Over a hundred academics recently signed a letter to the British Journal of Sports Medicine and BMJ editors criticising their pro-butter stance and for allowing a “biased” editorial comment which claimed that the effect of saturated fats on cardiovascular health was grossly exaggerated and that we had been misled. A rebuttal followed. What should have happened was a sensible scientific debate. Instead, this has degenerated into a dispute over bias, integrity, and beliefs.

There is certainly a touch of religious wars about these debates, and much of this is because of who controls the sacred nutritional guidelines. The current guidelines in the US and UK are similar. Both have areas of common consensus that few disagree with, such as eating fewer calories, eating more plants and vegetables, eating less processed food, and drinking fewer sugary drinks. But with saturated fat, there is much less consensus on the evidence of the harm, and what we should reduce or replace it with.

The fat-diet-heart hypothesis is far from simple. The original idea that cholesterol in food was to blame for heart disease has been disproven, and was replaced by the total fat hypothesis, and then the saturated fat hypothesis. This is based on largely observational (and some genetic) data that LDL cholesterol in blood is correlated with heart disease and that these levels are slightly increased as dietary saturated fat increases. Observational studies in poorer countries in different environments (e.g.PURE study) have shown the opposite, with higher saturated fats (and dairy) associated with lower mortality. No study has successfully shown that changing to a low total or saturated fat diet can reduce heart disease or mortality, and large trials like the PREDIMED study using high fat nuts and olive oil have shown the opposite effects, though mainly via total fat.

Another problem is deciding what practical advice to give people, when many foods considered healthy contain some of these fats. The recommendation is that foods containing saturated fats should be replaced with either starchy foods or unsaturated fats. This means, for example, swapping butter for low fat spreads (the rebranded name for margarines). The desperate wish to have a simple message applicable to everybody—such as “ reduce all saturated fats” creates the problem. This ignores the complexity and quality of foods, dietary patterns, and individual food choices—and it totally neglects individual variation. Research thinking is shifting rapidly from food not merely being about macronutrients and calories, but about the hundreds of chemicals and metabolites that interact with each other and our bodies and our trillions of gut microbes (our microbiome)—which are unique for each of us. Meta-analyses of epidemiological studies by independent experts show that eating butter containing saturated fat does not on average cause harm. There are no long term data on eating modern spreads, although the early trans fat versions we were encouraged to eat were harmful for most of us.

Telling people simplistically to eat low saturated fat spreads instead of dairy may force people to consume cheap highly processed items with multiple additives and novel (interesterified) fats that we know little about. Some high quality vegetable-based spreads with minimal processing may actually be healthy, but globally people are confused by the messages and ignore these guidelines. Unilever, the market leader, recently sold their spread business, and sales of “natural” butter are increasing at the expense of “artificial” low fat spreads.

Two things need to change if we are to progress. First, demonising one major food group (or type of dietary fat) is a mistake. Foods contain a wide range of saturated, mono- and poly- unsaturated fatty acids in varying proportions, and the different fatty acids never exist in isolation, meaning fats in food can have contrasting good and bad effects on the different functionality of lipoprotein particles. Second, we need to say farewell, once and for all, to the idea of the “standard human.” We are more individual in our food responses than most of us believe. A recent large trial found an equal number of people responded well to a low fat high carb diet as to a low carb high fat diet for weight loss. So one size is highly unlikely to fit all.

It is dangerous to vilify doctors and scientists who criticise guidelines or question population approaches. In the last year, we have seen major clinical dogmas debunked by new data, such as aspirin for primary heart prevention, vitamin D supplementation for fracture, low salt diets for heart failure, and omega 3 supplements in diabetes. We need critics and debate more than we need outdated inflexible guidelines or eatwell plates.

Pass the butter—or the marg, (depending on what is good for you).

Tim Spector is a Professor of Genetic Epidemiology, King’s College London.

Competing interests: TS is a consultant to Zoe Global Ltd (involved in personalised nutrition) and author of “The Diet Myth: the science behind what we eat.” Orion 2016