To my ear, healthful does not sound right. This could well be geographical bias on my part. Is it now a valid alternative to healthy? Does it have another meaning?
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Healthy means "in a state of well-being."
Healthful means "promoting or contributing to one's healthiness." (Apparently it used to be a suitable synonym for "healthy" but I don't believe it is widely considered such anymore.)
So, to re-word the old adage, you could say that an apple a day is healthful; but you wouldn't say that you are healthful unless you're trying to convince a reluctant cannibal to consume you.
To my ear, "healthful" refers to something that promotes health, like a "healthful diet" or a "healthful climate," whereas someone who eats a healthful diet has a better chance of being "healthy." In everyday speech, we often hear "healthy" used (incorrectly?) in place of "healthful."