30.
Attenuated familial adenomatous polyposis, or AFAP, is a subtype of FAP. People with AFAP have fewer adenomatous polyps in the colon than people with FAP, and these polyps tend to develop later in life. Colorectal cancer usually develops in people with AFAP by the age of 56.
Lynch syndrome (also called hereditary non-polyposis colorectal cancer, or HNPCC) is an inherited condition caused by mutations in the genes that correct mistakes when cells divide and DNA is copied.
Lynch syndrome causes polyps to develop in the lining of the colon, rectum or both, but not as many polyps as in FAP. Lynch syndrome may be suspected if a person between 40 and 50 years of age develops colorectal cancer or has relatives who developed colorectal cancer around that age.
Lynch syndrome can occur as 2 types:
- Type A (Lynch type I) increases the risk of colorectal cancer only.
- Type B (Lynch type II) increases the risk of colorectal cancer as well as cancers of the stomach, small intestine, pancreas, kidney, ureter, ovary, uterus, breast, bladder, bile duct and skin.
A personal or family history of polyps in the colon, rectum or both significantly increases the risk of developing colorectal cancer. Colorectal polyps usually start in the inner lining, or mucosa, of the colon or rectum. Adenomas (adenomatous polyps) are polyps that have the potential to become cancer.
The risk of developing colorectal cancer from adenomatous polyps is related to the type, size and number of polyps:
- Tubular polyps are more common than other types of polyps but are usually smaller and less likely to become cancerous.
- Villous polyps are less common but are more likely to become cancerous.
- Polyps larger than 1 cm are more likely to be cancerous.
- The more polyps, the more chances that one of the polyps will become cancerous.
There are some rare genetic conditions that can increase the risk of developing colorectal cancer.
Turcot syndrome is a variant of both Lynch syndrome and FAP. People with Turcot syndrome develop many colon polyps that can become cancerous. With Lynch syndrome, Turcot syndrome is associated with mutations in the MLH1 and PMS2 genes. This variation also has an increased risk for brain tumours, specifically glioblastoma. With FAP, Turcot syndrome is linked to a mutation of the APC gene. This form of the syndrome also has an increased risk for brain tumours, specifically medulloblastoma.
MYH-associated polyposis, or MAP, is an inherited condition that increases the risk of colorectal cancer. People with MAP develop many adenomatous polyps in the inner lining, or mucosa, of the colon. MAP is caused by a mutation in the MUTYH DNA repair gene.
Juvenile polyposis syndrome is an inherited condition that causes hamartomas to develop. Hamartomas are usually a non-cancerous (benign) type of polyp, but they have the potential to develop into cancer.
Peutz-Jeghers syndrome is an inherited condition that involves a mutation of the STK11 (also called PJS or LKB1) gene. People with Peutz-Jeghers syndrome often develop hamartomas in the digestive tract. They also tend to develop dark blue or brown freckles that are most common on the face, fingers and toes and inside of the mouth. Peutz-Jeghers syndrome is also associated with a higher than average risk of developing other types of cancers including breast, pancreatic, stomach, ovarian, lung and small intestine cancers.
Hereditary mixed polyposis syndrome (HMPS) is an inherited condition that causes many different types of polyps to grow. This is why it is called a “mixed” polyposis syndrome. The most common type of polyp to grow is a hamartoma.
Cowden syndrome and Bannayan-Riley-Ruvalcaba syndrome are rare genetic conditions and part of a group of disorders that have a mutation in the tumour suppressor gene PTEN. People with a PTEN mutation are at an increased risk of developing benign and cancerous tumours, including colorectal cancer.
People who are not physically active are at a higher risk for developing colorectal cancer. You can lower your risk by becoming more active.
Colorectal cancer occurs more often in people who are overweight or obese than in those who have a healthy weight. Having a higher body mass index (BMI) increases the risk in both men and women, although men seem to be most at risk.
Drinking 50 g (about 3.5 drinks) of alcohol per day can increase the risk of developing colorectal cancer by 1.5 times compared with non-drinkers. Alcohol increases the risk for both men and women, but the effect appears to be stronger for men.
Smoking tobacco increases the risk of developing colorectal cancer. Researchers think that it may take a long time (perhaps up to 20 years) for colorectal cancer to develop due to smoking. It appears that the risk increases with both the length of time a person smokes and the amount smoked.
Eating a diet high in red meat (beef, pork, lamb and goat) increases the risk of developing colorectal cancer. The risk increases with the amount of red meat in a person’s diet. Eating red meat has also been shown to increase the risk of dying from colorectal cancer.
