The word 'healthful' is used in the only US and Canada. The British, Australians and New Zealanders don't use it at all. If you were to say 'healthful' most people in those countries, they'd think you were making a mistake with your English.

Healthy is used most often here in the US. Some people make this difference: Healthy is used to describe the condition: a healthy body, a healthy plant, a healthy business. Healthful is used to describe things that make someone healthy: healthful vegetables, healthful nutrients, healthful exercise. But really, most people just use healthy for everything.

Healthful is supposed to be the correct way of using the adjective. Vegetables are healthful for you but Many people use healthy instead Vegetables are healthy for you. In theory you can use either or, but healthy is much more common.