A row has erupted over claims that official guidance to cut down on saturated fat had been based on shaky evidence.
In the 1970s and 80s, there was a big push away from saturated fats in the US and UK as experts cautioned they were bad for the heart.
Scientists writing in the journal Open Heart have now argued this advice lacked the solid scientific trials needed to back it up.
But many experts have come to the defence of the original guidelines.
Prof Christine Williams, at the University of Reading, says the claim that evidence was insufficient is "misguided and potentially dangerous".
And in recent years there has been more scientific data supporting the case that saturated fat is bad for health.