What Causes Bowel Cancer?

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Bowel cancer, also known as colorectal cancer, is a type of cancer that starts in the colon (the large intestine) or the rectum. These cancers can also be referred to separately as colon or rectal cancer, depending on where they start.

Cancer starts when cells in the body begin to grow out of control. In the case of bowel cancer, this uncontrolled cell growth typically begins as small, noncancerous (benign) clumps of cells called polyps that form on the inside of the colon or rectum. Over time, some of these polyps can become colon cancers.

What Are The Symptoms Of Bowel Cancer?

The symptoms of bowel cancer can include changes in bowel habits (such as diarrhoea, constipation, or narrowing of the stool that lasts for more than a few days), feeling that your bowel does not empty completely, blood (either bright red or very dark) in the stool, constant tiredness or fatigue, unexplained weight loss, and persistent abdominal discomforts, such as cramps, gas, or pain.

What Increases The Risk Of Bowel Cancer?

The cause of bowel cancer remains unknown, but current research suggests that various risk factors, including age, genetics, diet, and lifestyle, may increase the likelihood of developing the disease.


Most people with colon cancer are older than 50. More than 90% of cases of bowel cancer develop in adults over the age of 50, and nearly 60% of cases develop in people aged 70 or older. The rates of colon cancer in younger people have been increasing, but doctors are unsure why.

Family History

Having a family history of bowel cancer in a first-degree relative (a mother, father, brother or sister) under 50 may increase your risk of developing the condition yourself.

Most people who develop bowel cancer do not have a family history. Having only one family member who developed bowel cancer at an older age does not increase your risk.

Genetic Conditions

About 5% of bowel cancers are linked to inherited genes. Two rare inherited genetic conditions can increase your risk of developing bowel cancer: Familial Adenomatous Polyposis (FAP) and Non-Polyposis Colorectal Cancer (HNPCC), also known as Lynch syndrome.

FAP is a condition that triggers the growth of non-cancerous polyps inside the bowel. Although these polyps are non-cancerous, there is a high risk that at least one will turn cancerous over time. Most people with FAP have bowel cancer by the time they are 50. HNPCC is an inherited gene fault or mutation that increases your risk of developing bowel cancer.

People with FAP and HNPCC are sometimes advised by their doctor to have their large bowel removed as a precautionary measure.

Your doctor can refer you to a genetics specialist, who can advise you in more detail about your level of risk and recommend any necessary tests to check for the condition periodically.


Bowel cancer may be associated with a typical Western diet, which is low in fibre and high in fat and calories. Up to 30% of bowel cancer is estimated to be caused by eating too little fibre.

Growing evidence also suggests that a diet high in red and processed meat can increase your risk of developing bowel cancer.

Healthy eating choices could help reduce your bowel cancer risk. A healthy diet should be high in fibre and contain a variety of fruits, vegetables and whole grains. Fruits, vegetables and whole grains contain vitamins, minerals, fibre and antioxidants, which may play a role in cancer prevention. It might help to swap red meat for chicken, fish, beans, and pulses in non-meat meals. People who eat more than 90g (cooked weight) a day of red and processed meat should cut down to 70g a day.

Lifestyle Factors

People who smoke have an increased risk of developing bowel and other types of cancer and other serious conditions, such as heart disease. Up to 7% of bowel cancers are estimated to be linked to smoking.

Drinking alcohol is associated with an increased risk of bowel cancer, particularly if you regularly drink large amounts.

Being overweight or obese is linked to an increased risk of bowel cancer. Strong evidence suggests that physically inactive people, with a sedentary lifestyle have a higher risk of developing bowel cancer.

People who are obese have an increased risk of dying of colon cancer when compared with people considered normal weight. This is particularly true in men.

You can help reduce your risk of bowel and other cancers by maintaining a healthy weight, being physically active, and eating a balanced diet.

Digestive Disorders

Inflammatory intestinal conditions such as Crohn's disease or ulcerative colitis may increase your risk of developing bowel cancer.

If you have one of these conditions, you need regular check-ups to look for signs of bowel cancer from about ten years after your symptoms first develop. These check-ups usually include examining your bowel with a colonoscope.


People with diabetes or insulin resistance have an increased risk of developing bowel cancer.

Previous Cancer

You have an increased risk of developing bowel cancer again if you have already had bowel cancer in the past.  You might also have a slightly increased risk of developing bowel cancer if you have had any cancer in the past.

Radiation Therapy

Radiation therapy, directed at the abdomen to treat previous cancers, increases the risk of developing bowel cancer. Around 2% of bowel cancer is linked to radiation exposure. Some of these cases are due to radiotherapy treatment for previous cancer, while the rest are linked to radiation used in X-rays and CT scans.


The risk of rectal cancer is increased in people with gallstones. They also have double the risk of developing polyps in the large bowel (colon).


People with acromegaly also have an increased risk of bowel cancer. With this condition, the pituitary gland produces too much growth hormone, which causes an overgrowth of bones, usually in the face.

Helicobacter Pylori Infections

Limited evidence suggests that the risk of bowel cancer is higher in people with a Helicobacter pylori (H. pylori) infection. This bacteria can cause stomach ulcers, but most infections do not develop into cancer.

Understanding The Risks Of Developing Bowel Cancer

Bowel cancer begins in the colon or rectum and typically starts from benign growths known as polyps. Symptoms include changes in bowel habits, fatigue, and unexplained weight loss. Risk factors for bowel cancer are multifaceted, including older age, certain genetic conditions, family history of bowel cancer, and lifestyle factors such as a high-fat, low-fibre diet, smoking, and heavy alcohol use.

Other medical conditions, like diabetes, inflammatory bowel disease, previous cancers, and radiation exposure, also raise the risk of developing bowel cancer. Preventive measures include awareness of these factors, regular screenings, and a healthy lifestyle.

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