Shulchan Arukh, Orach Chayim 618:5

דין חולה ביום הכפורים ובו י סעיפים
חולה שצריך לאכול אם יש שם רופא בקי אפי' הוא עכו"ם שאומר אם לא יאכילו אותו אפשר שיכבד עליו החולי ויסתכן מאכילין אותו על פיו ואין צריך לומר שמא ימות אפי' אם החולה אומר אינו צריך שומעים לרופא ואם החולה אומר צריך אני אפילו מאה רופאים אומרים אינו צריך שומעים לחולה: “The law concerning one who is sick on Yom Kippur” - Containing ten paragraphs.
A sick person who needs to eat, if there is an expert doctor there, even if he is a non-Jew,^125aThe following is a comment given by Magen Avraham, (see footnote 33.): 618:1 - "Even if he is a Gentile": The Rosh, הרא״ש, (see footnote 62) wrote in the name of Riba, ריב״א, (Riba are the initials of Rabbi Isaac Ben Asher who was also known as Isaac ben Asher ha-Levi. He lived in the second half of the eleventh and the first half of the twelfth century. He was a talmudist of Speyer and the first of the German tosafists. He was a pupil of Rashi. He was well respected by his contemporaries and was consulted on many halakhic questions. His death came as the result of chosing to fast on Yom Kippur even though he was warned that if he did so he would die. He compiled tosafot to most of the tractates of the Talmud and his halakhic decisions are quoted in later halakhic works; Shlomoh Zalman Havlin, E. J., v. 9, pp. 16-17), that a Gentile or a woman are believed to effectively contradict a Jew (in this situation). who says^125bThe following comment is given by Magen Avraham, (see footnote 33.): 618:2 - "Who says": Even if he does not say that he will endanger himself, only that he says that it is possible that the illness will worsen on him, we give him (food) since we fear that perhaps he will endanger himself (by not eating), (Bayit Ḥadash, ב״ח, see footnote 20b.; Maharash, מהר״ש, Morenu ha-Rav Rabenu Shimshon of Chinon (France); Levush, לבוש, see footnote 20a., and Maharil, מהרי״ל, see footnote 8.). that if this person is not fed it is possible that the illness will worsen on him and he will be in danger, they feed him on the doctor’s orders, and he does not have to say that he does not need (the food) we still listen to the doctor, but if the sick person says, “I need (the nourishment),” even if one hundred doctors say that he does not need (the nourishment) we listen to the ill person.

רופא אחד אומר צריך ורופא אחד אומר אינו צריך מאכילין אותו: הגה והוא הדין לשנים נגד שנים ואפילו קצתן יותר בקיאים מקצתן כנ"ל: One doctor says that the sick person needs (food) and another doctor says that he does not need (the food), we feed him (the sick person).
Hagah: The same holds true if two (doctors) disagree with two others even if some of them are better experts than the others (you listen to those who say the patient should eat even if they are the inferior doctors who say this) and so it seems to me (to be the correct decision).

אם החולה ורופא אחד עמו אומרים שאינו צריך ורופא (אחר) אומר צריך או שהחולה אינו אומר כלום ורופא אחד אומר צריך ושנים אומרים אינו צריך אין מאכילין אותו: If the sick person and one doctor with him say that he (the sick person) does not need (food), and another doctor says he does need (food), or if the sick person says nothing and one doctor says he needs (food) and two say he does not need (the nourishment), we do not feed him.

