There is a very technical distinction between these two that is slowly wearing away. In their strictest senses, healthy means “possessing good health” and healthful nmeans “conducive to good health,” but these words have become quite interchangeable.
Healthy once applied only to life forms and healthful to the conditions or variables that allowed good health to develop.
It is acceptable now to use phrases such as “healthy food,” which technically should be incorrect since most food is either already dead or just about to be and so it can hardly be described as “possessing good health.”
Bryan Garner has this to say about healthy replacing healthful:
Such a development would be unhealthful, since it would lead to a less healthy state of the language. Garner, Oxford, p. 167.