The question of when a person is allowed to eat on Yom Kippur, and how much, arises every year, and to some extent each case has to be treated individually. In broad terms, however, we can note that halakhah requires us to balance two vital principles: insuring people’s general safety and observing Yom Kippur with the proper seriousness.

The first half of the statement cannot be stressed enough. The halakhah is very clear that as long as there is a viable medical reason indicating that a person needs to eat, that person should eat as much as is necessary to remove any possible danger to that person’s health. Viable medical reasons include both a doctor’s orders, even if the person who is ill claims that he does not need to eat, and the other way around, the ill person’s claim that he/she needs to eat, even if the doctor cannot identify a specific medical reason that would require that person to eat. (Of course, the sincerity of the person involved is assumed; if a person is ill and feels that without eating there is danger to his/her life, that person is allowed to eat as well.).

The requirement to protect oneself from harm led most posqim to rule that it is impermissible to accept a voluntary stringency upon oneself in this matter. That is, supposing that a doctor required a particular patient to eat on Yom Kippur, that person could not say, "well, I think I can fast." There were posqim who permitted such personal stringencies, on the theory that God protects those who put their trust in Him, but the majority reject this position.

I stress that the doctor whose advice we are following is discussing this particular patient, because there are some doctors who make blanket rules that are not empirically correct, such as that pregnant women may not fast on Yom Kippur. While there are specific pregnant women whose condition requires that they drink or eat on Yom Kippur, others can do just fine while fasting; it is important to get patient-specific advice from a doctor.

Along the same lines it is worth noting that a slew of authorities permitted not only those activities specifically required for saving the patient’s life, but also any activities that would simply help the patient feel better. In a famous story (in a Shabbat context), R. Hayyim of Brisk once ordered his son to adjust a light (or turn it on) in the room of a patient who was seriously ill simply because the doctor said it would not be a bad idea to do so. That principle presumably applies to Yom Kippur as well, although specific rulings would have to be applied to each situation.

I would also point out that in such situations, it is only permissible to have a non-Jew conduct these activities for the patient (thus allowing the healthy Jews attending that patient to observe the day fully) if we can be sure that the messenger understands and shares the feeling of urgency that we have. For example, the Tsits Eliezer only allows sending a non-Jew (or even a non-religious Jew) to a pharmacy to pick up medication for someone who is ill if we can be sure that the person fully understands the seriousness of assuring this person’s health. Otherwise, it would be incumbent, and part of the mitsvah of pikuah nefesh, for an observant Jew to do those activities him or herself.

On the other hand. What happens if the person has eaten or drunk as much as the doctor and the patient agree was necessary to stave off danger; at that point, can the person still eat, or must the person return to fasting? There are several approaches to the question that have been taken. The most obvious approach is to discuss whether Yom Kippur is hudhah or hutrah by the necessity of saving someone’s life.

Hudhah would mean that the prohibitions of Yom Kippur are still in full force, just that we were allowed to violate them for a certain reason. Hutrah would mean that, in terms of this person, the notion of Yom Kippur has ceased, at least in terms of eating, for that year. Since it is fairly clear (although there is a minority view that disagrees) that we see it as hudhah, it would mean that one is only allowed to eat as much as necessary to be certain there is no danger.

That view can be problematic, since it suggests that one has to be extremely careful about how much one eats; anything extra would violate Yom Kippur. There is, however, room for some leeway, since (as we just noted) one can do acts for an ill person that will simply make that person feel better even if not absolutely necessary to save the person's life. It can easily be argued, therefore, that if the person needed to worry that a bit of extra food would violate the day of Yom Kippur, that knowledge would get in the way of the healing. Under those grounds, the leeway to eat a little extra might exist.

Another issue that Tsits Eliezer points out is that, while the person is clearly not fasting any more (a fast can only be for a whole day), that person can clearly still have a sense of inui. Eating breakfast doesn’t keep me full for the entire day, and refraining from eating could create at least some sense of inui, and therefore might be incumbent upon the person. In fact, there was a report that the Hatam Sofer was of the opinion that someone who ate later in the day would be liable for karet, as if that person had never had any heter at all.

Again, by way of summary, it is important to keep in mind both aspects of this discussion in dealing with people who are ill. Of primary importance is insuring that fasting on Yom Kippur will not create even the possibility of danger to the person's life, and if it will, the person should not fast. On the other hand, people should not eat more than necessary to remove all doubt of danger, since that excess might be considered a violation of the Torah's requirements regarding Yom Kippur.