We are sustained by God's energy not by food alone:
Jesus testified that man shall not live by bread alone, but by every word that proceedeth out of the mouth of God. (Matthew 4:4.) This memorable passage signifies that man's body does not depend only on external sources of life force—distillations from; breath, oxygen, sunshine, solids, and liquids—but also on a direct inner source of cosmic life that enters the body through the medulla, flowing then to the subtle centers in the brain and spine.
Unlike the ordinary man, the yogi realizes that his life is not conditioned by exhalation and inhalation, but that the steady life force in the brain is continuously reinforced through the medulla from the omnipresent cosmic current.
The breathless yogi, however, realizes the state of conscious "death" as a far deeper and more blessed state than that bestowed by the deepest blissful semi-superconscious sleep. When breath ceases in the Kriya Yogi, he is suffused with an incomparable bliss. He realizes then that it is the storm of human breath that is responsible for the creation of the dream wave of the human body and its sensations; it is breath that causes body consciousness.
By experiencing the reality of the body as prana or lifetrons, controlled by the thought of God, the yogi becomes one with Him.
A man sees himself in a dream; the power of his mind creates the consciousness of a real physical body. Similarly, by materializing His thought, the Lord has made dream men walking about a dream creation in dream bodies of flesh. The body is nothing but a materialized dream of God.
The Lord surrounded man's soul first with an idea body. Then He encased the idea body with a very fine or subtle light (the astral body). The third or final encasement was the electroatomic dream body, the illusion of a fleshly form.
The reason, therefore, that the Gita advises devotees to practice the pranayama life-control technique is to enable them to realize that the body is made not of flesh, but of life force condensed from the thought of God.
.....the truth in the Bible— that man's body-battery can live by the Word or vibratory current coming from God. The life in the body is directly sustained by the cosmic prana flowing through the medulla. Through the operation of maya or delusion, however, man believes that he cannot live without food and other outward aids. He becomes erroneously dependent on the gross sources of energy reinforcement through oxygen, sunshine, solids, and liquids. Man has formed a bad mortal habit in feeling that he cannot exist without a supply of energy from these material substances. That is why, if oxygen, sunshine, solids, and liquids are denied to the body, man's frightened consciousness permits the life force to depart.
The Spiritual Eye: http://www.yogananda.com.au/