Yoma, Chapter Eight, Mishnah Five
As important as Yom Kippur may be, the value of life precedes the observance of all mitzvot. Our mishnah teaches that if fasting might cause danger to a persons life, that person must eat. I should note that the mishnah is not addressed to the person looking to cheat the system by feigning illness and eating on Yom Kippur. With God watching one cant really cheat the system. The mishnah is addressing the religious fanatic who might risk his/her own life in order to observe the fast. The mishnah is not allowing someone to eatit is forbidding them from fasting.
1) If a pregnant woman smelled [food on Yom Kippur], they feed her until she feels restored.
2) A sick person is fed at the word of experts.
a) And if no experts are there, they feed him upon his own request until he says: enough.
Section one: Pregnant women are obligated to fast on Yom Kippur, unless they are told by a doctor that they may not. Contrary to what some people seem to think, there is no blanket exemption. However, if a pregnant woman gets a craving to eat something because she smelled a certain food, they must feed her until she is satisfied. Her cravings are seen to be potential matters of life and death.
Section two: A sick person should check with a doctor/expert before he eats or drinks on Yom Kippur. If they say he must drink or eat than he must do so, even if he doesnt want to. The Talmud notes that if the experts say he does not need to eat or drink but he believes that if he doesnt his life might be endangered, he must eat. In other words, his own word trumps that of the experts if he wants to eat and they dont think it is necessary. However, if he doesnt want to eat and they think he should, he must eat. In this case, their words trump his.
If there are no experts around, then we take him at his word and we must feed him or give him water until he says that he has had enough.