אם שנים אומרים צריך אפי' מאה אומרים אינו צריך ואפי' החולה אומר עמהם שאינו צריך מאכילים אותו מאחר ששנים אומרים צריך: הגה וה"ה אם החולה ורופא א' עמו אומרים צריך אע"פ שמאה רופאים אומרים אינו צריך מאכילין אותו (טור) ולא חיישינן דהחולה אומר צריך משום דמאמין לרופא זה שאומר צריך: (ב"י בשם מהרי"א): If two (doctors) say that he (the sick person) needs (food) and even if one hundred (doctors) say that he does not need (food) even if the sick person agrees with them (who say) he does not need (the food) we feed him since two say that he does need (the food).¹²⁶According to Judaism, two witnesses in any situation are legally just as weighty as one-hundred witnesses, and therefore two witnesses decide the issue as is the case in this instance. In this situation when two say yes, feed him and one hundred say no, do not feed him, the one hundred are legally considered the same as the two. Two against two in this case decides the issue for leniency in that the man should be fed so as not to endanger the health of the sick man. (See B. Yoma 83a.).
Hagah: The same holds true (if the sick person and one doctor with him say he needs (the food) even though one hundred doctors say he does not need (the food) we feed him.¹²⁷ibid., for the same reason when the sick person himself wants the food and counts as the two against the one hundred who are legally considered as two, and two decides the issue to feed the man. (טור)¹²⁸Tur, טור; see footnote 23.), and we are not afraid that the sick person says that he needs (food) because he believes this doctor who says that he needs (the nourishment),¹²⁹In matters of sickness, the statement of the sick person is taken as a sincere and honest statement and not merely as the echo of the doctor's words who the sick person probably respects. Therefore the sick person's word counts as heavily as the doctor's when the man asks to be fed on Yom Kippur. (See B. Yoma, loc. cit.). (ב״י בשם מהרי״א).¹³⁰Beit Yosef in the name of Moreiynu ha-Rav Rabbi Israel Isserlein, ב״י בשם מהרי״א.
For the Beit Yosef, בית יוסף; see footnote 20.
For Israel Isserlein, מהרי״א, see the footnote on Terumat ha-Deshen, footnote 96.

אם הרופא אומר שאינו מכיר החולי הרי הוא כאדם דעלמא ואין דבריו מעלין ולא מורידין: הגה מיהו אם נחלש הרבה עד שנראה לרוב בני אדם שאצלו שהוא מסוכן אם לא יאכל מאכילין אותו (א"ו הארוך): If the doctor says that he is not familiar with the disease, behold he is considered the same as a layman (a non-professional) and his stated opinion does not raise or lower (does not mean anything).
Hagah: However if he (the sick person) becomes very weak so that he appears to the majority of people with him that he is in danger, if he will not eat, we feed him, (א״ז הארוך).¹³¹Issur ve-Hetter he-Arokh was published in Ferrara in 1555. Its authorship has been attributed to Jonah ben Abraham Gerondi. It contains laws on forbidden foods which is the meaning of the title, but it also has laws which are connected with the duty of saving lives.
The term issur ve-hetter designates all halakhic rulings on forbidden foods and topics related to that. From the second half of the twelfth century onward it came to specify a literary genre dealing with the subject of forbidden foods. Many books dealing with the subject were produced. As Jews moved and settled in Germany, different customs arose which had not been dealt with before and this produced a need, especially relating to food, as to what was and what was not permitted to Jews.
Many of the issur ve-hetter books are anonymous and many glosses and notes have been added to the original texts to make them relevant to new customs. Some of the best known works in this category are: Sefer ha-She'arim or Sha'arei Dura, called "Issur ve-Hetter" by Isaac b. Meir of Dueren; thirty-six She'arim on laws of issur ve-hetter by Israel Isserlein (see footnote 96) along with his glosses to Sha'arei Dura; the laws of issur ve-hetter at the end of Minhagei Maharil (see footnote 8) which is an abridgment of the Sha'arei Dura as is the Torat Ḥattat by Moses Isserles, and the most well known, the Issur ve-Hetter he-Arokh.
Jonah Gerondi was born around 1200 and died in 1263. He was a Spanish rabbi, author and moralist. He studied in France where he learned from his teacher Solomon b. Abraham of Montpellier who had an intense hatred for the philosophical works of Maimonides. He helped burn the books of Maimonides but repented later when Talmud books were burned in the same square a few years later. He saw this as divine retribution.
Jonah was influenced by Kabbalah and he later returned to his home town of Gerona in Spain where he publically preached his doctrine of ethics and morality, torat ha-musar. He finally settled in Barcelona and became a very popular teacher. He established a large yeshivah in Toledo and died there.
He was known as a scholar involved with piety, humility, and ascetism. He wrote on ethics and the bad state of Spanish Jewry which was ignoring the mitzvot especially in matters of sexuality. He inspired public responsibility for each person for his fellow man. He based his ideas and decisions on the halakhah and popular aggadah. He wrote ethical works on social justice and social ethics. He was given credit for writing the Issur ve-Hetter he-Arokh and the laws of the saving of life attached to it.
Israel Moses Ta-Shma, E. J., v. 9, pp. 1085-86; Ephraim Kupfer, E. J., v. 10, pp. 179-80.

