'Healthfully' (as in healthful) or 'healthily' (as in healthy) are both fine according to the OED. Personally I've never heard of 'healthful'. Looking around F

'Healthfully' (as in healthful) or 'healthily' (as in healthy) are both fine according to the OED. Personally I've never heard of 'healthful'. Looking around Fowler's Modern English Usage suggests preference for 'healthful' is a U.S. thing (an even there it seems people are unaware of it, judging by the comments at a U.S. dictionary site). Fowler's also notes 'healthful' is considered 'old-fashioned and literary' or 'formal' by some teaching dictionaries.