Bigu

The reasoning behind bigu rests on the doctrine of the sanshi, the three corpses or three worms, spirits believed to inhabit the upper, middle and lower parts of the body and to feed specifically on the grains a person eats. Left well fed, the three corpses were believed to ascend periodically to report a person's misdeeds to the celestial bureaucracy, shortening the lifespan allotted to them, and eventually to hasten death outright so that they could be freed to wander as independent spirits. Denying them grain was understood as a way to starve and weaken them, one technique among several, alongside specific rituals and talismans, aimed at expelling them from the body entirely.

This theory is set out in early medieval Daoist texts, most influentially in Ge Hong's fourth century Baopuzi, which treats bigu as one method among many, including alchemical elixirs, breath control and sexual cultivation techniques, all aimed at the same goal of physical transcendence and long life. Grain avoidance did not mean total fasting from the outset; adepts substituted specific herbs, tree resins, minerals and other supposedly purer substances thought not to nourish the three corpses, following recipes recorded across a substantial body of self-cultivation literature.

Bigu was understood as a graduated practice rather than a single step: an adept moved from an ordinary diet to one built around specific substituted foods, and from there, in the most extreme accounts, toward the ideal of subsisting on qi alone, breath and vital energy taken to replace food entirely. This end point was described more often as an aspiration credited to legendary immortals than as a literal daily practice most adepts expected to reach, and it sits at the far end of a spectrum that in ordinary practice looked more like an unusual, restrictive diet than a claim to require no food at all.

The practice is documented among the fangshi, the technique specialists and masters of esoteric methods active from the Han dynasty onward, whose repertoire fed directly into what became organized Daoist religion, and it remained part of the vocabulary of neidan, internal alchemy, in later centuries, where physical practices such as bigu were increasingly read as outer supports for an inner, meditative transformation of the body's energies. It survives today mainly as a historical category within the study of Daoist self-cultivation rather than as a living, widely kept discipline.