כשמאכילין את העוברות או את החולה מאכילין אותם מעט מעט כדי שלא יצטרף לשיעור הילכך מאכילין אותו כשני שלישי ביצה בינונית וישהו כדי אכילת ארבעה ביצים והשתיה יבדקו בחולה עצמו כמה היא כדי שיסלקנו לצד אחד ויראה כמלא לוגמיו: When one feeds the pregnant woman or the sick person, we feed that person little by little so that it (the food) will not combine to the prescribed amount of food which is prohibited to eat, therefore we feed him (the equivalent of) two-thirds of a medium (sized) egg and (then) wait as long as the time that is required for the eating of four eggs, and as to the drink, they should examine the sick person himself how much is the liquid so that he can, when it is moved to one side (of his mouth), appear as if he had a mouthful.¹³²See the Shulḥan Arukh, Oraḥ Ḥayyim, chapter 612, all, above, for the rules that apply in this case.

וישקוהו פחות מאותו שיעור וישהו בין שתיה לשתיה כדי אכילת ארבעה ביצים ולפחו' ישהו בין שתיה לשתיה כדי שיעור שתיית רביעית ואם אמדוהו שאין השיעורים הללו מספיקים לו או שהחולה אומר כן או שנסתפקו בדבר מאכילים ומשקים אותו כל צרכו (מיד): One should give him to drink (the above amount¹³³The important element here is the amount of time that passes while eating or drinking on Yom Kippur. "The above amount" refers to the time measurement. It should take longer than the time required to consume four eggs. This applies to both eating and drinking. Food or drink on Yom Kippur must take longer to consume than one would normally take to ingest them. If one must eat on Yom Kippur, then one must eat and drink slowly so that the transgression which results is a minor transgression and not a more serious one. The prohibition of eating and drinking is within a certain time period. A transgression does result, but not as severe a one as would result from eating quickly or even normally. The amount of time one must wait between one drink and the next is at least as long as it takes to drink two revi'it, the amount of one revi'it being one and one-half egg. Therefore the least amount of time one must wait between drinks is the length of time it takes to drink the equivalent of the contents of three (medium) eggs.), and they should wait between one drink and the next one as long as the time that it would take to eat four eggs. And if he (the doctor) estimated that these amounts are not enough for him (the sick person), or if the sick person says thus, or they are doubtful about the matter, we give him food and drink according to his need (immediately).

