As per allopathy, one cannot live without air or oxygen for more than 3 minutes, without water for more than three days and without food for more than 3 weeks. According to the Chandogya Upanishad, food is responsible for the making of motor organs (Karmendriyas), sensory organs, Manas (mind, intellect, memory and ego) and Prana. The fiery foods are responsible for making the motor indriyas, earthy foods for sensory indriyas and manas and water for making Prana Vayu. Therefore, it is possible for a person to live on water for up to few weeks because he will keep making Prana and keep breathing but absence of food on 14th day onwards will start affecting his Gnanaindriyas and Manas. The person will start losing power of hearing, touching and tactile sensation and will start showing impairment in mental status, memory, intellect functions and egoistic behavior.

(Disclaimer: The views expressed in this write up are my own).