healthy: possessing or enjoying good health or a sound and vigorous mentality; a healthy body; a healthy mind.

That means a healthy apple describes the health of the apple - the term has nothing to do with how the apple affects our health.  Enter the term healthful which dictionary.com defines as:

healthful: conducive to health; wholesome or salutary: a healthful diet.

A healthful apple denotes that the apple has a positive affect on our health.

But nothing is simple with the English language.  If you read a bit further along in the dictionary.com definition of healthy you'll find the third definition to be:

healthy:  conducive to good health; healthful: healthy recreations.

Argh!  What does this mean?  (Other than the English language is confusing that is) 

Which is right?  Or perhaps the real question is which is most accurate?  Strictly speaking  healthy refers to the apple not to our health.  Yet, English is constantly evolving.  Ask 50 people what the terms "a healthy apple" vs "a healthful apple"  mean and your're likely to get a wide range of answers.   Perhaps context and the person's health awareness all play a part in how they perceive the statements. It seems the research community is going with 'healthful".  Now I admit the first time I heard the term healthful used in place of healthy I found it awkward.  I think the term is growing on me... but in either case it's food for thought!