Fasting in a Healthy Person

The Yellow Emperor said: I wish to hear you explain why a person dies after seven days without eating. What is the reason for this? Bo Gao replied: Allow me to explain the cause. The circumference of the stomach is one foot five inches, its diameter five inches, its length two feet six inches, and it curves transversely, capable of holding three dou and five sheng of water and grain. Of this, grain usually remains at two dou, and when water reaches one dou and five sheng, it is full. The upper burner disperses and discharges the essence qi, sending forth those subtle, swift, and slippery substances, while the lower burner irrigates downward into the various intestinal organs. The circumference of the small intestine is two and a half inches, its diameter eight and one-third fen, its length three zhang and two feet, capable of holding two dou and four sheng of grain and six sheng and three he and a little over half a he of water. The circumference of the ileum is four inches, its diameter one and one-third inches, its length two zhang and one foot, capable of holding one dou of grain and seven and a half sheng of water. The circumference of the large intestine is eight inches, its diameter two and two-thirds inches, its length two feet and eight inches, capable of holding nine sheng and three he and one-eighth he of grain. The total length of the stomach and intestines is altogether five zhang, eight feet, and four inches, capable of holding nine dou, two sheng, and one he and a little over half a he of water and grain—this is the amount of water and grain that the stomach and intestines can contain. But in a healthy person, this is not the case: when the stomach is full, the intestines are empty; when the intestines are full, the stomach is empty. They alternate between fullness and emptiness, so that qi can move upward and downward, the five zang organs are stable, the blood vessels are harmonious and unobstructed, and the spirit can dwell within them. Therefore, what is called spirit is the essence qi of water and grain. Hence, within the stomach and intestines, there should remain two dou of grain and one dou and five sheng of water. Thus, a healthy person defecates twice daily, each time discharging two and a half sheng, for a total of five sheng per day. In seven days, five times seven makes three dou and five sheng, so the retained water and grain are completely exhausted. Therefore, a healthy person dies after seven days without eating or drinking, because the essence qi and fluids of water and grain have all been used up.