It is well established that one who is dangerously ill on Yom Kippur need not fast (see Yoma 82a). It is also well known that the dangerously sick individual should try to limit his eating and drinking to very small quantities of food on Yom Kippur. In this essay we will attempt to give the source for this practice, the scope of this practice, and review the dispute as to whether this rule applies to Tishah B'Av as well.
The Torah teaches that one who eats on Yom Kippur is punished with Kareit. This punishment takes effect, according to the Mishna (Yoma 73b), only if he has eaten the volume of food equivalent to the size of a large date ("kakotevet hagasa") or if he drinks an amount of liquid that could fill one's cheek ("maleh lugmov").
The Gemara (Yoma 74a) records a celebrated dispute between Rav Yochanan and Reish Lakesh regarding one who ate or drank less than these amounts ("chatzi shiur"). Reish Lakesh argues that although it is rabbinically forbidden to eat or drink a tiny amount of food on Yom Kippur, he has not violated a Torah prohibition since the Torah does not consider him as having "eaten," since he consumed such a small amount. On the other hand, Rav Yochanan agrees that one who eats or drinks a tiny amount violates a Torah prohibition. He reasons that since the tiny amount of food has the potential to be combined with more food ("chazi l'itztaroofi")he violates a Torah prohibition. According to Rav Yochanan, one who consumes these tiny amounts is not punished with Kareit, albeit he violates a Torah prohibition. The position of Rav Yochanan is accepted as normative Halacha (Rambam Hilchot Shevitat Assor 2:3).
Eating Less than the "Shiur"
The Gemara (Kritut 13a), though, mentions a circumstance in which one may eat less than the shiur. The Gemara writes that "the Rabbis permitted a pregnant woman to eat less than a 'shiur' due to the danger." The Gemara clarifies that, of course, if for emergency health considerations she must eat more than the "shiur," she is permitted to do so. It should be noted that the shiur must be consumed in intervals of less than the time it takes to consume half a loaf of bread ("k'dei achillat pras"). Acharonim debate as to how many minutes this refers to. The Chatam Sofer (Teshuvot Chatam Sofer 6:16) is cited by the Mishna Berurah (618:21) as ruling that "k'dei achillat pras" is the equivalent of nine minutes. The Aruch Hashulchan (O.C. 618:14) writes that it is approximately six to seven minutes. For a review of the opinions on this matter see Shmirat Shabbat K'hilchata (39:18).
The Behag (end of Hilchot Yom Hakippurim), the Ramban (in his work Torat Ha-Adam), and the Rosh (Yoma 8:13), extrapolate from the Gemara in Keritut 13a to any sick individual. They rule that every ill individual should try to limit their eating on Yom Kippur to less than a shiur. The reason for this is we want to avoid the ill individual from having to violate a transgression which is punishable by Kareit. The Rosh explains that only if the doctor states that eating less than a shiur will suffice will we require the patient to eat less than a "shiur." If the doctor believes that more than a "shiur" is required the individual should eat more than a "shiur." The Shulchan Aruch (618:7) rules in accordance with the view of the Behag, Ramban, and Rosh. See, however, the Netziv Haamek Sheila to Sheilta 167 who demonstrates that the Rif and Rambam do not agree with the ruling of the Behag, Ramban, Rosh, and Ran.
Rav Chaim's Understanding of this Rule
Rav Chaim Soloveitchik (cited by his son Rav Yitzchak Zev Soloveitchik in his work on the Rambam Hilchot Shvitat Asor 2:8 and by his grandson Rav Yosef Dov Soloveitchik, Halachic Man p. 39) greatly limited the applicability of this rule. Rav Chaim felt that it applied only to someone who is not dangerously ill but must eat on Yom Kippur to avoid becoming dangerously ill. Once, however, one has the status of a "choleh sheyesh bo sakanah" (dangerously ill person) he may eat on Yom Kippur without limiting his eating to less than a "shiur."
