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When fasting endangers the life of a person it is a mitzva for that person to eat and drink as needed, since danger to life takes precedence over fasting (just as it does over other mitzvot). Indeed, those whose life may be in danger from fasting, but who decide to be “stringent” and fast anyway, are actually committing a sin.
The general rule is that doctors determine the level of danger. Unfortunately, some doctors mistakenly tell everyone sick to eat and drink minimal quantities on Yom Kippur (see section 12 below). Perhaps these doctors are overly cautious, or perhaps they do not take the mitzva of fasting seriously enough. In fact, even eating a small quantity on Yom Kippur is forbidden by the Torah. It is only the dangerously ill who are permitted to eat and drink (in which case it may be preferable for them to eat minimally).
Therefore, questions about fasting must be posed to a doctor who is ethical, has integrity, and will reach a decision responsibly, factoring in both the holiness of the fast and the holiness of human life. Doctors who meet these criteria can be relied upon, whether they are Jewish or not.
Nevertheless, if a sick person is convinced that he or she is in a life-threatening situation, they should eat and drink, even if the doctors disagree. This is because sometimes a person can sense the seriousness of their condition before the doctors can.
