Bhagavad Gita 15.14

अहं वैश्वानरो भूत्वा प्राणिनां देहमाश्रित: |
प्राणापानसमाकार: पचाम्यन्नं चतुर्विधम्

Translation

It is I who take the form of the fire of digestion in the stomachs of all living beings, and combine with the incoming and outgoing breaths, to digest and assimilate the four kinds of foods.