Eat food

From the Herald, based on this paper

Dietary advice issued to tens of millions had warned that fat consumption should be strictly limited to cut the risk of heart disease and death.

But experts say the recommendations, which have been followed for the past 30 years, were not backed up by scientific evidence and should not have been issued.

Firstly, the “not  backed up by scientific evidence” actually means “not backed up by randomised trials”. When there’s a shortage of randomised trials on a topic it doesn’t mean there is no evidence. Randomised trials are ideal, but they are very hard to do usefully for effects of diet.  The same issue of the scientific journal has a useful commentary piece talking about the evidence and policy questions.

Second,  it’s true that there were real gaps in knowledge on the difference between types of fat back then. All fat isn’t the same, and neither is all saturated fat, or all polyunsaturated fat. Since I wasn’t in epidemiology back then, I don’t know how much this was a known unknown that should have led to more caution versus an unknown unknown.

Third, in the US at least, people didn’t really reduce their fat consumption as a result of the guidelines. For example, in a paper in the American Journal of Clinical Nutrition

In a comparison of NHANES 2005–2006 with NHANES I, men had a decreased absolute daily fat intake (by 20 ± 23 kcal, from 909 to 889 kcal), whereas women had an increased absolute daily fat intake (by 27 ± 14 kcal, from 577 to 605 kcal).

Fat intake as a proportion of calories decreased quite a lot, because calories went up, but absolute fat intake stayed fairly stable. Saying the recommendations ‘have been followed for the past 30 years’ is misleading.

Fourth, as this shows we don’t know a lot about how to make recommendations that translate to the right sort of behaviour changes. This is another area where there’s shortage of randomised trials. And of scientific evidence generally.

And finally, there was a good story by Martin Johnston in the Herald in December that gives more background on the issue. There’s genuine disagreement, but the establishment view isn’t what the caricatures suggest:

Professor Jackson reckons the Japanese and traditional Mediterranean diets offer insights. He says the balance of carbs and fats is probably unimportant as long as most fat is not saturated and most carb is the complex variety, not sugar and white flour-based refined carbs.