In my opinion, observational studies are not science. Or at least not the science that they think they are studying. They are more along the lines of psychology and sociology than biology or nutrition.
What observational studies are is a starting point for the real science. Eating X or doing Y is associated with outcome Z. The first thing to ask is “is this plausible?” i.e. does it make sense from an evolutionary biological viewpoint that the behavior could cause the effect? For example, being a basketball player is positively associated with height. So does playing basketball cause people to be tall? If you can come up with a biological reason for that then the real science would be to take a group of people, assign them to basketball playing and non-basketball playing groups and see if one group gets taller than the other. I’m going to guess that the answer will be no, playing basketball does not make you taller and that the association is backwards. Taller people tend to play more basketball. But i don’t know which is true merely from the observation that basketball players tend to be tall. Or if there is any causal relationship at all.
Great summary @richard. Confirmation bias is a problem for everyone.