Bhagavad Gita Chapter 15, Text 14

Bg 15.14

aham vaisvanaro bhutva
praninam deham asritah
pacamy annam catur-vidham

Word for word: 
aham — I; vaisvanarah — My plenary portion as the digesting fire; bhutva — becoming; praninam — of all living entities; deham — in the bodies; asritah — situated; prana — the outgoing air; apana — the down-going air; samayuktah — keeping in balance; pacami — I digest; annam — foodstuff; catuh-vidham — the four kinds.

Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
I am the fire of digestion in the bodies of all living entities, and I join with the air of life, outgoing and incoming, to digest the four kinds of foodstuff.

Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
According to Ayur-vedic sastra, we understand that there is a fire in the stomach which digests all food sent there. When the fire is not blazing there is no hunger, and when the fire is in order we become hungry. Sometimes when the fire is not going nicely, treatment is required. In any case, this fire is representative of the Supreme Personality of Godhead. Vedic mantras (Brhad-aranyaka Upanisad 5.9.1) also confirm that the Supreme Lord or Brahman is situated in the form of fire within the stomach and is digesting all kinds of foodstuff (ayam agnir vaisvanaro yo ’yam antah puruse yenedam annam pacyate). Therefore since He is helping the digestion of all kinds of foodstuff, the living entity is not independent in the eating process. Unless the Supreme Lord helps him in digesting, there is no possibility of eating. He thus produces and digests foodstuff, and by His grace we are enjoying life. In the Vedanta-sutra (1.2.27) this is also confirmed. Sabdadibhyo ’ntah pratisthanac ca: the Lord is situated within sound and within the body, within the air and even within the stomach as the digestive force. There are four kinds of foodstuff – some are drunk, some are chewed, some are licked up, and some are sucked – and He is the digestive force for all of them.