Answer: Healthy vs. Healthful
neiths_arrow would like to know the difference between healthy and healthful.
In many cases nowadays, these two words can be used interchangeably. Some of the blame (or praise) for this situation can be laid at the feet of advertisers. It's not surprising, given the modern concern with proper diet, that the idea of foods that are good for one's health comes up often in commercials. These ads tend to use the word healthy in its third-choice definition from Webster's: "conducive to health."
However, the primary definition of this word is "enjoying health and vigor of body, mind, or spirit":
"Here you go, sir," said Willikins. "Porridge a healthy breakfast."
Sir Samuel Vimes looked dubiously at the bowl of gray goo. "Willikins, that's not my idea of breakfast. For one thing, it doesn't have any crunchy brown bits in it."
"Lady Sybil is trying to keep you healthy, sir."
"That takes fried eggs and streaky bacon, Willikins, not a bowlful of glue."
So where does that leave healthful, which is defined as "beneficial to health of body or mind"?
Actually, Vimes' butler could have said "healthful" instead:
"Here you go, sir," said Willikins. "Porridge a healthful breakfast."
In fact, you could make the argument that Willikins, who uses formal, slightly stuffy diction, would be more likely to use healthful in this case. It's the more educated and less casual choice.
Advertisers, however, aren't aiming to sound formal or educated, especially when they're trying to persuade a television audience to try a new snack product or frozen dinner. Neither, presumably, would Ankh-Morpork's most famous salesman:
"Step right up for a very, very special sausage-inna-bun, guaranteed no rat!" crowed Cut-Me-Own-Throat Dibbler. "You won't find a bite more healthy than this, nowhere!"
In summation, then: use healthful to mean "leading to good health" when your writing is formal or when you're writing dialog or viewpoint for a character who would be likely to choose words carefully. Use healthy for a more casual sound or when you mean the condition of someone's mind, body, or spirit.