Learn Italian: In quanto nutrizionista, non farei mica questa dieta! - As a nutritionist, I wouldn't do this diet at all!

When using in quanto to mean "in the capacity of" or "as a," Italian drops the article before the noun, which is usually a profession or role. Adding un or una here would actually be grammatically incorrect.

Could I use come instead of in quanto here to say "As a nutritionist"?

Yes, you could say come nutrizionista (also without the article). Both express your role or capacity. However, in quanto is a bit more precise because come can also mean "like." Using in quanto leaves no doubt that the speaker actually is a nutritionist, essentially meaning "Because I am a nutritionist."

What exactly is mica doing in this sentence?

mica is a very common spoken adverb used to strongly emphasize a negative statement. It translates loosely to "at all," "by any means," or "hardly." When used with non, it reinforces the speaker's absolute rejection of the idea.

In English we say "go on a diet." Is fare una dieta the standard phrasing in Italian?

Yes, the standard collocation in Italian is fare una dieta (literally "to do a diet") or seguire una dieta ("to follow a diet"). You cannot translate "to go on a diet" word-for-word.