Butter, for example, was declared a major health hazard a couple of decades ago. However, as evidence of the detrimental effect of certain plant oils came to light, the prevailing wisdom has changed. These days, butter is seen as the healthier alternative to margarine – a complete reversal of the belief held a few years ago.

This makes it rather difficult for health-conscious people to stay ahead of the dangers posed by common foodstuffs. The ongoing arguments about banting, paleo and keto diets versus plant-based diets are a perfect illustration of this. Is a high fat, meat-heavy diet good for us or bad for us? One side argues that it’s a recipe for heart attacks, while the other side maintains that the weight loss and energy people experience while eating these foods is proof of the health benefits.

Unfortunately, there is scientific evidence to support both claims, further muddying the waters. In fact, the evidence is strong enough on both sides (as are vested interests) that a number of lawsuits have been filed against professionals promoting one diet over another, with all parties claiming they are acting in the interests of the public.

The latest research findings have done nothing to clarify the situation. A new series of studies published in the journal Annals of Internal Medicine found that the health impact of cutting back on meat is either non-existent or small, and the evidence of any harm is so weak that it would be misleading to suggest people should avoid meat for health reasons – even processed meat.

These findings challenge the guidelines from just about every major national and international health group. The World Health Organisation’s International Agency for Research on Cancer (IARC) has for years recommended that people should cut back on processed meats to avoid certain types of cancer. The American Heart Association and the US government’s dietary guidelines panel have also long suggested curbing meat consumption for better health.

The new research, however, argues that previously published guidelines have been bad science. Focusing on the impact of red meat consumption on cancer, cardiovascular disease, and mortality, among other effects, as well as people’s values and preferences regarding red meat, the studies concentrated on what the researchers refer to as “the highest quality evidence” available, as opposed to the very broad range of research that has traditionally been used to underpin nutritional advice.

In the past, guidelines for whether or not humans should cut back on meat were based on a variety of research categories, including randomised trials, animal evidence, and case-control studies. The new study relied on a research-rating system called GRADE, or the Grading of Recommendations Assessment, Development, and Evaluation.

GRADE was developed for creating summaries of research evidence to help guide health decision-making, and is currently the most widely used tool for evaluating the quality of science, with more than 110 organisations endorsing the method. GRADE identified those studies that provided the most certain evidence available, eliminating things like animal studies, in order to deliver the results.

Unsurprisingly, the outcome of the research has engendered mixed reactions. Stanford meta-researcher John Ioannidis publically praised the studies, saying: “These papers provide a nice counterbalance to the current norm in nutritional epidemiology where scientists with strong advocacy tend to overstate their findings and ask for major public health overhauls even though the evidence is weak.” The US Physicians Committee for Responsible Medicine, however, filed a petition with the Federal Trade Commission, asking the researchers to “correct false statements” contained in the report, which is a “major disservice to public health”.

These conflicting viewpoints are unlikely to be reconciled any time soon, but a recent microbiological study may have found evidence that all sides of the argument are actually right. Tracking the effects of different foods on more than 1 000 people, including hundreds of identical twins, this research found that blood glucose responses to a standardised food could vary tremendously from the mean, suggesting that each individual may need personalised diets to limit blood sugar spikes. They got the same results for how quickly people cleared the fats from their blood after a meal. This applied even to identical twins, who metabolised the same food differently.

More research is already being conducted, but until some definitive answers are found, the healthiest diet available will remain a contentious issue.