Jīng (Chinese: 精; Wade–Giles: ching1) is the Chinese word for "essence", specifically kidney essence. Along with qì and shén, it is considered one of the Three Treasures (Sanbao 三寶) of traditional Chinese medicine or TCM.
According to Traditional Chinese Medical theory, Jīng or Essence can be summarised in two parts: the Yin, being congenital or prenatal, and the Yang, being postnatal or acquired. Prenatal Jing is acquired at birth from the parents: the father's sperm and the mother's ovum. This is a similar concept to DNA. Postnatal Jing is acquired after birth through food, water, oxygen, as well as environmental and social conditions—very much like the concept of epigenetics. The concept is expounded in the Taoist cosmological Bagua.
The Yin and Yang Jing transform to create and replenish each other. The Yang Jing circulates through the eight extraordinary vessels and transforms to become and replenish yin; in turn the marrow becomes blood, body fluid and semen.
Jīng (精; essence) should not be confused with the related concept of jìn (勁; power), nor with jīng (經; classic/warp), which appears in many early Chinese book titles, such as the Nèi Jīng, yì jīng and Chá Jīng, the fundamental text on all the knowledge associated with tea.
The characteristics which constitute signs of good Jing (e.g. facial structure, teeth, hair, strength of adrenals or kidneys) share the embryological origin of neural crest cells. These cells undergo immense and challenging cellular migrations requiring great organisation. As such, Jing may simply represent the strength of embryological self-organisation in the organism. This will be manifested most strongly in those cells which require most organisation; that is, the neural crest cells. 
One is said to be born with a fixed amount of jīng (prenatal jīng is sometimes called yuanqi) and also can acquire jīng from food and various forms of stimulation (exercise, study, meditation.)
Theoretically, jīng is consumed continuously in life; by everyday stress, illness, substance abuse, sexual intemperance, etc.
Prenatal jīng is very difficult to be renewed, and it is said it is completely consumed upon dying.
Jīng is therefore considered quite important for longevity in traditional Chinese medicine (TCM); many disciplines related to qìgōng are devoted to the replenishment of "lost" jīng by restoration of the post-natal jīng and transformation of Shen. In particular, the internal martial arts T'ai chi ch'uan, the Circle Walking of Baguazhang and the middle path of Wuxingheqidao may be used to preserve pre-natal jīng and build post-natal jīng, if performed correctly. Ginseng, particularly Korean and Chinese, is said to bolster the jīng.
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