There are operations like 'Brand Power'[1] which masquerades like consumer fact checking service but in fact they just 3rd party advertisers for brands.
When I first saw their Ads in India, I thought it's disingenuous and then I learnt that they do the same even in developed countries.
[1] https://www.youtube.com/watch?v=d7N-ABnVPZk&t=13s (Tamil)
The internet happened, and now you can discuss things and get facts and opinions from sides you previously couldn’t, with not much more than a click.
I have spent a bunch of time on that site and read Dr. Greger's book "How Not to Die". He certainly works hard to understand how studies could be flawed, but he is still limited by the fact that really good underlying studies are hard to come by.
A few things stood out to me reading that book:
1) So, so many studies are hopelessly flawed.
2) Even ones that seem relatively good often have other studies showing the opposite result.
3) The evidence for a plant-based diet seemed fairly overwhelming, but some of the specific recommendations felt more questionable.
4) If you combine all of the claimed increases/decreases in disease risk for a given set of foods, you can fairly quickly get to a ridiculous result where you have something like a 1 in a billion chance of getting a relatively common ailment or are almost guaranteed to get 3 different cancers tomorrow.
To be a little fair, we don't really understand much about nutrition or have easy ways to do high quality studies. A "gold standard" study is just the best of a bad lot. It's the rarest of humans that can say, "I spent 3 years of my life doing a study and didn't find anything significant".
Put yourself in the shoes of someone who wants to study this, and you rapidly see the problems. Effects of nutrition take many years to show, so you're generally going to need to track people for years. People move, get bored of talking to you, die, etc. Their diet also varies greatly over time. They don't always remember what they ate, and certainly can't quantify it with any degree of accuracy.
If you perform some intervention, it's hard to know that the people actually complied with it. You can do it in the short term by having an in-patient setting, but again you miss the long term effects, which are really what matters to most people.
I feel like people reach too easily for conspiracy theories these days…
People are boxed into this "healthiness is a one-dimensional scale" thinking. Are eggs healthy or not healthy? Are potatoes healthy or not healthy? Is pizza healthy or not healthy?
But reality is way more complex and nutrition science knows very little about "healthiness" (to the extent that you can distill diet into a one-dimensional scale). Scientists only claim, "X is maybe correlated with Y (in Z population)".
So this simplification, plus the lack of definitive known truth, leads people to latch onto B.S. like "eating eggs is unhealthy because eggs have cholesterol".
Sure, there are food allergies - I have some myself - but this site seems like it’s trying to get me to buy the book and give the video views. I want information in text quickly. Then if I want to learn more, more information in longer text. This is how almost all valuable information is conveyed across all topic areas.
Plus different activity levels make different diets healthy - if you're a professional athlete needing 6,000+ calories per day, "healthy" for you is very different to healthy for me, who sits at their desk all day.
This isn't even accounting for personal preferences like being vegan, or religious restrictions.
There is no "healthy diet" that is universally applicable. Go back and look at the list of different possible restrictions in the parent post.
Then add the fact that many people are allergic to all kinds of different things.
Then add to that the fact that people of different ethnic backgrounds are adapted to do better with different balances of nutrients.
Any "hypothetical person" such as you describe would, fundamentally and inevitably, "normalize" one specific type of person and "other" everyone else. Claiming that "this is a healthy diet" in such a way would be grossly irresponsible, even if you then went on to list the features of a person it is "healthy" for.
Finally, even if you wanted to make such a description, the whole point of this thread is that we do not have a scientifically-rigorous-enough idea of what it would be to be able to do so with a high degree of certainty, even for a clearly-defined set of parameters for who would be eating it.
Because if we can’t answer this much much simpler problem then we have no hope of giving nutritional advice to a population that is as varied as you describe.
I strongly recommend going to https://nutritionfacts.org/ for your information -- a nonprofit that never sells anything or promotes any products, just tries to distill research on nutrition in easy-to-understand videos and posts.
Here's an example: "How the Egg Board Designs Misleading Studies" https://www.youtube.com/watch?v=2s25RaOZsFM