Fairy taboos around food are complicated and layered, and each aspect tends to have its own rules and repercussions. For this blog we will break the prohibitions around food down into three categories and try to summarize each one as concisely as possible.
1 - Eating Food from Fairy.
The most well known prohibition around food and fairies is certainly the rule not to eat fairy food. The general belief is that to eat the food of fairies is to be irreversibly bound to them and their world. We see a wide range of anecdotes centered on this idea, usually featuring a human who has encountered a group of fairies and been invited or inveigled to join them, been offered food or drink, and is then cautioned by a human among the group (often recognized as a recently deceased community member) not to take the offered meal. The warning always includes the explicit message that if the food or drink is accepted the person will not be able to leave and return to the mortal world or their family. In the ballad of 'Childe Rowland' the protagonist is advised to "bite no bit and drink no drop" when he goes to Fairy to rescue his sister if he wants to succeed and return again to earth with her. There are some exceptions to this, particularly in situations when the food is being offered by one of the monarchy of the Otherworld, but overall this is one of the most consistent prohibitions we find.
2 - Giving Food to Fairies.
There is a long standing and deep seated understanding that fairies were entitled to a portion of the human harvest, including both crops and animals. We see this beginning in Irish mythology where the Dagda negotiates an agreement with the Gaels to give the Gods - who have gone into the sidhe to live - a portion of all their grain and milk in exchange for the Gods allowing the crops to flourish and cows to be in milk. Over time this concept was extended and shifted to the fairies more generally. In the modern period we find examples in MacNeill's book 'Festival of Lughnasa' that discuss the fairies being given a tithe of the crops during the harvest, with an understanding that such a tithe is due to them. While this may not at first seem like a taboo it should be understood in the context of an action that had to be taken in order for humans to prosper.
3 - Fairies Claiming Food.
Related to point #2 is the idea that fairies will claim food they want, under different circumstances; this may be an extension of the idea that they are owed, by longstanding agreement, a portion of what humans harvest. Evans-Wentz relates anecdotes in 'The Fairy Faith in Celtic Countries' of the belief that if food fell or was dropped it was being claimed by the Good People and should be left to them. Along those lines Campbell in 'The Gaelic Otherworld' and Kirk in 'The Secret Commonwealth' both discuss the fairies removing the substance from food items, either in the fields or on the stove. This theft of the essence of food, rather than its physical presence, is attributed by Campbell to the owner of the item speaking badly of it. Another widespread folk belief in both Ireland and Scotland was that any berries left unpicked after Samhain belonged to the Good Folk and that eating them was unhealthy as they had been either spit on or urinated on by the púca, as a means of claiming them. Food that had been given to the fairies, or claimed by them, should not be eaten by humans as it was thought to have no value to it, although there are accounts of animals eating it. This falls into the area of a taboo as it was believed that taking what the fairies had claimed for themselves was at best very unlucky.