Public health officials for years have urged Americans to limit consumption of red meat and processed meats because of concerns that these foods are linked to heart disease, cancer and other ills.

But on Monday, in a remarkable turnabout, an international collaboration of researchers produced a series of analyses concluding that the advice, a bedrock of almost all dietary guidelines, is not backed by good scientific evidence.

If there are health benefits from eating less beef and pork, they are small, the researchers concluded. Indeed, the advantages are so faint that they can be discerned only when looking at large populations, the scientists said, and are not sufficient to tell individuals to change their meat-eating habits.

'The certainty of evidence for these risk reductions was low to very low,' said Bradley Johnston, an epidemiologist at Dalhousie University in Canada and leader of the group publishing the new research in the Annals of Internal Medicine.

The new analyses are among the largest such evaluations ever attempted and may influence future dietary recommendations. In many ways, they raise uncomfortable questions about dietary advice, and what sort of standards these studies should be held to.

Already they have been met with fierce criticism by public health researchers. The American Heart Association, the American Cancer Society, the Harvard T.H. Chan School of Public Health and other groups have savaged the findings.

Some called for the journal's editors to delay publication altogether. Scientists at Harvard warned that the conclusions 'harm the credibility of nutrition science and erode public trust in scientific research'.

Physicians Committee for Responsible Medicine filed a petition against the journal with the Federal Trade Commission. Dr Frank Sacks, past chair of the American Heart Association's nutrition committee, called the research 'fatally flawed'.

While the new findings are likely to please proponents of popular high-protein diets, they seem certain to add to public consternation over dietary advice that seems to change every few years. The conclusions represent another in a series of jarring dietary reversals involving salt, fats, carbohydrates and more.

The prospect of a renewed appetite for red meat also runs counter to two other important trends: a growing awareness of the environmental degradation caused by livestock production, and longstanding concern about the welfare of animals employed in industrial farming.

Beef in particular is not just another foodstuff: it was a treasured symbol of postWorld War II prosperity, set firmly in the centre of America's dinner plate. But as concerns about its health effects have risen, consumption of beef has fallen steadily since the mid-1970s, largely replaced by poultry.

'Red meat used to be a symbol of high social class, but that's changing,' said Dr Frank Hu, chair of the nutrition department at the Harvard T.H. Chan School of Pub- lic Health in Boston. Today, the more highly educated Americans are, the less red meat they eat, he noted.

The new reports are based on three years of work by a group of 14 researchers in seven countries, along with three community representatives, directed by Dr Johnston. The investigators reported no conflicts of interest and did the studies without funding. In three reviews, the group looked at studies asking whether eating red meat or processed meats affected the risk of cardiovascular disease or cancer.

To assess deaths from any cause, the group reviewed 61 articles reporting on 55 populations, with more than 4 million participants. The researchers also looked at randomised trials linking red meat to cancer and heart disease (there are very few), as well as 73 articles that examined links between red meat and cancer incidence and mortality.

In each study, the scientists concluded that the links between eating red meat and disease and death were small, and the quality of the evidence was low to very low.

That is not to say that those links don't exist. But they are mostly in studies that observe groups of people, a weak form of evidence. Even then, the health effects of red meat consumption are detectable only in the largest groups, the team concluded, and an individual cannot conclude that he or she will be better off not eating red meat.

A fourth study asked why people like red meat, and whether they were interested in eating less to improve their health. If Americans were highly motivated by even modest heath hazards, then it might be worth continuing to advise them to eat less red meat. But the conclusion? The evidence even for this is weak, but the researchers found that 'omnivores are attached to meat and are unwilling to change this behaviour when faced with potentially undesirable health effects'.

Taken together, the analyses raise questions about the longstanding dietary guidelines urging people to eat less red meat, experts said.

'The guidelines are based on papers that presumably say there is evidence for what they say, and there isn't,' said Dr Dennis Bier, director of the Children's Nutrition Research Center at Baylor College of Medicine in Houston and past editor of the American Journal of Clinical Nutrition.