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.