On Yom Kippur, if a person is sick and must eat, when possible he should have small amounts of food (shiurim). Since he is not consuming a satisfying quantity of food, he is not considered to have completely broken the fast. On other fast days Chazal did not obligate a sick person to consume smaller amounts of food.
Some halachic authorities apply this concept to eating before prayer. Even if a person is sick, if he can eat less than the amount that is considered a shiur (k’zayis), then he should eat the smaller amount (Rav Shlomo Zalman Auerbach as cited in Halichos Shlomo, Tefillah 2,1). However, since most authorities do not mention this stringency, we may conclude that the accepted custom permits a sick person to eat regularly without concern for the size of the portion (heard from Rav Moshe Sternbuch).
When a person must eat before tefillah, need he be concerned about the type of food he eats? A sick person can eat whatever will put him in the healthiest state so he can pray properly. He may even eat tasty foods, which in other situations would be considered haughtiness, since in this instance he is eating for health reasons (Mishnah Berurah 89,24).
Text Copyright © 2013 by Rabbi Daniel Travis and Torah.org