Eat How?

I heard a phrase in a television commercial today that one hears often. The phrase is “eat healthy,” presumably meaning eat lots of fruits and vegetables, avoid saturated fats, etc. Walking back to the kitchen to clean up the dinner dishes, however, it suddenly occurred to me that this is an odd phrase. A check of the dictionary confirmed my intuition. “Healthy” is an adjective and needs to modify something, as it does in a phrase like “healthy food.” If one is told to “eat healthy,” the obvious question to ask is “healthy what?” Of course, what is really meant is something like “eat in a manner that promotes good health.” Our phrase need an adverb, not an adjective. (I do hope that no one will try to convince me that “healthy” is—or ever can be—a noun.) In other words, we should say “eat healthily.” That doesn’t sound very idiomatic, of course. “Eat healthfully,” which can mean the same thing, sounds a bit more natural, though still a trifle strange. Perhaps if we used a gramatically correct phrase more often, however, it would not sound so odd.