133:15 When feeding a pregnant woman, a woman who has given birth, or a sick person, one puts the food in front of them and says, “If you know that you might be in danger if you don’t eat as much as you need, then eat as you normally would until you feel satisfied. However, if it is possible for you not to eat the full quantity at once, then act as follows: Eat a volume of about two-thirds of an egg at a time.” This is because the quantity of food that makes one liable to the penalty of kareis (spiritual excision) on Yom Kippur is the size of a large dried date, which is a slightly smaller than the average egg without its shell. “Take a break, then eat again, so that the pause in the middle of eating makes the time between the end the first portion and the start of the second portion is at least as long as it takes to eat a piece of bread.” One may eat this way repeatedly, so long as the portions are separated by the time it takes to eat a piece of bread (“k’dei achilas pras” – a pras is the volume of four eggs – OC 618:7. This is in the vicinity of 8 or 9 ounces). If two portions are eaten within this time span, they are considered combined.
When it comes to drinking, one should drink slightly less than a cheekful, then wait at least k’dei achilas pras – or at the very least, the time it takes to drink a reviis (3.3 oz.). One should measure these times before Yom Kippur so that he will know in advance how long to wait between portions. (The time of k’dei achilas pras is generally accepted as nine minutes. A cheekful of liquid, however, is a subjective measure and should be determined in advance of the fast. One need not wait at all in between eating and drinking because they do not combine to form requisite-sized portions – MB 618:21.)
133:16 The sign that a person has become overwhelmed by a craving for food and ill from his hunger is that his eyes become dim and he cannot see. If this happens, he is fed until his eyes become bright again.