Q. I was also wondering, one that can’t fast the complete day on Yom Kippur, but has an option of eating shiurim or small amounts from the morning on, and that will be enough for him to fast the whole day. Should he do that or better don’t eat at all, until he has to eat a complete meal later on during the day?
A. Igrois Moshe (4: 121), Shemiras Shabbos K’ (39: n. 69) quoting RS’Z Auerbach zt’l, Mikroei Kodesh (40) quoting RZ’P Frank zt’l and others maintain that it is better to begin eating shiurim early in the day. Others assert that it depends on the case. If the patient is already in a state of illness that carries some danger to life, it is better to begin eating shiurim early in the day. However, if the patient is not at all in life danger now, but if he fasts there may be a possibility, that he may intensify his illness to that degree, since now he is not in any real danger, he should avoid eating even shiurim, which carry a Biblical prohibition. After all, if his illness does increase, he can then eat as he needs.
Horav Shlomo Miller’s Shlit’a opinion is that in doubt, one should be machmir on pikuach nefesh and eat or drink shiurim to avoid a later danger.
Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit’a