What Causes Colorectal Cancer?

Cancer occurs when a normal cell divides and multiples uncontrollably. Normally cells are regulated in their growth, maturity, division and death by the DNA present in their nucleus. With damage to the DNA the cells may become rogue and multiply uncontrollably to give rise to tumors.

Most cases of colorectal cancer first develop inside clumps of cells on the inner lining of the bowel. These are called polyps. Polyps may be multiple in number and do not necessarily indicate bowel cancer. However, those with polyps are at a greater risk of bowel cancer.

Risk factors for colorectal cancer

The lifetime risk of developing colon cancer in the United States is about 7%. There are several factors that raise the risk of getting colorectal cancer. These include:


Colorectal cancer risk increases with age. Most cases occur in the 60s and 70s, while cases before age 50 are uncommon unless a family history of early colon cancer is present.

Genetic conditions

Certain genetic conditions predispose to getting colorectal cancers. For example, those with familial adenomatous polyposis (FAP) and hereditary non-polyposis colorectal cancer (HNPCC), also known as Lynch syndrome are at a higher risk.

FAP affects 1 in 10,000 people. These individuals develop several non-cancerous polyps inside the intestines. These are not cancerous but at least one of them may turn cancerous. Almost all people with FAP will have bowel cancer by the time they are 50 years of age. These individuals are thus advised to have their large bowel removed by surgery before they reach the age of 25.

HNPCC affects at least 2.5% of all cases of colorectal cancer. It is caused by a genetic mutation. Around 90% of men and 70% of women with the HNPCC mutation will develop bowel cancer by the time they are 70 years of age. Individuals with HPNCC are also advised to get their large bowels removed by surgery to prevent the cancer. People with HNPCC are at increased risk of other cancers too, including womb, ovary, stomach, pancreas and bladder.

Inherited factors and family history

Around 20% of people who develop colorectal cancer have a first degree relative (parent or a sibling) or a second-degree relative (grand parent, uncle or aunt or cousin) with the same cancer. Having a blood relation (first or second degree) raises the risk by two fold and having two relatives with bowel cancer raises the risk by four-fold.

Dietary factors

There are several studies that have shown that a diet high in red and processed meat and low in fibers can increase the risk of developing bowel cancer. As per the Department of Health people who eat more than 90 grams (cooked weight) of red and processed meat a day need to reduce their consumption to 70 grams.


Obesity is associated with an increased risk of colorectal cancer. Obese men are 50% more likely to develop bowel cancer than people with a healthy weight. Those with a body mass index (BMI) of over 40, are twice as likely to develop bowel cancer. Obese women have a small increased risk of developing bowel cancer but morbidly obese women are 50% more likely to develop bowel cancer than women with a healthy weight.

Smoking and alcohol

Smokers are at a 25% higher risk of colorectal cancers. Chemicals in tobacco are found to be carcinogenic or raise the risk of cancers in general. Large studies have shown that excessive alcohol intake is associated with bowel cancer. According to a landmark study in this aspect called the EPIC study for every two units of alcohol a person drinks each day, their risk of bowel cancer goes up by 8%.

Physical inactivity

Sedentary life and no exercise has been linked to colorectal cancers. Risk of bowel cancer could be reduced by up to one-fifth if an individual involves an hour of vigorous exercise in his or her daily routine or exercises moderately for at least 2 hours a day.

Other digestive diseases

Individuals with conditions like inflammatory bowel disorders (e.g. Crohn’s disease and ulcerative colitis) are at a higher risk of developing bowel cancer. Those with Crohn’s disease have a 2-3 times higher risk while among those with ulcerative colitis as many as 1 in 20 will go on to develop colorectal cancer.

Further Reading

Last Updated: Jun 5, 2019

Dr. Ananya Mandal

Written by

Dr. Ananya Mandal

Dr. Ananya Mandal is a doctor by profession, lecturer by vocation and a medical writer by passion. She specialized in Clinical Pharmacology after her bachelor's (MBBS). For her, health communication is not just writing complicated reviews for professionals but making medical knowledge understandable and available to the general public as well.


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The opinions expressed here are the views of the writer and do not necessarily reflect the views and opinions of News Medical.
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