From: Angelic Scanlations:

A practitioner's Dan energy originates from the moon, the sun and the five elements. The human body has the five elements, since all living things do.

Regarding the issue if humans can train the Dan, there are two main views: One group thinks that using people will result in many difficulties, thus they only use elemental energy from external sources. They are called true practitioners. The other group thinks that, since the body is the supreme thing under the heavens, if they want to increase their strength exponentially, they should use other people's elemental energy. They are called the nefarious practitioners.

The true and nefarious practitioners' battle is now unfolding!!!