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.