מי שאחזו בולמוס והוא חולי שבא מחמת רעבון וסימנו שעיניו כהות ואינו יכול לראות מאכילין אותו עד שיאירו עיניו ואם אין שם מאכל של היתר מאכילין אותו מאכל איסור ואם יש כאן שני מיני איסורים אחד חמור מחבירו מאכילין אותו הקל תחלה: הגה אם צריך לבשר ויש כאן בהמה שצריכין לשוחטה ובשר נבילה מוכנת ע' לעיל סי' שכ"ח סעיף י"ד: He who catches ravenous hunger and he (has) a disease that comes from hunger, the symptoms of it are that his eyes are dark and he is not able to see, we feed him until his eyes light up. If there is no permitted food (Kosher¹³⁴Kosher food is food which is ritually pure, and which a Jew is permitted to eat. Basically this restricts a Jew to meat from an animal with split-hoofs and which chews its cud, a fish with scales and fins, and certain fowl and locusts, (Leviticus II, Deuteronomy 14). The animals must be killed in a specially prescribed way and be prepared so that all the blood is removed. (See Shulḥan Arukh, Yoreh De'ah I-III, including other relevant laws).) there then we feed him with food that is forbidden (non-Kosher). If there are there two types of forbidden foods, one more strictly prohibited than the other, we feed him the lesser prohibited one first.¹³⁵An example of this might be a selection between beef that was not slaughtered according to Jewish law or a cut of beef not permitted and a piece of pork. The pork is more strictly forbidden and therefore of the available choices, the non-Kosher beef would be used instead of the pork.
Hagah: If he needs meat and if there is here an animal (a Kosher type) that needs to be slaughtered, and there is also ready non-Kosher meat, see above in chapter 328, paragraph fourteen.¹³⁶In the Shulḥan Arukh, Oraḥ Ḥayyim, 328:14 the same situation is discussed as is discussed in 618:9 except in the earlier law the Sabbath is being discussed and not Yom Kippur, but in this case, according to Isserles, the same law applies. A person is sick and for his illness he needs to eat meat. On the Sabbath as on Yom Kippur it is forbidden to slaughter an animal for food under normal circumstances, nor is it permitted to cook food. The question is whether one is able to violate the Sabbath on Yom Kippur to save a life (which is permitted) by killing an animal according to the laws of Kashrut for this person's needs, or is it better to permit the sick person to eat non-Kosher meat that would not require a slaughtering on the Sabbath or Yom Kippur. Which would be more preferable among these two options? The law is that if there was time to do it, that is, if the sick person could wait for the meat, they would slaughter the animal and eventually feed him Kosher meat and not meat from a ritually impure animal. But if the sick person needed the meat immediately and the ritually impure meat was ready, and the slaughtering of a Kosher animal would postpone the sick person from being able to eat too long, then they were permitted to feed him meat from a ritually impure animal in that case to save his life.

