Aerobics - but even less strenuous activity could help
Scottish experts have gathered up all the evidence on the issue, and say that it points more firmly than ever to this conclusion.
Bowel cancer is one of the UK's biggest killers - there are approximately 17,000 deaths from it every year.
The causes are still not fully understood, although eating fresh fruit and vegetables also appears to have a protective effect.
The research, carried out by the Cancer Research Campaing and Scottish Cancer Foundation, combined and analysed the results of more than 50 separate studies into bowel cancer.
It found that people who exercised regulatly were 50% less likely to develop the disease.
She said: "This is exciting, because it provides a relatively easy way for us all to reduce our risk of this terrible disease, which claims the lives of over 300 people each week in the UK.
"We've analysed a wide range of studies looking at many different types of physical activity, and taken together they provide convincing evidence that exercise really does protect against bowel cancer."
It is not clear yet why exercise should help - it may alter levels of hormones which are key to cancer development, or perhaps alter the speed at which food moves through the bowel, which may also have a bearing.
Exercise avoiding
Other surveys have suggested that a large proportion of men and women avoid the physical exercise that could keep them disease -free.
Just under a quarter of men, and just over a quarter of women confessed to taking no moderate or vigorous physical activity.
Professor Gordon McVie, Director General of the Cancer Research Campaign, said: "This research is important, because it suggests that by making a simple lifestyle change, people could substantially reduce their chances of developing bowel cancer.
"Regular physical activity doesn't have to mean running a marathon each week.
"A brisk walk for half an hour, five times a week, is as good a way as any of getting the exercise we need."
Exercise is also well recognised as protective against other illnesses such as heart disease.