See, this is exactly what I'm talking about. I'm willing to bet that just like me, you're not an expert in that field. And there are people who are experts in the field, reviewing the literature, and have come up with a pretty good consensus.
In fact, I've recently done so owing to some recent health issues. As best I can tell there's almost no consensus among experts aside from "everything in moderation" and "eat a balanced diet" which isn't directed advice as much as hedging.
I have asked the various experts I've talked to why advice is so varied and their answer is that there is no consensus on many of these issues. Some had their own opinions about which parts might be right, others were clearly in the "it's a mess, just don't eat too much and wait and see" camp.
This is not a well developed field.
Particularly when consensus is not unanimous.
I'm basically regurgitating the Bad Food Bible book (written by Aaron Carroll, the guy who hosts HealthCareTriage).
https://nutritionfacts.org/video/gut-flora-obesity/
Sources are under the video.
The guy is fighting the good fight (against USDA and so on), but this is still the HN is hilariously bad at diet and related issues.
Recommending reading one guys opinion on it sorta proves the point.
And as for diet, the studies are out there. I'm no expert, but I listen to people who could credibly be argued are (as one example Dr. Greger who reads tons of studies and cites them in his videos), and typically their advice comes down to common wisdom that people just don't want to hear: eat food, not too much, mostly plants.
It's been this way for years. HN is really quite good when it comes to things in their wheelhouse (software, tech companies), but hilariously bad about diet and related issues, I suspect because it's a sore spot for many and they are in denial.