For a regular human being, the sustaining or the energising mechanism is through food. It has been like that for generations and so, we are not aware of the alternative ways.

When the human body is properly prepared with its meridians activated, the mechanism of energising the physical body changes from gross food to the subtle energy or the prana or the cosmic energy.

A human body is not just what is visible (the physical). The physical is ruled by the subtle reality which is like the blue print that you can access to alter you mind-body. This subtle reality has energy meridians. These meridians connect to the physical body through glands. The entire chakra system and the three meridians ida, pingala and sushumna are part of the subtle body. Susuhumna is the major merdian which is of our primary concern. When one is able to feel/see this meridian and move prana through it, it gets activated. Upon its activation, it brings to life the entire subtle body. When the subtle body is strengthened enough, it obtains the cosmic energy and sustains the physical body. This eventually moves you into Beingness ~ The Source!