For years we’ve been told how terrible fat is for our health, but with different types out there, the story is never going be so black and white. New research has now shown how trans fats are linked to a greater risk of death and coronary heart disease, while there was no evidence that saturated fats are associated with risk of death, heart disease, stroke, or Type 2 diabetes. This might seem like the green light to eat as much butter as you’d like, but as always, it's not quite that simple.

Industrial trans fats are produced by hydrogenating plant oil – basically turning the oil from a liquid into a solid – and found in margarine and processed foods like cakes and biscuits. Saturated fats on the other hand are found in large quantities in butter, milk, meat, and egg.   

“For years everyone has been advised to cut out fats. Trans fats have no health benefits and pose a significant risk for heart disease, but the case for saturated fat is less clear,” said Russell de Souza, who led the study published in the British Medical Journal. The analysis reviewed 50 studies of dietary fat and cardiovascular health, in what’s called a meta-analysis. They found that consumption of industrial trans fats was associated with a 34% increase in overall mortality, and was specifically linked to an increase in coronary heart disease.

While this new research supports that trans fats are really harmful for your health, increasing your risk of death, it misses the point that few people still eat them. In the U.K., almost all trans fats have been removed from food, with the majority of people only eating half of the recommended daily limit. And in the U.S., the Food and Drug Administration has already committed to removing all artificial trans fats from the American food supply by 2018.

So while trans fats might be worse than saturated fats, we don’t eat much of them anyway, and it is for this reason that saturated fat is considered a greater risk simply because we eat far more of the stuff. This new study found that there was no evidence to link saturated fats with the previous claims that they harm your health, but according to Professor Jeremy Pearson from the British Heart Foundation (BHF), this is due to limitations in the data, rather than the fact that no link exists. And it certainly doesn’t mean we should be eating more of them, in fact the BHF recommends that you swap dietary saturated fat for polyunsaturated fat.

“This is a large piece of research in terms of analyzing as many data as they could get hold of, which is good, and they’ve done it very carefully, which is also very good. But they have limitations,” explained Pearson to IFLScience. “It’s not the gold standard for understanding whether anything is causally related to anything else, because you need a trial for that.” Despite this, he does praise the study, and the strong link it found between trans fats and health. He simply warns that just because they failed to find any evidence that saturated fat causes bad health, it doesn’t mean it doesn’t exist.