Fairy food consists principally of things intended for human food, of which the elves take the substance, fruit, or benefit, leaving the semblance or appearance to men themselves. In his manner they take cows, sheep, goats, meal, sowens, the produce of the land, … Cattle falling over rocks are particularly liable to being taken by them, and milk spilt in coming from the dairy is theirs by right.
They have, of food peculiar to themselves and not acquired from men, the root of silver weed (brisgein), the stalks of heather (cuiseagan an fhraoich), the milk of the red deer hinds and of goats, weeds gathered in the fields, and barleymeal. The brisgein is a root plentifully turned up by the plough in spring, and ranked in olden times as the `seventh bread'. Its inferior quality and its being found underground, are probably the cause of its being assigned to the Fairies.
One of the most important warnings a traveller to faery land must heed is to never eat the fairy food. To do so is to risk almost certain enchantment and entrapment in the lands of faery. That is a common element of fairy stories around the globe.