Risks and causes of bowel cancer
Your risk of developing bowel (colon and rectal) cancer depends on many things including age, genetics and lifestyle factors.
What is a risk factor?
Anything that can increase your risk of getting a disease is called a risk factor.
Different cancers have different risk factors. Having one or more of these risk factors doesn't mean you will definitely get that cancer.
Diet
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.
The role of some parts of our diet remains unknown or uncertain.
Eating a healthy, balanced diet is more important than most individual foods when it comes to cancer. But we do know that some foods can affect the risk of bowel cancer on their own.
Eating too much red and processed meat
Many studies have shown that eating lots of red and processed meat increases the risk of bowel cancer. It is estimated that around 13 out of 100 bowel cancers (around 13%) in the UK are linked to eating processed meats.
Processed meat is any meat that's been treated to make it last longer, or taste better, by adding chemical preservatives such as nitrates. This could be through smoking, curing, or salting the meat. Processed meat includes ham, bacon, corned beef, and some sausages like salami, chorizo and hot dogs.
Red meat is darker in colour than white meat and includes fresh, minced, and frozen beef, pork and lamb. 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 3 slices of sandwich ham.
It might help to swap red meat for chicken or fish. Or use beans and pulses in meals instead of meat.
Eating too little fibre
Eating lots of fibre reduces your risk of bowel cancer. Eating too little fibre causes 28 in 100 bowel cancers (28%) in the UK.
You can boost the fibre in your diet by choosing wholegrain versions of foods. To get more fibre in your diet try:
- swapping to wholegrain rice, pasta or bread
- swapping your snack to low calorie popcorn rather than crisps
- choosing wholegrain breakfast cereals
- eating more fruit and vegetables high in fibre, such as peas and raspberries
Calcium in your diet
Calcium is found in dairy foods such as milk or yoghurt. There are non-dairy alternatives available, such as plant milks, which have calcium added.
A recent study has shown that having an extra 300mg of calcium per day, which is about the amount in a large glass of milk, is linked to a lower risk of bowel cancer.
More research is needed to know if calcium supplements can reduce the risk of cancer.
Being overweight and obese
Obesity is a cause of bowel cancer. It is estimated that 11 out of 100 bowel cancers (11%) in the UK are linked to being overweight or obese.
For adults with a South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean ethnic backgrounds a BMI of 23 or more is overweight and a BMI of 27.5 or more is obese.
For adults not with these ethnic backgrounds, a BMI of 25 or more is overweight and a BMI of 30 or more is obese.
The risk of bowel cancer is higher in people who are obese compared to those who have a healthy BMI.
Physical activity
Being active can help you keep a healthy weight and reduce your risk of cancer.
Being physically active may also lower your risk of bowel cancer in a way that is not related to being a healthy weight. But it isn’t clear yet how much.
Smoking tobacco
7 out of 100 bowel cancers (7%) in the UK are 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.
Alcohol
Alcohol increases the risk of bowel cancer. It has been estimated that around 6 out of 100 bowel cancers (around 6%) in the UK are linked to drinking alcohol.
Age and getting older
Bowel cancer is more common in older people. More than 40 out of 100 bowel cancers (more than 40%) in the UK each year are diagnosed in people aged 75 or over.
Globally and in the UK, we’re seeing a small increase in rates of early onset cancers. This means they are affecting people aged 25 to 49. It isn't clear what is contributing to the rise. But preventable risk factors, 
and improvements in early detection might all play a part.
Family history
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 50 years old.
Talk to your GP if you think you have a family history of bowel cancer. They might refer you to a genetics clinic if appropriate. A geneticist carries out a detailed assessment and can confirm whether people need screening or genetic testing.
Inherited or genetic 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)
Familial adenomatous polyposis (FAP) is one of these conditions. FAP is responsible for less than 1 in 100 (less than 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. The surgeon usually removes the colon to prevent the development of bowel cancer.
Lynch syndrome
Another condition is Lynch syndrome, or hereditary non polyposis colon cancer (HNPCC). People with this gene change have a higher risk of getting bowel cancer and other cancers.
We know that aspirin can be useful to reduce the risk of bowel cancer in people with Lynch syndrome. It's not appropriate for everyone. Speak to your doctor if you have Lynch syndrome and would like to talk about taking aspirin to reduce your risk of bowel cancer.
People with gene changes that increase bowel cancer risk might need screening more often than the general population.
Examples of other inherited gene changes or genetic conditions that may increase your risk of developing bowel cancer include:
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MUTYH associated polyposis (MAP)
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Peutz Jeghers syndrome (PJS)
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Juvenile Polyposis syndrome (JPS)
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PTEN Hamartoma tumour syndrome
Ulcerative colitis and Crohn’s disease
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.
Previous bowel cancer
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.
Medical conditions
Type 2 diabetes
Type 2 diabetes is a condition that causes the level of sugar (glucose) in the blood to become too high. People with type 2 diabetes have an increased risk of bowel cancer. We don't know why this is and scientists are looking into it.
Acromegaly
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.
Benign polyps in the bowel
Growths in the bowel, called polyps are not cancerous. Certain types of polyps called adenomas can develop into cancer over a long period of time. In fact, most bowel cancers develop from an adenoma. Your risk of developing cancer depends on how many polyps you have, and how large they are.
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 these 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.
Ionising radiation
Around 2 in 100 (around 2%) of bowel cancers in the UK are linked to high energy (ionising) radiation exposure.
Some of these 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. Background radiation is the low level of radiation which is always present in the environment, and includes radon, a naturally occurring gas.
Infections
There is some evidence that the risk of bowel cancer is higher in people who have an infection called Helicobacter pylori (H. pylori). But we need more research to fully understand how H.pylori could affect bowel cancer risk.
This is a type of bacteria which can cause stomach ulcers. It is a common infection which many people have. Most do not develop into cancer.
Other possible causes
There are often stories about potential causes of cancer in the media. 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.
More information on risk factors for bowel cancer
We have more detailed information for health professionals about bowel cancer cancer risks and causes.
Reducing your risk
There are ways you can reduce your risk of cancer.
