People who eat moderate amounts of red and processed meat are still at increased risk of bowel cancer, according to a major new study.

‘Diet and colorectal cancer’, published today (Wednesday 17 April) in the International Journal of Epidemiology, also found that every bottle of beer or small glass of wine raises bowel cancer risk, while eating a lot of fibre from breads and breakfast cereals lowers it.

“Aotearoa New Zealand has one of the highest bowel cancer rates in the world; this study shows we could prevent some of these cancers by changing our diets, consuming less red and processed meat and alcohol, and more wholegrains,” says lead author Dr Kathryn Bradbury, a senior research fellow in the School of Population Health at the University of Auckland’s Faculty of Medical and Health Sciences.

“Think less beer and bacon, more bran and brown bread.”

There were 1,268 deaths from bowel cancer in 2016 (latest available data), making it the second leading cause of cancer death in New Zealand.

Dr Bradbury and co-authors Professor Tim Key, University of Oxford, and Dr Neil Murphy, International Agency for Research on Cancer, studied the diets of nearly half a million British women and men, aged 40 to 69 when the research began, over more than five years. During this time 2,609 of them developed bowel cancer.

They found that:

• people eating around 76g of red and processed meat a day on average – barely a mouthful above the New Zealand-recommended upper limit of 71g daily average - had a 17 percent higher chance of developing bowel cancer than those who only ate about 21g a day
• risk rose 20 percent with every 25g of processed meat (roughly equivalent to a rasher of bacon or slice of ham) people ate per day, and 19 percent with every 50g of red meat consumed (a thick slice of roast beef or the edible bit of a lamb chop)
• each bottle of beer or small glass of wine raised bowel cancer risk by eight percent
• people in the highest fifth for fibre intake from bread and breakfast cereals had a 14 percent lower risk of bowel cancer
• no link was found between bowel cancer risk and fish, poultry, cheese, fruit, vegetables, tea and coffee
• Current evidence points to an increased bowel cancer risk for every 50g of processed meat a person eats per day, but this research found that risk increases at just 25g per day, showing a similar rise in risk at smaller intervals. This is one of the largest single studies in the field and one of few to measure meat quantities and associated risks so precisely.

“There’s substantial evidence linking red and processed meat to bowel cancer, and the World Health Organization classifies processed meat as carcinogenic and red meat as probably carcinogenic,” says Dr Bradbury. “But most previous research looked at people in the 1990s or earlier, and diets have changed significantly since then. Our study gives a more up-to-date insight that is relevant to meat consumption today.”

Professor Key, deputy director at the University of Oxford’s cancer epidemiology unit and joint study funder Cancer Research UK’s expert in diet and cancer, says: “Our results strongly suggest that people who eat red and processed meat four or more times a week have a higher risk of developing bowel cancer than those who eat red and processed meat less than twice a week.”

The researchers drew on the UK Biobank study, a rich source of data open to all approved researchers, which is advancing understanding of how many illnesses develop. On joining the study, participants had various measurements and samples taken, and filled in a questionnaire that included a section on diet. The new study’s researchers linked this diet information to cancer and death registries.

“We looked at whether the participants’ diets at the start of the study predicted whether they developed bowel cancer during the follow-up period. Although we used UK data, the results are relevant to New Zealand as we have broadly similar diets and the UK also has quite a high rate of bowel cancer,” says Dr Bradbury.

“You don’t have to cut out red and processed meat altogether, but this study shows that reducing how often and how much you eat meat, you have can reduce your risk of bowel cancer. You can try having meat free lunches, or days, and swapping red meat for chicken, fish or legumes.”