Unfortunately for these studies you have multiple selection criteria that are nonrandom:
(1) interest in the study
(2) adherence to protocol of the study
(3) reporting back in
If nutrition science wants to be serious, their N should not be in the 10s but rather the 10,000s.
That has an expense, but for important things it is absolutely the right thing to do.
Plus it doesn't even matter what is true for the statistical average, given the infinite amount of variables and outcomes one glass of wine might be statistically beneficial but absolutely terrible for your own health because you have one specific gene combination or one specific microbiome mix. Which means you'd have to go through the same regimen of analysing and tracking all the parameters for yourself for it to be applicable
No one can monitor or measure everything, whether philosophically (Heisenberg uncertainty principle) or prosaically (cost). But if something is true, we can often probe it enough to get at least a low-res idea of the nature of it. This moves us light years ahead of primarily using our personal experience, gut, and vibe to establish epistemologically sound assertions.
It is true in the limit but not useful in practice.
When it comes to studying food / diet studies really do need to be a lot more careful about trying to control their variables.
Nutrition science as a field seems to have few absolute truths and many many overturned papers / results.