This is based on the ruling of the Magid Mishna (to Rambam Hilchot Shabbat 2:14) and Shulchan Aruch (O.C. 328:4) that one may violate Shabbat in order to take care of even the non-critical needs of a "choleh sheyesh bo sakanah." Similarly, explains Rav Yitzchak Zev Soloveitchik, once someone is considered dangerously ill to the extent that he must eat on Yom Kippur, all of his needs must be met in an unlimited manner. Thus he need not limit his eating to less than a shiur.
Criticism of Rav Chaim's Approach
Rav Shlomo Yosef Zevin (Moadim Bahalachah, p.28) notes that the general practice among rabbis is not to rule like Rav Chaim. The Aruch Hashulchan (O.C. 618:15) clearly supports Rav Zevin's assertion. Similarly, Rav Yehoshua Neuwirth (Shmirat Shabbat Kehilchata 39:6) and Rav Ovadia Yosef (Teshuvot Yechave Daat 6:39) do not rule in accordance with Rav Chaim. Indeed, Rabbi Moshe Snow (a Rebbe at the Rav Teitz Mesivta of Elizabeth who is a student of Rav Moshe Feinstein) informed this author that Rav Feinstein did not rule in accordance with Rav Chaim's approach. Dr. Avraham Avraham (Nishmat Avraham (O.C. 618:2) reports, however, that Rav Yechezkel Abramsky did rule in accordance with Rav Chaim's view. Those who disagree with Rav Chaim point to the plain straightforward reading of Shulchan Aruch 618:7, which states that a sick person should try to eat less than a "shiur". The Shulchan Aruch does not state that this rule applies only to one who is not dangerously ill. It seems that the Shulchan Aruch believes that this rule applies even to a "Choleh Sheyeish Bo Sakana".
Furthermore, the Mishna Berurah (328:14 and Beware Halacha ad. loc. s.v. kol) demonstrates that many Rishonim disagree with the aforementioned Maggid Mishna which the ruling of Rav Chaim is based on. The Mishna Berura concludes that when Torah prohibitions are involved, the Maggid Mishnah's ruling should not be followed. Hence, since eating on Yom Kippur is a Torah prohibition, it would appear that the Mishna Brura should be added to the list of those authorities who do not concur with the ruling of Rav Chaim. One should ask his Rav for guidance when this issue arises in practice. For a philosophical perspective on Rav Chaim's ruling see Rav Yosef Dov Soloveitchik, Halachic Man p. 39.
Application to Tish'ah B'av
Acharonim dispute as to whether this rule applies to Tish'ah B'av. The Mishna Brura (Beware Halacha 554:6 s.v. D'Bimakom Choli) rules that it does apply, but the Aruch HaShulchan (554:4) rules that it does not. It would seem, perhaps, that the Aruch Hashulchan's ruling is more persuasive. Only on Yom Kippur is there a reason to distinguish between eating less than a shiur and a full shiur. Although eating less than a shiur is biblically prohibited, one is not punished with Kareit by doing so. Therefore, it is halachically worthwhile to eat less than a shiur.
On the other hand, it seems that one accomplishes nothing by eating less than a shiur on Tish'ah B'av. It is rabbinically forbidden to eat on Tish'ah B'av and there is no distinction between eating more or less than a shiur.
One may, however, find a basis for the ruling of the Mishna Brura. The Gemara (Taanit 30b and Pesachim 52b) compares the rules of Tish'ah B'av with that of Yom Kippur. The Gemara states that a number of stringentcies that apply to Yom Kippur apply on Tish'ah B'av as well. This is because Chazal patterned Tish'ah B'av on Yom Kippur. Accordingly, it would be reasonable to say that the requirement to eat less than a shiur, if possible, should apply on Tish'ah B'Av as it does on Yom Kippur.
It is possible to suggest a compromise between the Mishna Berura and the Aruch Hashulchan. Perhaps only one who must eat on Tish'ah B'Av to avoid becoming dangerously ill, must strive to eat less than a "shiur". In other words, perhaps even those who do not accept Rav Chaim's ruling regarding Yom Kippur, may find it acceptable for Tish'ah B'Av. One should ask his Rav for guidance when such an issue arises.