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Risks and causes

Find out what causes bowel cancer, including lifestyle factors and other medical conditions, and see what you can do to reduce your risk.

We don’t know what causes most bowel cancers. But there are some factors that may increase your risk of developing it.

Bowel cancer is more common in older people. Almost 6 in 10 bowel cancer cases in the UK each year are diagnosed in people aged 70 or over.

Your risk of bowel cancer is increased if you have a first degree relative diagnosed with bowel cancer. A first degree relative is a parent, brother or sister, son or daughter.

The risk is increased further if you have more than one relative diagnosed with bowel cancer. Or you have a first degree relative diagnosed at a young age, for example, under the age of 45 years old.

Talk to your GP if you think you have a strong family history of bowel cancer. They might refer you to a genetics clinic appropriate. A geneticist carries out a detailed assessment and can confirm whether people need screening or genetic testing.

Inherited conditions

There are some rare inherited conditions or syndromes associated with certain gene changes. Family members have an increased risk of bowel cancer if they inherit these gene changes. 

Familial adenomatous polyposis (FAP) is one of these conditions. FAP is responsible for less than 1 in 100 cases (1%) of all bowel cancers.  If left untreated, all individuals with this syndrome will almost certainly develop bowel cancer by their 40s.

Specialists recommend that people with FAP have bowel surgery in their 20s. They have their colon removed to prevent the development of bowel cancer.

Another condition is Lynch syndrome, or hereditary non polyposis colon cancer (HNPCC). People with this gene fault have a higher risk of getting bowel cancer and other cancers.

The BRCA1 gene fault increases the risk of breast cancer in those who have it. This gene might also increase the risk of bowel cancer in women under 50.

People with an Ashkenazi Jewish background have a higher risk of bowel cancer. This might be due to a gene fault that is more common in this group. Around 6 in 100 Ashkenazi Jews (6%) have a faulty gene called I1307K. BRCA1 is also common in Ashkenazi Jews.

People with gene changes that increase bowel cancer risk might need screening more often than the general population.

Growths in the bowel, called polyps or adenomas, are not cancerous. But they can develop into cancer over a long period of time. In fact, most bowel cancers develop from an adenoma. Adenomas are quite common. Around a third of the population will have at least one polyp by age 60. But only a small fraction of adenomas develop into cancer and it takes years to happen. 

The bowel cancer screening programmes in England, Wales, Scotland and Northern Ireland aim to find and remove polyps in the general population before they become cancerous.

Ulcerative colitis and Crohn's disease are chronic bowel diseases causing inflammation in the bowel. Having either of these diseases for many years increases your risk of bowel cancer by up to 70%. 

You have an increased risk of developing another bowel cancer if you have already had a bowel cancer in the past. Your specialist will talk to you about how often you may need screening.

You might have a slightly increased risk of getting bowel cancer if you have had certain cancers in the past. Studies have shown an increased risk of bowel cancer for individuals who have had:

  • lymphoma
  • oesophageal cancer
  • kidney cancer
  • bladder cancer
  • breast cancer
  • cancer of the uterus (womb cancer)
  • cervical cancer
  • head and neck cancer
  • prostate cancer

This might be due to genetic changes or an effect of the treatment for the first cancer. In some cases, it may be due to shared risk factors such as smoking or being overweight.

X-radiation and gamma radiation are classified by the International Agency for Research on Cancer (IARC) as causes of colon cancer. There is limited evidence to say radiation causes rectal cancer.  

Around 2 in 100 cases (2%) of bowel cancer in the UK are linked to radiation exposure. Some of these cases are due to radiotherapy treatment for previous cancer. The rest are linked to radiation used in tests such as x-rays and CT scans (diagnostic radiation) and background radiation. 

If you have diabetes, your pancreas doesn't make enough of a hormone called insulin. People with diabetes have an increased risk of bowel cancer. We don't know why this is and scientists are looking into it.

The risk of rectal cancer is increased in people with gallstones compared to those without. Also, people with gallstones have double the risk of developing polyps in the large bowel (colon) compared to those without gallstones.

