Learn Italian: In quanto nutrizionista, sconsiglio questa dieta a chiunque. - As a nutritionist, I advise anyone against this diet.

When you use in quanto to mean "in the capacity of" or "as a [profession/role]", it must be followed directly by the noun without any article. You say in quanto nutrizionista, never in quanto un nutrizionista.

Could I use come instead of in quanto here?

Yes, you can absolutely say Come nutrizionista... and it means the exact same thing. Using in quanto is just a bit more formal and strongly emphasizes the authority or professional capacity of the speaker (literally "In my capacity as a nutritionist...").

Does the word nutrizionista tell us if the speaker is male or female?

No, it doesn't. Professions ending in -ista (like dentista, giornalista, nutrizionista) use the same ending for both masculine and feminine. Normally, the article would reveal the gender (il nutrizionista vs. la nutrizionista), but since in quanto requires you to drop the article, the speaker's gender is completely hidden in this sentence.

Why does chiunque get the preposition a if the English translation is "I advise anyone"?