Nutrition is just a really difficult field for robust studies. There is an enormous amount of variables you have to control for and the effects you're looking for are likely small or only appear after decades. Strict adherence to the experimental protocol usually can't be supervised, leaving you with observational studies or self-reported data. Blinding is pretty much impossible. Funding is very limited, especially if you don't want to take money from "powerful corporations".

As a side-project me and a friend have been trying to formalize a set of different rules and models from which to establish a diet. But, as the consumption of certain micronutrients drive the need for others up it's pretty hard to beat just eating a varied diet of mostly clean food.

Some people on here mention organ meats, but that does not mean that it's safe to eat liver everyday either.

To make matters worse, varying gut biomes and environments tend to yield different nutritional requirements. Amongst other things. I'm not sure we'll ever nail that down.

Again though, traditional medicines were aware of this and it was typically recognized that diet was specific to an individual and their environment. There were (and still are) complex systems outlining how foods may affect different people and what it could mean.

Still, I'm not claiming anything here other than that it's interesting and worth considering. I take advil when my head hurts and I go to the hospital when I'm sick, and I'm not going to start treating cancer with fungus from my back yard. I just suspect there's a lot of useful knowledge in this tradition.

"Clean food" is such a vague term as to be nearly meaningless.