So... I think I have 3 points.
First is worst. Looking back over a few generations, standard nutritional advice informing doctors, institutions and the public have been poor. Not only inaccurate, but de facto pushing people down bad paths. Low fat & low cholesterol labels led to foods, with processed foods with high sugar & many additives. Cereal instead of a boiled egg.
Second is that many norms, like the food pyramid hanging in classrooms during the 80s & 90s were based on long forgotten, often arbitrary decisions. Self correction was fatally weak. Challenging norms had a higher burden for proof than the norm had faced. Proof is really hard, it turns out.
Third is that "what's a healthy diet?" was never answered better by science than book gurus or old cultural understandings of nutrition. That is, a practicable, science based understanding of nutrition wasn't achieved. Lots of things about nutrition have been learned but it doesn't really add up to a Big Theory.
I think the real issue here is "what to do when we bust." Science doesn't predictably produce answers to questions we think it should answer. We discover what we discover. But, what do you do when hospitals, health ministries and such need scientific guidelines? IDK is not an acceptable answer. I do know is not a scientific one. "Lets not do science " doesn't (didn't?) work either. The only choice left is between a lie and a delusion. Postmodernism is inevitable.