John Calvin Commentary Psalms 102:4

"My heart is smitten like grass, and withered; For I forget to eat my bread." — Psalms 102:4 (ASV)

My heart is smitten, and dried up like grass. Here he employs a third comparison, declaring that his heart is withered, and completely dried up like cut grass. But he intends to express something more than that his heart was withered, and his bones reduced to a state of dryness.

His language implies that as the grass, when it is cut down, can no longer receive juice from the earth, nor retain the life and vigor which it derived from the root, so his heart, being, as it were, torn and cut off from its root, was deprived of its natural nourishment.

The meaning of the last clause, I have forgotten to eat my bread, is that his sorrow has been so great that he has neglected his ordinary food. The Jews, it is true, during their captivity in Babylon, did eat their food; and it would have been evidence of their having fallen into sinful despair had they starved themselves to death.

But what he means to say is that he was so afflicted with sorrow as to refuse all delights and to deprive himself even of food and drink. True believers may, for a time, abstain from their ordinary food when, by voluntary fasting, they humbly beseech God to turn away His wrath, but the prophet does not here speak of that kind of abstinence from physical nourishment.

He speaks of an abstinence that is the effect of extreme mental distress, which is accompanied by a loathing of food and a weariness of all things. At the end of the verse, he adds that his body was, as it were, consuming or wasting away, so that his bones clung to his skin.