If a person is dangerously ill, but, according to the doctor’s instructions, need not eat and drink large quantities urgently, then according to several of the greatest Rishonim he should eat and drink less than the minimum punishable quantities (shi’urim) intermittently, to minimize the prohibition (as we will explain). Although eating or drinking even tiny amounts is prohibited by the Torah, ingesting a full shi’ur increases the gravity of the transgression: Doing so knowingly incurs the punishment of karet (extirpation) and doing so unknowingly obligates him to bring a sin offering. Therefore, ingesting less than a shi’ur is preferable.
However, if there is any concern that eating and drinking this way will cause at-risk patients to neglect the recovery of their strength, they must eat and drink normally. For example, if a postpartum woman is exhausted, it is better that she drink normally so that she can have uninterrupted sleep than that she stay awake to drink small quantities intermittently.
Likewise, diabetics whose condition has no stable treatment must be very cautious. If there is concern that eating and drinking le-shi’urim will lead them to be neglectful and not eat as much as they need, they should eat normally. It is also better that they pray in the synagogue with a minyan and eat substantial amounts every few hours, rather than eating minimal amounts over an extended period of time and thus be unable to come to synagogue.
Let us now explain the details of eating and drinking minimal quantities (le-shi’urim). For drinking, the minimum punishable quantity is a mouthful, that is, the interior of the mouth plus one cheek is filled with the liquid. This amount varies from person to person. Therefore, the patient must determine how much water fills his mouth by spitting a mouthful into a cup and marking where the water reaches. Le-khatḥila, he should do this before the fast begins. On the fast, he should drink less than this amount each time.
For solid food, the minimum punishable quantity is the volume of a large date (kotevet) – smaller than an egg but larger than an olive, it is approximately 30 cc or one ounce (SA 612:1-5, 9-10).
These shi’urim also contain a time component. That is, to be punishable, one must eat or drink the requisite shi’ur in the time it takes to eat a half a loaf of bread (akhilat pras). Some maintain that this is nine minutes, and le-khatḥila it is good to follow this opinion. One who must eat and drink more frequently may suffice with a seven-minute break. When it comes to drinking, one may even suffice with a break of one minute, because some maintain that for drinking this is enough of a break (SA 618:7-8). There is no halakhic difference between water and other liquids; therefore, if drinking le-shi’urim suffices for a patient, it is recommended that he drink high-calorie beverages, which may make it unnecessary for him to eat.
It seems that since the status of the dangerously ill person is known before Yom Kippur, and we can prepare in advance in an orderly fashion, there is no concern for negligence. On the other hand, it is possible that those Rishonim who do not mention eating le-shi’urim are concerned that precise instructions to minimize the prohibition would adversely affect people’s lifesaving efforts. This provides an answer to a question raised by R. Yaakov Ettlinger. He writes that we permit the sick to eat only precisely what is necessary; anything more than that is biblically prohibited. He then expresses surprise that the poskim do not mention this (Binyan Tziyon §34). We can answer that they do not mention it because in practice it is difficult to establish exactly how much a patient must eat or drink, so to avoid any possibility of endangering life, we permit someone dangerously ill to eat and drink as much as he needs. Hilkhot Ḥag Be-ḥag quotes R. Elyashiv as saying this (ch. 26 n. 33), and it is similar to the ruling that we take care of the sick on Shabbat exactly as we would on a weekday (SA 328:4; see Peninei Halakha: Shabbat, ch. 27 n. 4). It should be noted that there have been tragedies in which diabetics who needed to eat on Yom Kippur did not do so and died as a result. Since many diabetics can function normally, they come to synagogue, but there it is difficult for them to eat le-shi’urim. Some have become weak and, without realizing the danger they were in, ended up passing out and dying. When there is any shadow of a doubt concerning danger, diabetics should eat as usual and then go to the synagogue, as the value of eating le-shi’urim does not take precedence over the value of going to the synagogue.
. For more on “akhilat pras,” see Peninei Halakha: Pesaḥ 16:25 and Peninei Halakha: Berakhot 10:7. There are many opinions about its duration, ranging from 4 to 9 minutes. MB 618:21 says that on Yom Kippur we should be stringent in accordance with the opinion of Ḥatam Sofer and consider it 9 minutes. However, in times of need, one may be lenient and wait 7 minutes, which is longer than most opinions require. (In Peninei Halakha: Berakhot 10:7 I rule that eating a shi’ur within 7 minutes requires a berakha aḥarona.) In terms of drinking, Rambam uses a different criterion: the time it takes to drink a revi’it in a relaxed and continuous manner (MT, Laws of Resting on the Tenth 2:4). This is no longer than a minute. However, SA 618:8 states that le-khatḥila one should be stringent and follow Raavad, who equates drinking with eating. (Accordingly, one should wait 7 minutes.) In times of necessity, it is better to follow Rambam rather than drink more than shi’ur at once.