Bradbury and co-authors Professor Tim Key, of the 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.
They were aged 40 to 69 when the research began, and over more than five years, 2,609 of them developed bowel cancer.
The researchers 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 20 per cent higher chance of developing bowel cancer than those who only ate about 21g a day.
Risk rose 19 per cent with every 25g of processed meat - roughly equivalent to a rasher of bacon or slice of ham - people ate per day, and 18 per cent with every 50g of red meat, or the equivalent of a thick slice of roast beef or the edible bit of a lamb chop.
Further, each bottle of beer or small glass of wine raised bowel cancer risk by eight per cent.
In contrast, people in the highest fifth for fibre intake from bread and breakfast cereals had a 14 per cent lower risk of bowel cancer.
The researchers found no link between bowel cancer risk and fish, poultry, cheese, fruit, vegetables, tea and coffee.
Current evidence pointed 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 was 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," Bradbury said.
"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."
Key, who is the deputy director at the University of Oxford's cancer epidemiology unit, said the 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 the 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," Bradbury said.
"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.
"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."
Key was also the co-author of a major report last year that examined the consequences of a high meat diet.
That found that, although meat could be an important source of nutrients, high consumption could increase the risk of some types of diseases - particularly bowel cancer.
Coupled with environmental impacts, including greenhouse gas emissions, they suggested more work should be done to look at how best to convince people to change their diets and cut back on meat.
CANCER LINK 'NOT NEW'
Based on the last New Zealand adult national nutrition survey published 10 years ago, the average New Zealand adult consumed about 9.3g of lamb and 41.1g of beef each day.
Current working figures showed Kiwis were eating 17.2kg beef, 5kg of lamb and 0.7kg mutton per capita.
But industry data indicated a downward trend of red meat consumption in New Zealand over the last 10 years, with beef, lamb and mutton down 38 per cent, 45 per cent and 72 per cent respectively.
Responding to today's study, Beef + Lamb New Zealand's head of nutrition Fiona Greig said the association between red meat and cancer wasn't new.
Rather, she noted, the shift to a holistic lifestyle approach when it came to reducing cancer risk was emphasised by the World Cancer Research Fund (WCRF)'s most recent report.
That highlighted that overall dietary and exercise patterns were more important than individual foods or the components that make up those foods, which were favourable to reducing cancer risk, Grieg said.
"Further, the WCRF downgraded the evidence on red meat from convincing to probable, which highlights there are other factors at play for cancer risk," she said.
"In other words, rather than looking at meat in isolation, which it is not typically eaten by itself, it is the company that red meat keeps. Is it eaten with plenty of fibre-rich vegetables and wholegrains?
"Is alcohol, exercise and a healthy body weight part of the lifestyle? Given the many benefits red meat in a healthy diet provides, the authority on cancer recommendations recognises red meat does have a place in the diet, hence its current recommendation of up to 500g cooked per week."
She also pointed out that red meat was an important source of essential nutrients required for growth, brain development and general wellbeing.
The amount consumed should meet dietary goals as well as nutrient requirements, particularly in infants, toddlers and women of child-bearing age who are at risk of iron and zinc deficiency, Grieg said.
"Latest statistics indicate New Zealand has concerning rates of iron deficiency including eight out of 10 toddlers don't meet the recommended daily intake or iron, 14 per cent of children under two years are iron deficient, one in 14 women are low in iron and over a third of teenage girls don't achieve their daily iron requirements," she said.
"With red meat being one of the richest source of dietary iron, considerations need to be given to how the removal of red meat from the diet will have implications on the health status of New Zealanders, particularly for those who are most at risk."
