Definition of eating and drinking by “measures”

What does it mean to eat and drink “by measures” on Yom Kippur?

Eating and drinking “by measures” means eating quantities of food that do not satiate, or drinking quantities of liquid that do not quench thirst.

It says in the Torah about Yom Kippur (Leviticus 16:31): “and you shall afflict yourselves”. The Oral Torah infers from this that a person has to fast by abstaining from food and drink. (This is based on the fact that the word “affliction”, “עינוי”, and the word “fast”, “תענית” are derived from the same root.) The sages determined that the amount of food that prevents a person from being afflicted is “the size of a large date”, which is somewhat smaller than an average egg. As for drink, the amount is “a full cheek”, which is the amount of liquid that fills one side of the mouth.

On Yom Kippur, it is forbidden to eat and drink even less than these amounts. However, eating and drinking smaller amounts is a lesser prohibition than eating these amounts or more. Therefore, if someone’s medical condition forces him to eat or to drink on Yom Kippur, but his health does allow him to eat and drink in portions that are less than abovementioned amounts, then he has to eat and drink “by measures”, so as not to transgress the graver prohibition.

Shulchan Oruch, section Orach Chaim, chapter 612 and Mishnah Berurah ibid.