The Compendium of Bheḷa 4.4.2

4.4.2

athātra praśno bhavati | ko ' tra khalvasyāhāraṃ pacati vātaḥ pittaṃ śleṣmānupānaṃ veti | netyāha bhagavān punarvasurātreyaḥ | yadyete pākahetavaḥ syustarhi na kaścidiha durbalāgniḥ syādvātādīnāṃ sannihitatvāt , sānupānatvāc ca | athāsyoṣmā tejaś ca śarīrasthamāhāraṃ pacataḥ | te kāyāgnir iti[-agne] vidyāt ||

"Now, on this point a question arises: what indeed digests his food here—wind, bile, phlegm, or an after-drink?" "No," said the venerable Punarvasu Ātreya. "If these were the causes of digestion, then no one here would have weak fire, because wind and the others are present, and because of [the presence of] an after-drink. Rather, his bodily warmth and fiery energy digest the food dwelling in the body. One should know them as the body-fire."