It is a positive commandment to fast on Yom Kippur, as we read:
And this shall be to you a law for all time: In the seventh month, on the tenth day of the month, you shall deprive yourselves; and you shall do no manner of work, neither the citizen nor the stranger who resides among you. For on this day, atonement shall be made for you to purify you of all your sins; you shall purify yourselves before the Lord. It shall be a Shabbat of complete rest for you, and you shall deprive yourselves; it is a law for all time. (Vayikra 16:29-31)
One who does not fast fails to fulfill a positive commandment and violates a negative one. Even though the mitzva of inui (deprivation) is primarily about refraining from life-sustaining food and drink, it also includes four other prohibitions, discussed in the next chapter.
The mitzva is to refrain from eating even the tiniest amount and from drinking even a drop of water. One who eats or drinks in any amount violates a Torah prohibition. One who knowingly eats the volume of a large date (kotevet) or drinks a cheek-full of water is punishable by karet (extirpation), as we read, “Any person who does not deprive himself throughout that day shall be cut off from his kin” (Vayikra 23:29). If one transgresses unknowingly, he is liable to bring a sin offering. These punishments apply only if one eats or drinks an amount large enough to put his mind at ease, as then he is no longer considered to be experiencing inui. These shi’urim (minimum quantities, namely a cheek-full of drink and a kotevet of solid food) were received by Moshe at Sinai as the amounts that put one’s mind at ease. Still, one who eats or drinks anything at all transgresses a Torah prohibition.
One who eats something that is not generally considered edible – such as leaves, twigs, an extremely sharp spice, or rotten food – does not transgress a Torah prohibition, as this is not the normal way to “eat.” Similarly, one who drinks a non-potable beverage – such as an extremely bitter or rotten liquid – does not transgress a Torah prohibition (SA 612:6-8; Rema ad loc. 9). Nevertheless, all of the above are prohibited rabbinically. Since this person has chosen to eat or drink them during the fast, he has made it clear that from his perspective they are edible or potable. Therefore, it is forbidden for him to eat or drink them. Le-khatḥila one must even avoid consuming less than a shi’ur (MB ad loc. 15).
When it comes to all other prohibitions pertaining to eating, one who eats an olive’s bulk (kezayit) is liable, as a kezayit is the minimum quantity that is considered “eating.” However, on Yom Kippur the criterion is what amount puts one’s mind at ease and negates the state of inui. The Sages have a tradition passed down by Moshe that for food this amount is that of a kotevet, and a cheek-full for drink (Yoma 80a-b).
. Several Aḥaronim write that one who is fed via feeding tube due to a damaged esophagus may continue such feeding on Yom Kippur. They maintain that the prohibition applies only when someone gets pleasure from the taste and from the feeling of fullness that one obtains from food (Ḥelkat Ya’akov, OḤ 52; Nishmat Avraham 612:7 n. 2, based on Eglei Tal, Minḥat Ḥinukh, and others). But this is problematic because, in fact, this person does not experience inui, and the Torah does not directly command us not to eat or drink, but to experience inui. It seems, therefore, that one who is fed this way violates a rabbinic prohibition because he has negated the experience of inui, albeit in an irregular way (shinui). Maharsham (1:124) is of a similar opinion. Ḥatam Sofer OḤ 127 seems to say that one fed in this way violates a Torah prohibition as he does not experience inui. Aḥiezer 3:61 limits Ḥatam Sofer to cases where one derives pleasure from ingestion. Nevertheless, it seems to me that since the person ingests food in an irregular way, the prohibition is rabbinic.
Some claim that one may not take pills before Yom Kippur to ease the fast, as the mitzva is to experience inui, and doing this negates that experience (minority opinion in Sdei Ḥemed; R. Ḥayim David Halevy, Mayim Ḥayim 2:40). However, the leading view is the permissive one, since these pills have only a mild effect in creating a feeling of satiety. Their purpose is the same as eating before the fast, namely, to minimize the difficulty of the fast (majority opinion in Sdei Ḥemed; Ḥelkat Ya’akov 2:52; Tzitz Eliezer 7:32:4; Yabi’a Omer 9:54). Nevertheless, it would seem to be prohibited to take pills before Yom Kippur which make one feel especially good (such as strong painkillers which contain opioids), if the purpose is to alleviate the inui. However, one who must take such pills for medical reasons, to avoid severe pain, may take them before the fast and even on the fast (without water). Even though they alleviate some of his suffering, this is not his intention; this will be explained in the next section, in the discussion of caffeine pills.