Processed meats are meats preserved by smoking, curing, salting or adding preservatives such as nitrates or nitrites. Eating processed meats increases the risk of developing colorectal cancer, and the risk increases with the amount eaten. It has also been shown to increase the risk of dying from cancer.
Eating a diet low in fibre increases the risk of colorectal cancer. The evidence shows that eating foods containing dietary fibre protects against colorectal cancer.
Higher amounts of sedentary behaviour (sitting for long periods of time) have been linked to an increased risk for colorectal cancer, particularly cancer in the distal colon and rectum.
IBD is a disease in which the lining of the digestive tract becomes inflamed, causing sores and bleeding. Ulcerative colitis and Crohn’s disease are the 2 most common types of IBD. Ulcerative colitis increases the risk of colorectal cancer more than Crohn’s disease.
People with IBD are at an increased risk of developing colorectal cancer. Their risk increases with the length of time they have IBD and how much of the colon is affected by IBD.
People who have type 2 diabetes have higher rates of colorectal cancer. There is an association because 2 of the risk factors for developing diabetes – being physically inactive and obesity – are also risk factors for colorectal cancer.
Jewish people of Eastern European descent (Ashkenazi Jews) have a higher than average risk of developing colorectal cancer. This group tends to have a specific mutation in the APC gene that increases the risk. This genetic mutation isn’t common among other ethnic groups.
Women who have had breast, ovarian or uterine cancer have an increased chance of developing colorectal cancer. This increased risk may be due to common risk factors including Peutz-Jeghers syndrome, Ashkenazi Jewish ancestry, Lynch syndrome or some other genetic syndromes.
Several studies show that being tall is a risk factor for colon cancer but probably not for rectal cancer. Researchers think this increased risk may be due to the influence of growth and puberty hormones.
Cooking meat (including beef, pork and poultry) and fish at high temperatures (such as well done, fried, broiled or barbecued) creates chemicals called heterocyclic amines (HCAs). Some HCAs are associated with an increased risk of developing colorectal cancer.
Fat from meat, poultry or fish dripping onto hot coals or stones creates other potentially cancer-causing substances called polycyclic aromatic hydrocarbons (PAHs). PAHs are added to food by smoke and flare-ups. Some PAHs are associated with an increased risk of developing colorectal cancer.
People who have been exposed to ionizing radiation, such as during atomic bomb explosions in Japan, have an increased risk of developing colorectal cancer, mainly colon cancer.
People who have been treated with radiation therapy for a previous cancer are at increased risk for colorectal cancer.
The following factors have been linked with colorectal cancer, but there is not enough evidence to show they are known risk factors. Further study is needed to clarify the role of these factors for colorectal cancer.
Asbestos is a group of minerals that occur naturally. Asbestos has been widely used in building materials and many industries. It is known to cause cancer in people. Long-term and frequent exposure to asbestos might cause colorectal cancer.
Cystic fibrosis is a genetic condition that affects glands that make mucus and sweat, which can affect how well the lungs and digestive tract function. A large study that followed people with cystic fibrosis for 20 years has shown that they may have an increased risk of developing colorectal cancer.
Unknown risk factors
It isn’t known whether or not the following factors are linked with colorectal cancer. It may be that researchers can’t show a definite link or that studies have had different results. Further study is needed to see if the following are risk factors for colorectal cancer:
- diets high in fat
- gallstones and surgery to remove the gallbladder (cholecystectomy)
- Helicobacter pylori (H. pylori) infection
- chlorinated drinking water by-products
- night shift work
Questions to ask your healthcare team
To make the decisions that are right for you, ask your healthcare team questions about risks.
A small growth on a mucous membrane, such as the lining of the colon, bladder, uterus (womb), vocal cords or nasal passage.
Most types of polyps are non-cancerous, but some have the potential to become cancer.
A non-cancerous, or benign, tumour or growth that starts in epithelial cells that line the inside of organs and glands. Adenomas may become cancerous if they are not treated.
An adenoma may be called an adenomatous polyp when it is in the gastrointestinal (GI) tract.
A measure that relates body weight to height (calculated by dividing weight in kilograms by height in metres squared).
BMI is used to find out if people are underweight, overweight, obese or in the normal weight range for their height.
A type of bacteria that causes inflammation and ulcers in the stomach and small intestine.
Infection with H. pylori is associated with an increased risk of developing stomach cancer.