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.

Anything that can increase your risk of cancer is called a risk factor. Those that lower the risk are called protective factors.

Having one or more risk factors doesn’t mean that you will definitely get bowel cancer. Risk factors for bowel cancer include:

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. But we do know that some foods can definitely affect the risk of bowel cancer.

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 cancer cases (around 13%) in the UK are linked to eating these meats.

Processed meat is any meat that has been treated to preserve it and/or add flavour - for example, bacon, salami, sausages, canned meat or chicken nuggets. And a portion is about 2 sausages or 3 slices of ham.

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.

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 around 30 in 100 bowel cancer cases (around 30%) 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 brown 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

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.

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 higher in people who are obese compared to those who have a healthy BMI.

Exercise

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.

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

Bowel cancer is more common in older people. More than 40 out of 100 bowel cancer cases (more than 40%) in the UK each year are diagnosed in people aged 75 or over.

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 45 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) is one of these conditions. FAP is responsible for less than 1 in 100 cases (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.

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.

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

Ulcerative colitis and Crohn’s disease

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.

Previous 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.

You might have a slightly increased risk of getting bowel cancer if you have had another type of cancer in the past. 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.

Medical conditions

Diabetes

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

Gallstones

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.

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 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.

Radiation

Around 2 in 100 cases (around 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.

Infections

There is some evidence that the risk of bowel cancer is higher in people who have an infection called Helicobacter pylori (H. pylori). This is a type of bacteria which can cause stomach ulcers. This is a common infection which many people have. Most do not develop into cancer.

Other possible causes

There are often stories about potential causes 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.