I'm just gonna say it: epidemiological studies suck and we need something better. Humans and their lives are just too multi-variate, even with nearly ~500k people in a study. All you end up with are correlations – I've basically never seen a study like this that manages to demonstrate causality convincingly.
To infer causality you need interventions. Although there are some methods to infer causal relationships from observational data (e.g. see Pearl or Spirtes' work, which is ignored by most epidemiologists). I think the biggest problem is that they are not measuring low level molecular variables like metabolites, or abundances of gut bacteria. Hence, it's pretty hard to infer a causal chain. And since controlling for c…
Especially when the purported effects are small/clinically insignificant.