Those who refine their bodies need a lot of life energy, or what is known as blood energy or vitality. This is the energy stored within cells for their operation.
For example, in muscle cells, life energy is used for movement. Other cells use life energy for a host of other functions. Without life energy, the body will shit down.
It can be said that without life energy, there won't be life. So the body produces life energy naturally. But the life energy produced naturally in the body won't be enough for cultivation.
The body needs the life energy it produces to live. Those who cultivate the path of body refining can only use the excess life energy that the body produces.
There has to be a minimum amount of life energy in the body to maintain life. If cultivators want extra, they must get it from external sources.
