healthy diet = 4,390,000 hits
healthful diet = 144,000 hits
Traditionalists say that "healthy" means "in good health." They say that, if you mean "promoting good health," you need to use "healthful." That is, a healthy person eats a healthful diet.
That's no longer true. From Webster's New World College Dictionary:
1 having good health; well; sound
2 showing or resulting from good health -- "a healthy color"
This was on my mind today because the Los Angeles Times uses "healthful" this way most or all of the time. Or at least its Health section does. I'm sure that's a rule based on years of dealing with misinformed grammar-snob readers. Frankly, if I were them, I'd do the same. It's not worth the fight.
So I figured that, while a Google search showed a preference for "healthy diet," a Google News search was likely to show a preference for "healthful diet." I was wrong.
healthy diet = 735 hits
healthful diet = 28 hits