They are not very knowable. It is expensive to design a study that would work. All too often a real world attempt to figure this out conclude "despite our best efforts we couldn't get people to behave in the needed way". So we have proxy studies that we hope mean something, but might not. Mixed in are lots of people making data fit their conclusion, and then selling it as fact.
replyFew people have the time to figure things out and so it isn't knowable even though all the steps are easy to lay out.
There are lots and lots of studies and metastudies etc. We know lots about nutrition and sports performance, and learn more and more
replyBut we produce a lot more made up stuff to generate views and sell products
You are also making the assumption that studies and metastudies are not trying to sell you something or that the authors aren't trying to prove something that the science doesn't support.
replyI don't think every study is completely corrupt. I wouldn't call it an assumption as much as an assessment that a lot of studies seem to agree with reality pretty well.
replyPragmatically I suspect this might be because most of the market don't actually read these things, and are just as happy with made up bs.