Q. My husband is unable to fast this Tisha Beav. Should he eat small amounts as one does during Yom Kippur?
A. Regarding eating less than the subscribed amounts (shiurim) for someone who is unable to fast on Tisha Beav. One of the differences between fasting on Tisha Beav and Yom Kippur is eating pachos m’keshiur, or less than the minimum amount. If fasting might endanger a person’s life, one is forbidden to fast. On Yom Kippur, if a small amount of food or beverage removes the danger as it is common, one should only eat very small amounts of food and beverage at one time.
Many Poskim maintain that concerning eating small quantities applies only to Yom Kippur and not to Tisha B’Av (Shulchan Aruch O.H. 554:6). A sick person is completely excluded from the mitzvah of fasting on Tisha Beav. Therefore, he is not required to try to consume less than the minimum amount. However, anyone who is eating on a fast day because of medical necessity should eat only enough for his essential needs and not eat in excess (see Shulchan Aruch 554:5).
However, Biur Halacha (ibid.) quotes Pischei Olam who maintains that someone eating on a fast day because of the danger created by a cholera epidemic should eat only small amounts (even other than Yom Kippur). Tzemach Tzedek (8-9),Maharam Shik (289) and others rule similarly.
Yet the accepted Halacha is that if someone is seriously ill, on Tisha Beav one does not need to eat shiurim. (Maharam Shik 290. Kaf Hachaim 554: 31, Avnei Ezer 540, Piskei Teshuvos 554 n.44 and others).
Horav Shlomo Miller’s Shlit’a opinion is similar.
Rabbi A. Bartfeld as advised by Horav Shlomo Miller and Horav Aharon Miller Shlit’a