Cutting back on red and processed meat brings few if any health benefits, according to a review of evidence drawn from millions of people, but the finding contradicts dietary advice of international agencies and has prompted criticism from many experts.

The researchers who conducted the review said their findings suggest most people can eat red and processed meat at current average intake, typically three or four times a week for adults in North America and Europe, without significant health risks.

"Based on the research, we cannot say with any certainty that eating red or processed meat causes cancer, diabetes or heart disease," said Bradley Johnson, an associate professor at Dalhousie University in Canada who co-led the review published on Monday in the Annals of Internal Medicine journal.

However, in what amounts to a scientific food fight, experts from Harvard, Yale, Stanford and elsewhere, including one of the review authors, said guidelines that could lead people to eat more red and processed meats were irresponsible.

They asked in a letter to the journal that it "pre-emptively retract publication" of the papers pending further review.

A statement by the Harvard School of Public Health, shared with Reuters by Frank Hu, a doctor and chair of the nutrition department, said: "From a public health point of view, it is irresponsible and unethical to issue dietary guidelines that are tantamount to promoting meat consumption, even if there is still some uncertainty about the strength of the evidence."

The World Health Organisation's International Agency for Research on Cancer (IARC) and the World Cancer Research Fund (WCRF) both say red and processed meat may or can cause cancer.

The WCRF advises eating "little, if any" processed meat and only "moderate amounts" of red meat, such as beef, pork and lamb — with a weekly limit of 500 grams (17.6 ounces) cooked weight.

Giota Mitrou, the WCRF's director of research, said people should not misinterpret the review as saying meat is risk-free.

"The public could be put at risk if they interpret this new recommendation to mean we can continue eating as much red and processed meat as they like without increasing their risk of cancer," she said. "This is not the case."

"Grave concerns"

In the analysis published on Monday, researchers from Canada, Spain and Poland conducted a series of reviews of both randomised controlled trials and observational studies looking at the possible health impact of eating red and processed meat.

Among the randomised trials they selected for analysis, which included around 54,000 people, they found no statistically significant link between eating meat and the risk of heart disease, diabetes, or cancer.

Among the observational studies, which covered millions of people, they did find "a very small reduction in risk" in those who ate three fewer servings of red or processed meat a week, but said this association "was very uncertain".

"Our bottom line recommendation [...] is that for the majority of people, but not everyone, continuing their red and processed meat consumption is the best approach," Johnson said.

David Katz, director of the Yale-Griffin Prevention Research Center at the Yale University School of Medicine, said he had "grave concerns" about the potential of the new review "for damage to public understanding, and public health".

But other experts said the work was a comprehensive, well-conducted analysis of the available evidence on eating meat and human health.

"This study will, I hope, help to eliminate the incorrect impression [...] that some meat products are as carcinogenic as cigarette smoke, and to discourage dramatic media headlines claiming that bacon is killing us," said Ian Johnson, a nutrition expert at Britain's Quadram Institute of bioscience.

Christine Laine, editor in chief of Annals of Internal Medicine, noted that nutrition studies are challenging.

"To be honest with our patients and the public, we shouldn't be making recommendations that sound like they're based on solid evidence," she said.

"There may be lots of reasons to decrease meat in your diet, but if you're decreasing it to improve your health, we don't have a lot of strong evidence to support that."