What the Bible says about Spiritual Heal
(From Forerunner Commentary)
The principle stated here is the key to improving one's physical health just as surely as it will improve one's spiritual health. The sense of the usage of "heart" here is synonymous with the English word "within," thus, what is within a person's mind is hidden from view when one looks from outside. Appearances can be very deceiving because what is within may be far different from what another can perceive. A person may appear smiling and jovial, but he may only be acting to achieve a scheme that ultimately profits him. Therefore, he is using others to achieve his end.
John W. Ritenbaugh
Eating: How Good It Is! (Part Five)