People with acromegaly also have an increased risk of bowel cancer, although studies are still trying to find out why. Acromegaly is a condition where the pituitary gland produces too much growth hormone and causes an overgrowth of bones, especially in the face.

The risk of bowel cancer is higher in people who have Human papilloma virus (HPV) or helicobacter pylori (H pylori). HPV is a common virus, and there are many different types. H pylori is a bacteria which causes stomach ulcers. Both are common infections which many people have. Most do not develop into cancer.

Studies are still looking into why some of these infections turn into cancer and why others don't.

Alcohol increases the risk of bowel cancer. It has been estimated that about 1 out of 10 bowel cancers (10%) in the UK are linked to drinking alcohol.

Around 1 out of 10 bowel cancers (10%) in the UK is linked to smoking. The risk increases with the number of cigarettes smoked a day.

It is never too late to stop smoking but the sooner you stop the better.

Obesity is a cause of bowel cancer. It is estimated that 13 out of 100 bowel cancers (13%) in the UK are linked to being overweight or obese.

Obesity means being very overweight with a body mass index (BMI) of 30 or higher. And being overweight is a BMI of between 25 and 30. The risk of bowel cancer is a third (33%) higher in people who are obese compared to those who have a healthy BMI.

Try to keep a healthy weight by being physically active and eating a healthy, balanced diet. There is strong evidence which shows that people who are more physically active have a lower risk of bowel cancer.

It is very difficult to research the link between diet and cancer. Scientists need to carry out very large studies to see which specific foods might reduce the risk of cancer, and which could raise the risk.

Eating too much red and processed meat

The role of some parts of our diet remain unknown or uncertain. But researchers can be more certain about the link between red and processed meat and bowel cancer.

Many studies have shown that eating lots of red and processed meat increases the risk of bowel cancer. It is estimated that around 1 in 5 bowel cancers in the UK are linked to eating these meats.

The government recommends that people eating more than 90g of red and processed meat a day should reduce it to 70g or less. 70g is the cooked weight. This is about the same as  2 sausages. 

Eating too little fibre

Bowel cancer is less common in people who eat lots of fibre. You can boost the fibre in your diet by choosing wholegrain foods such as whole wheat pasta. Or fruit and vegetables high in fibre, such as peas and raspberries.

It might help to swap red meat for chicken or fish. Or use beans and pulses in meals instead of meat.

Other possible causes

Stories about potential causes are often in the media and it isn’t always clear which ideas are supported by evidence. There might be things you have heard of that we haven’t included here. This is because either there is no evidence about them or it is less clear.

For detailed information about bowel cancer risks and causes

Last reviewed: 
20 Jan 2016
  • Cancers attributable to consumption of alcohol in the UK in 2010.
    DM Parkin
    British Journal of Cancer, 2011 Dec.Vol. 6; Issue105 Suppl 2:S14-8.

  • Cancers attributable to dietary factors in the UK in 2010. II. Meat consumption.
    DM Parkin
    British Journal of Cancer, 2011 Dec. Vol 6; Issue 105 Suppl 2:S24-26.

  • Cancers in 2010 attributable to ionising radiation exposure in the UK.
    DM Parkin and SC Darby
    British Journal of Cancer. 2011 Dec. Vol 6; Issue 105 Suppl 2:S57-65

  • Diabetes mellitus and incidence and mortality of colorectal cancer: a systematic review and meta-analysis of cohort studies.
    Y Jiang, Q Ben, H Shen and others
    European Journal of Epidemiology. 2011 Nov; Vol. 26, Issue11:863-76.

  • Human papillomavirus infection and colorectal cancer risk: a meta-analysis.
    DC Damin, PK Ziegelmann and AP Damin
    Colorectal Disease, 2013 Aug; Vol 15, Issue 8:e420-8

  • Obesity and risk of colorectal cancer: a systematic review of prospective studies.
    Y Ma, Y Yang, F Wang and others
    PLoS One, 2013;Vol 8, Issue 1:e53916.

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