> we do know that diet and exercise work.

Stricly speaking we don't.

This is "common sense" and the official recommendation, but fundamentally we don't have solid long term reproductible experiments[0], and due to the nature of the problem (humans living their life in a complex society) we'll probably never have a good answer.

I've read many many studies spanning a few months and calling it a day (did the subjects rebound ? who knows), other taking a very small and homogenic pathological group, making it follow a strict regimen and end the experiment right after the subjects are let free again. But nothing with an actually rigorous protocol that gives a clear undisputable result.

In a way I feel a lot of researchers are bound to their common sense and think they either don't need to prove the obvious, or it brings them nothing to let the room for controversial results ? (nobody's paying for research that says current policy is dumb)

[0] If you have any double blind study with more than a hundred subjects taken randomly from the general population (including "healthy" subjects), with a control group, spanning more than 3 years of observations I'd be dying to read it..

If you think that's a high bar, obesity is touted to be the worst health crisis the US has to deal with with tremendous impacts, putting at least that much effort into research doesn't seem outlandish.