What are the guidelines if one must eat on Yom Kippur?
By Rabbi Chaim Hillel Raskin, Rov of Anash in Petach Tikvah.
The primary biblical prohibition against eating on Yom Kippur—as part of the required self-affliction—applies to eating food the size of a fig (koseves) or drinking a cheekful (m’lo lugmav), for these amounts put one’s mind at ease when hungry or thirsty, and the punishment for doing so, G-d forbid, is kares. Less than those amounts (chatzi shiur) is still biblically prohibited, but it doesn’t carry the punishment of kares.
The lesser punishment is indicative of lesser severity. Thus one who is medically required to eat on Yom Kippur is often advised by the Rav—if it is medically sufficient—to eat and drink in small amounts, for in this way they can still be considered afflicting themselves to some degree as long as they haven’t consumed the primary shiur. Obviously, if this doesn’t suffice to alleviate the danger, one must eat normally.
When eating on Yom Kippur, kiddush isn’t recited and there is no need for lechem mishneh, since Chazal never established these for this day. However, if one ate the shiur for birchas hamazon not in installments, yaaleh veyavo is recited with the words “b’yom hakipurim hazeh” (the same is true for children eating on the fast day).
The measure of koseves is 30 cc (slightly over 1 fl. oz.) of food and a cheekful is measured by each person individually—by filling their mouth with water and dividing that in half—ranging around 35-45 ml. Practically, liquid with concentrated nutrition (e.g. grape juice) is more effective in having the desired effect with a small amount. Food and drink do not combine with each other for the shiur unless it is vegetables or meat with their own juice.
If additional food or drink is necessary, and it is medically safe to wait, one should space out the intervals with the time of k’dei achilas pras—the amount of time it would take to eat a half-loaf of bread— between each session so that they don’t combine with each other. On Yom Kippur we are stringent to wait the longer estimate of nine minutes, but in dire situations one could rely on smaller estimates of 7, 6 or even 4 minutes.
A sick person who drinks should likewise wait k’dei achilas pras, but in dire situations one may rely on an those who say that the combining time is k’dei shtiyas riviyis—the amount of time it would take to drink a reviyis of liquid—which is two minutes, or in desperate situations one may rely on waiting approx. 15 seconds.
It is noteworthy that one of the primary medical issues related to fasts is dehydration. Thus, those who have a hard time fasting, amongst them nursing or pregnant women, should sit in a cool room. Poskim rule that a sick person who acts stringently when he is medically required to eat are liable for taking their life in their own hands.
One may not rely on these rulings without consulting their personal doctor and rav.
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