The study, involving 500,000 middle-aged men and women, is one of the largest so far to look at the link between bowel cancer and our diets. Its findings come only days after the World Health Organisation warned processed meat was as big a cancer threat as cigarettes.
A typical portion size for red or processed meat is about 70g, official guidelines say – equivalent to two rashers of bacon, two slices of ham or one sausage.
But a quarter pounder burger is 200g – nearly three times this level – while a 10oz steak from a restaurant is equivalent to 284g, or more than four portions.
Bowel cancer is already the second-biggest cancer killer in the UK, leading to 16,000 deaths a year. Only lung cancer kills more patients.
Alarmingly, the research also found that adults who ate large amounts of fibre – found in fruit, vegetables or wholegrain foods – did not have a significantly lower risk of developing bowel cancer than those who consumed very little.
For years, NHS guidelines have urged Britons to eat five portions of fruit and veg a day, and opt for wholemeal bread and pasta to avoid cancer and other illnesses.
Mark Flannagan, chief executive of Beating Bowel Cancer, said: ‘We must not underestimate the importance of diet in reducing your risk of bowel cancer.
‘The evidence suggests there is a strong link between red and processed meat and bowel cancer, so we recommend eating both in moderation.’
Experts think the chemical haem in red meat damages the DNA of cells in the digestive system, triggering tumour growth.
But processed meat – including ham, sausages, bacon and burgers – is thought to be even more harmful as it contains cancer-causing additives as well as being high in fat and salt.
Professor Tim Key and Dr Kathryn Bradbury, of Oxford University, looked at the records of 500,008 British men and women aged 40 to 69. They had completed detailed questionnaires about how often they ate meat in a typical week, and were tracked over a four-year period between 2006 and 2010.
Over that time, 1,503 of the participants developed bowel cancer. The analysis showed that adults who had red or processed meat four times a week were 42 per cent more at risk than those who had it once or not at all.
And those who ate it at least twice a week were 18 per cent more at risk compared with vegetarians.
Officials classified processed meat on the same cancer-causing level as cigarettes, asbestos and arsenic.
Red meat was placed one grade lower as ‘probably cancerous’, although experts are less clear about the risks.
NHS guidelines also state adults should aim for 25g of fibre a day – five pieces of fruit or veg – to protect against bowel cancer, heart disease and obesity.