Sri Aurobindo translates Aitareya Upanishad, Chapter 1, Section 3 (part 1): “The Spirit thought, ‘These verily are my worlds and their guardians; and now will I make me food for these. The Spirit brooded in might upon the waters and from the waters brooded mightily over, Form was born. Lo, all this that was born as form, is no other than Food. Food being created fled back from His grasp. By speech He would have seized it, but He could not seize it by speech. Had He seized it by speech, then would a man be satisfied by merely speaking food. By the breath He would have seized it, but He could not seize it by the breath. Had He seized it by the breath, then would a man be satisfied by merely breathing food. …”
There follows a series of other analogous statements as various elements of the being try to seize food. The Upanishad then continues: “By the apana He would have seized it, and it was seized. Lo, this is the seizer of food which is also Breath of the Life, and therefore all that is Breath hath its life in food.”
We tend to take the question of eating food for granted, as also the operation of the force of desire. The Rishis of the Upanishads, however, explored these issues as part of the larger question of the nature and significance of the manifestation and the operation of the universal forces that first involved the Infinite and Unmoving into the material worlds, and then began the process of evolving successively through various sheaths or layers of consciousness.
In a static and unchanging environment, neither food nor desire would be at all necessary. If however, change is to occur, then there must be some kind of mechanism or process involved in bringing about change. If it is meant to be organic change, not simply “miraculous creation”, then this process must involve the creation, maintenance and dissolution of the forms and forces that carry out that change.
Matter in the form of food undergoes changes through the process of being devoured and incorporated into new forms. The life-force in the form of apana, one of the 5 Pranas, acts as the immediate cause of these changes in the material world. Desire is the energetic motivating force that moves the pranas in the world, with the entire play of attraction and repulsion, gravitational force, and in living beings the subtler energies of emotional attraction or repulsion, liking and disliking, being the actual expressions of this force in various forms. Hunger is considered to be the most outward form of the working of desire, but in fact, it is operative at each successive level of matter, life, and mind. That is why the Upanishad apportions Hunger and Thirst to have their place in the actions of all the universal forces that operate in the world.
Sri Aurobindo, The Upanishads, Aitareya Upanishad, pp.285-294