Causes

Like most types of cancer, colorectal cancer is more common in older people. More than 80% of colorectal cancers (8 in 10) are diagnosed in people over 60.

The exact cause of colorectal cancer is unknown. But certain things called risk factors can increase the chances of developing it. Having one or more risk factors doesn’t mean you will definitely get cancer. Equally, if you don’t have any risk factors, it doesn’t mean you won’t get colorectal cancer.

Diet

A diet containing a lot of red and processed meat increases the risk of colorectal cancer. Red meat includes beef, lamb and pork. Processed meat includes smoked meat, ham, bacon, sausages, pâté and tinned meat. Eating fried or grilled meat may also increase the risk.

Eating two or more portions of red or processed meat a day seems to increase the risk the most. People who eat less than two portions a week have the lowest risk.

No link has been found between colorectal cancer and eating poultry (such as turkey and chicken).

Not eating enough fruit and fresh vegetables may also increase risk.

Physical inactivity

People who aren’t physically active are more likely to develop colorectal cancer.

Body weight

Being overweight can increase the risk of developing colorectal cancer, especially in men.

Smoking

Colorectal cancer is more common in people who have smoked cigarettes for many years.

Inflammatory bowel conditions

Having an inflammatory colorectal condition such as ulcerative colitis or Crohn’s disease can increase the risk of colorectal cancer. People with these conditions may be offered regular bowel screening with colonoscopies.

Irritable bowel syndrome is not an inflammatory bowel condition and does not increase the risk of developing colorectal cancer.

Family history

Most people who get colorectal cancer don’t have a family history of it. Having one relative who developed colorectal cancer at an older age doesn’t significantly affect your risk. But, if several close family members on the same side of your family have had colorectal cancer, or if a close family member developed colorectal cancer before the age of 50, this may mean you have a higher risk. Close family members are parents, brothers and sisters.

People who have a history of colorectalcancer in their family can be referred to a specialist clinic to have their risk assessed. People at high risk of colorectal cancer are offered bowel screening. This involves regular tests to look at the inside of the large bowel (colonoscopy).

Familial conditions

About 5% of colorectalcancers (5 in every 100) are caused by an inherited faulty gene.

There are two rare conditions that can run in families: familial adenomatous polyposis (FAP) and Lynch syndrome (also called hereditary non-polyposis colorectal cancer or HNPCC). People with either condition have a very high risk of developing colorectalcancer.

Familial adenomatous polyposis (FAP)

People with FAP have many hundreds of thousands of benign tumours (polyps) in the lining of their colon. They may also have polyps in their rectum. They are regularly screened using colonoscopies to look for signs of cancer. Usually, it’s recommended that people with FAP have an operation to remove their colon and sometimes rectum. Unless the colon is removed, nearly everyone with FAP will develop colorectal cancer.

FAP causes about 1% of colorectalcancers (1 in every 100).

Lynch syndrome (HNPCC)

People with Lynch syndrome have an increased risk of developing colorectalcancer at a young age. If you have Lynch syndrome, you will usually have regular screening using colonoscopies. Screening usually starts from the age of 25, or five years before the age at which your youngest relative developed colorectal cancer.

Lynch syndrome causes about 3% of colorectal cancers (3 in every 100).

Colorectalcancer is not infectious and can’t be passed on to other people.