חולה שאכל ביום הכפורים ונתיישב דעתו בענין שיכול לברך צריך להזכיר של יום הכפורים בברכת המזון שאומר יעלה ויבא בבונה ירושלים: A sick person who ate on Yom Kippur and recovers to such a degree so that he is able to bless (the food he ate, to say the Grace after the meal¹³⁷The Grace after meals, Birkhat ha-Mazon, ברכת המזון, is a central part of the home liturgical service. Its obligation is considered biblical from the verse: "Thou shalt eat and be satisfied and bless the Lord thy God for the good land which He has given thee", (Deuteronomy 8:10). It has four parts to it and it should be said only after bread has been eaten. When bread has not been eaten, a shorter version of the Grace is recited.
The first blessing, Birkhat ha-Zan praises God for providing food for all His creatures. The second blessing, Birkhat ha-Areẓ, expresses Israel's special thanks for the good land God has given it. It speaks of the redemption from Egypt, the covenant of circumcision and the revelation of the Torah. The third benediction, Boneh Yerushalayim and also Neḥamah, consolation, asks God to have mercy on Israel and to restore the Temple and the Kingdom of David. These blessings also include a plea for God to always sustain and support Israel. This was the core of the Grace after Meals, but after the destruction of Bethar during the Bar Kokhba revolt in 135 C.E. a fourth blessing was added, Ha-tov ve-ha-metiv. It thanks God for His goodness praying that He may fulfill specific desires. This is followed by special petitions which begin with the word Ha-Raḥaman, "May the All-Merciful…" This part of the prayer was once for personal desires but it then became standardized. The number of petitions vary. The Sephardi rite has fifteen while the Ashkenazi only has nine.
The Talmud (Ber. 48b) ascribes the origin of each of the benedictions to different Jewish heros and events. The first was originated by Moses when he saw manna fall, the second by Joshua when he conquered Ereẓ Israel, the third by David and Solomon and the fourth by the rabbis at Jabneh although it might actually date back to as early as the reign of Hadrian.
The Birkhat ha-Mazon, on Sabbaths and festivals is usually preceded by Psalm 126, which reminds us of Zion and its restoration among our earthly delights. Whenever three or more men have eaten together one of the men begins the Grace by summoning the others, (Ber. 7:1-5), "Gentlemen, let us say Grace", (in the Sephardi rite it is "with your permission"), and the others reply "Blessed be the name of the Lord henceforth and forever." The leader repeats that statement and says, "With your consent (the Sephardim say, "With the permission of heaven"), let us now bless Him of whose food we have eaten." The others respond, Blessed be He whose food we have eaten and through whose goodness we live." This whole introduction is called zimmun (Ber. 45b). The Talmud states that this must be said also by three women who eat together. The zimmun becomes somewhat longer when the number of people together goes to ten, a hundred, a thousand, and ten thousand. Now we insert the word Elohenu, "Our God" in the third line when ten or more eat together. Grace used to be followed by a cup of wine and there was a discussion if this was only when the grace was said with zimmun or also individually, (Shulḥan Arukh, Oraḥ Ḥayyim, 182:1). The custom now is to have the wine only on Sabbaths, festivals, and special occasions.
On Sabbaths and festivals special sections are added to the third blessing, Boneh Yerushalayim, these are Reẓeḥ and/or Ya'aleh ve-Yavo and an additional petition is added in series of Ha-Raḥaman. Special Ha-Raḥaman petitions are inserted for New Moons, Rosh HaShanah, Sukkot, Passover, and Shavuot. On Ḥanukkah and Purim the special Al ha-Nissim, "about the Miracles", section is said during the second blessing which is devoted to the thanksgiving.
The Grace at a wedding feast is slightly different. The third line of the zimmun is supplemented by "Blessed he our God in whose abode is joy, of whose food we have eaten, and through whose goodness we live" and then the seven wedding benedictions are recited at the end of the Grace. The Grace at the house of a mourner is also changed. The end of the third benediction has a special prayer substituted, the text of the fourth blessing is changed as is the zimmun. At a circumcision ceremony, the wording of the zimmun changes to fit the occasion. The Ha-Raḥaman petitions also allow for passages to be inserted by children, guests or masters of the house.
The Birhat ha-Mazon is the only prayer commanded by the Torah but the words were developed by man. Therefore under certain circumstances and for children different, shorter versions have been developed which at least mention the land and Jerusalem. When bread is not eaten another form of Grace is said. It is called Berakhah Aḥronah, a "final benediction" and what is said depends on what is eaten. When one eats of food from the five species of grain; wheat barley, rye, oats, and spelt, wine, or fruits of Ereẓ Israel, a short Grace is said, which consists of one benediction with an insertion for the type of food eaten and for the special nature of the day if it is the Sabbath or a festival. It is called Berakhah Me'ein Shalosh, "a benediction summarizing the three" regular benedictions. For any other food a short benediction called Ve-Lo-Khelum, "nothing" is said. It is also known by its first two words, Bore Nafashot.
Editorial Staff, E. J., v. 7, pp. 838-41.) he has to mention Yom Kippur in “יעלה ויבא״¹³⁸Ya'aleh ve-Yavo, יעלה ויבא, "May our Remembrance rise and come and be accepted before Thee" is a special insertion made to the third blessing of the Grace after the Meal, Birkhat ha-Mazon, (see footnote 137) called Boneh Yerushalayim, which speaks about the restoration of the Temple and the Kingdom of David. It also asks God to have mercy on Israel. The blessing includes a plea for God to sustain and support Israel. The insertion of the Ya'aleh ve-Yavo is made on the festivals and New Moons and the Reẓeh on the Sabbaths. Both are inserted when the Sabbath falls on a festival. Since Yom Kippur is a fast day, there is no special insertion which specifically mentions Yom Kippur in the Birkhat ha-Mazon which is the Grace after Meals, of which there are not to be any on Yom Kippur. According to the Shulḥan Arukh if one must eat on Yom Kippur for reasons of health and then is able to, he must recite the Grace after Meals, and in the third blessing, the Boneh Yerushalayim he must insert the Ya'aleh ve-Yavo prayer which states that it is indeed a holiday, and at the time in the prayer when the specialness of the festival is mentioned, the person must also say "Yom ha-Zikharon ha-Zeh, "This Day of Remembrance" which indicates it is Yom Kippur.” in his Grace after the meal, namely in the benediction called “בונה ירושלים¹³⁹Boneh Yerushalayim, בונה ירושלים, the third blessing in the Grace after Meals; see footnotes 137 and 138.”