Since the body is physical and the soul is spiritual, there is a vast gap between them. Even the most refined limb in the body, the physical brain, which is a vessel for the most refined soul-faculty, the mind, is still physical, and so it cannot be compared to even the lowest level of the soul.
Since the body and the soul are so vastly different from each other, they need a force to combine them. This is the purpose of food—to bring the vitality of the soul to infuse the body with life, making one healthy. Thus, lack of food weakens a person’s vitality to the point that if he is deprived of food for a long time, he will die.
What gives food this power? It is written, “A man does not live on bread alone; rather, a man lives on that which comes out of the mouth of Havayeh.” “Mouth of Havayeh” refers to the divine vitality, the spark of G–dliness within the food, and the verse is telling us that it is this G–dly energy that truly sustains the person. We will discuss this process further in future posts, G–d-willing.
This is not to say that the body is not sustained by the nutrients and vitamins in the physical food, which it absorbs through the physical, chemical process of digestion. Rather, this is an external process that is merely a vessel for the deeper reality of the deeper, spiritual process by which the food is refined, which is primary.
 Sefer HaMa’amarim 5670, p. 335-336.
 Devarim 8:3.