(X) Doubt

Last time I was in a medical space, intermittent fasting was all the buzz. The idea that it somehow has opposition from mainstream medicine seems like the kind of thing that an internet community would invent to pretend at being brave rebels (and run defense if case they were caught getting frisky with unsubstantiated health claims.) If you search "intermittent fasting" on google, the results are full of very positive articles from all the big names in medicine:

    Johns Hopkins: Intermittent Fasting: What is it, and how does it work?
    Mayo Clinic: Intermittent fasting: What are the benefits?
    Harvard Health: Intermittent fasting: The positive news continues

So no, I don't buy for one second that "most doctors do not believe something like fasting can improve health."

Intermittent fasting is pretty much American invention with all the usual marketing.

There's a lot of vagueness in your last statement and I'm not sure what you're trying to imply.

"Most doctors" is an unspecific appeal to authority; which doctors can you cite that do not believe something like fasting can improve health?

"believe" was already mentioned by another commenter. An authority will make an assertion based on facts, not a belief based on ???

"something like" is it fasting or is it something else? Be specific.

"fasting" What kind of fasting do you mean? Between sunup and sundown like ramadan? Intermittent fasting? 30 day juice diet fasting?

"improve health" Improve how? And what aspect of health? "Health" is such a broad term that saying "this is good for your health" or "this food is healthy" are empty statements.

My point is that "auto brewery syndrome" is pretty well known at some type of community.

because believing is for religious people...

Believing something with 4000 years of tradition in different cultures, was validated in several studies is "religious".

Versus believing something that came out last year, and has only single study sponsored by manufacturer...

I'd say you're misinterpreting what belief implies here Tradition isn't proof, but also it doesn't harm credibility, at least to coherent people, and that's how it should be*

Eventually if there's enough proof, the consensus shifts, regardless of how much initially people try to deny it

Fasting goes against recent consensus, yet it's becoming both more researched and more recommended (At the very least at the layman level, I'm no dietician)

* Something working doesn't tell you why it works, so it's important to be rigorous and always check, else you risk cargo-culting unnecesary or even risky things

...and rational people when combined with justification and evidence.