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People who lead an active lifestyle are up to a third less likely to develop polyps which can develop into bowel cancer, according to a study.

The report pulls together 20 previous studies looking at the link between exercise and the development of large polyps.

Bowel cancer is the third most common cancer in the UK.

More than 38,000 people are diagnosed with the disease each year.

The work was done by scientists from the Washington University School of Medicine in St Louis in the US.

Publishing their findings in the , they say they have produced the most accurate figures yet that show low exercise levels are linked to bowel polyps.

They found that people who take regular exercise were 16% less likely to develop bowel polyps and 35% less likely to develop large or advanced polyps.

Polyps - also known as adenomas - are growths in the bowel and while they are not cancerous in themselves they can develop into cancer over a long period of time.

Cancer Research UK says most bowel cancers develop from a polyp and about 1 in 4 of us have one or more by the age of 50, while about half of us have them by the time we are 70.

But only a small fraction of polyps develop into cancer and it takes years for that to happen.

Professor Kathleen Wolin, one of the lead authors of the research, says the evidence now shows a clear link between exercise and a reduced risk of bowel cancer but it is not yet clear exactly why that link exists.

"There are a number of likely pathways but we don't know exactly. So for example exercise reduces inflammation in the bowel, which has been linked to bowel cancer.

"But exercise also reduces insulin levels and improves the body's response to hyperinsulinaemia (excess levels of insulin circulating in the blood), which again increases polyp risk.

"It also enhances the immune system and because people who exercise often do so outside, they get vitamin D, which is also important for bowel cancer.

"The reality is that exercise is acting through more than one mechanism. The upside is there are so many benefits all over the body, it is hard to pinpoint."

Sara Hiom, director of health information at Cancer Research UK, says the evidence shows that keeping active could help prevent thousands of cases of cancer every year.

"We'd recommend doing at least half an hour's moderate exercise a day - such as brisk walking or anything that leaves you slightly out of breath. Getting enough physical activity will also help you keep a healthy weight, which is one of the most important ways of reducing the risk of cancer."

Mark Flannagan, Chief Executive of Beating Bowel Cancer, backed the study and said it was clear that lifestyle was an important factor in protecting yourself from cancer.

"Although the majority of polyps are not cancerous, it is thought that almost all bowel cancers develop from polyps. Therefore we recommend taking 30 minutes of physical activity each day, along with a healthy diet and lifestyle, to reduce your risk of developing bowel cancer."

Deborah Alsina, Chief Executive of Bowel Cancer UK, said the report was good news.

"Evidence also shows that the combination of taking more exercise and having a healthy diet may protect against bowel cancer, as well as weight gain and obesity, so we encourage people to do both.

"It is also important that people take part in the screening programme, if eligible, as screening is an effective means of detecting polyps at an early stage. These polyps can easily be removed, reducing the risk of bowel cancer developing."