Comment by D-Machine

This. The amount of faith in nutrition "science" indicates severe science illiteracy in the public.

In general there are way too many confounds, and measurement is far too poor and unreliable (self-report that is wrong in quality and quantity; you can't track enough people for the amount of time where supposed effects would manifest), there is almost zero control over what people eat (diets and available foods even considerably over a decade for whole countries, never mind within individuals), and much of the things being measured lack even face/content validity in the first place (e.g. "fat" is not a valid category, and even "saturated vs. unsaturated" is a matter of degree, and each again with different kinds in each category).

We are missing so much of the basics of what are required for a real science here I think it is far more reasonable to view almost all long-term nutritional claims as pseudoscience, unless the effect is clear and massive (e.g. consumption of large amounts of alcohol, or extremely unique / restrictive diets that have strong effects, or the rare results of natural experiments / famines), or so extremely general that it catches a sort of primary factor (too much calories is generally harmful, regardless of the source of those calories).

Maybe it'll become actual science one day, but that won't be for decades.