It is formal indeed, but "здоровая пища" is a set phrase, it reminds every Russian of the famous book "Книга о вкусной и здоровой пище" which was first published in the USSR in 1950s and has come out in lots of editions since then. Besides, any noun derived from a verb sounds a bit formal in Russian; therefore, no Russian would ever say, "мне нравится полезная едa"; instead, we would say something like "я люблю есть то, что полезно для здоровья". In an informal conversation (and in good literature too) the preference is always given to the verb over a related noun.