Some time ago I discovered a health issue with me and it prompted me to spend a lot of time trying to understand how proper nutrition should look like.
I discovered that almost everything I have been taught, told by doctors or red on the internet about proper nutrition is lies. And those likes usually have intelligently sounding papers behind them that are not worth the paper they are printed on. And then repeated ad nauseam by thousands of health "experts" that do not seem to be really concerned about validity of any of it other than their personal life goals.
Like the lies that you should eat breakfast to start your day or multiple meals or that fats are bad for your health or that you should limit intake of salt as much as possible. Or that you have to avoid foods that have cholesterol in them. Or that whole grain bread is healthier than the one made from white flour (it is not). Or that you need to eat fiber to have healthy gut flora or to avoid constipation (I have mercilessly removed all carbs of any kind from my diet for over a month and my gut flora is perfectly ok and there is no constipation).
You know, all that stuff that we have been bombarded with from childhood.
I know all about correlation and causation (actually, I studied theoretical math and have a lot of experience with statistics. I have even given a lecture on logic to first year students once when I was asked to do it by my prof).
Let's just say, there is a lot more ways to make improper than proper inferences from your study. And that saying you know about causation absolutely does not mean the person who did the study actually understands it and took it into account properly. And also that a lot of people start with an opinion or a vested interest to show a particular result and then are trying hard to prove what they already "know".
In the end the problem is that all those people who make those studies are forced to crank out results or perish in their field. Conducting a study very carefully and honestly costs many times more effort than just getting it to a point where somebody will take and publish it.