My dear teachers I have a question and I know you will help me because you have never let me down. I have no words to describe how helpful you have been to my English learning. Thank you ever so much. You are really nice people.

My question is "what is the difference between healthy and healthful?" and "which one is more common if there is no difference?"

Thanks again.

Jose Rocha

I am a native speaker and cannot think of one instance when healthful would be used.
It is in the dictionary so it is a word, but healthy is the word that is used.

Healthy is a state of being. A person or an animal can be healthy (or unhealthy.)

Healthful is a property of a substance. Eating vegetables and vitamins is healthful.

Most people use healthy in all instances, but food is never healthy (it's dead), only healthful.
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Well written Barbara!
Brilliant answer, consise and accurate. Thanks.
The only problem is that most of us, including many dictionaries, have no problem with 'healthy food', There are 259 COCA citations for that, and only 35 for 'healthful food'.

See also: http://www.macmillandictionary.com/dictionary/american/healthy
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With all due respect to this succinct and excellent explanation, "healthy" can mean generously sized. So, a salad can be "healthy" if it has to be served in a very large bowl. The word "healthy" doesn't refer to the nutrients provided by the salad, but to the fact that it's a generous serving.