LONDON - Cutting back on red and processed meat brings few if any health benefits, according to a review of studies involving millions of people, a finding that contradicts dietary advice of leading international agencies and raised immediate objections from many health experts.

Most people can continue to eat red and processed meat at current average intake, typically three or four times a week for adults in North America and Europe, said a study's authors, who also made new recommendations based on the analysis.

"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, a group of doctors from Harvard, Yale, Stanford and elsewhere, including one of the study authors, requested in a letter to the journal that it "pre-emptively retract publication" of the papers pending further review, and said revised guidelines that could lead to increased consumption of red and processed meats would be irresponsible.

A statement scheduled for publication by the Harvard T.H. Chan School of Public Health, shared with Reuters by Dr. Frank Hu, chair of the Department of Nutrition, 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."

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

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 only "moderate amounts" of red meat, such as beef, pork and lamb - with an upper limit of 500 grams (17.6 ounces) cooked weight per week - and "little, if any" processed meat.

A panel of experts writing in The Lancet in January outlined an "ideal diet" for human health and the planet that said global average red meat consumption should be cut by 50 per cent and consumption of nuts, fruits, vegetables and legumes should double.

For the latest analysis, researchers from Canada, Spain and Poland conducted a series of reviews of both randomized controlled trials and observational studies looking at the possible health impact of eating red and processed meat.

Among the randomized 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 that this association "was very uncertain."

BEST APPROACH

"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.

Some experts not directly involved with the reviews 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 as they are usually not randomized controlled trials and often depend on participants' memories.

"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," Laine said.

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

Quadram's Johnson said people who choose to cut down their meat intake might still improve their health by doing so.

"There are (also) strong environmental and ethical arguments for reducing meat consumption in the modern world." 

Eating more plant-based foods can help to reduce emissions of carbon dioxide, scientists say.