Gemara

(Beraisa): "Machalah" refers to Marah. It sickens the whole body and causes 83 sicknesses (the numerical value of Machalah). Pas Shacharis (eating bread in the morning) with salt and a flask of water prevent all 83.

(Beraisa): There are 13 benefits of Pas Shacharis: it prevents heatstroke, chills, wind and damagers. It gives wisdom to the foolish, it helps him (be settled in Beis Din, which helps him) to win his case, to learn and to teach Torah. People heed his words, he remembers his learning, he does not sweat too much, he desires only his own wife, and it kills intestinal worms;

Some say, it even dispels envy and promotes love.

Question (Rabah): People say 'sixty runners cannot catch one who ate Pas Shacharis.' Chachamim say 'eat early in summer due to the heat, and in winter due to the cold. What is the source of these?

Answer #1 (Rava bar Mari): "They will not be hungry or thirsty, heat and sun will not strike them (because they already ate)."

Answer #2 (Rabah): "You will serve Hash-m" refers to Kri'as Shema and Tefilas Shacharis. "And He will bless your bread and water" refers to bread with salt and a flask of water. After this "I will remove sickness from your midst."

Shabbos 10a (Beraisa): Ludim (cannibals) eat in the first hour of the day... most people eat in the fourth, workers eat in the fifth, and Chachamim eat in the sixth. After this, it is like throwing a rock into a pouch with little wine.

(Abaye): This is only if he did not taste anything all morning. If he ate even a bit, there is no problem.

Sukah 26a (Mishnah): One may eat Arai (casually) outside a Sukah.

(Rav Yosef): This is the volume of two or three eggs.

Objection (Abaye): Sometimes this is enough for him, and it is a fixed meal! Rather, it is the amount that a Talmid eats before going to learn.

Rishonim

Rosh (Berachos 6:29): Ha'Motzi exempts drinks, since they come only due to the bread, even if they do not nourish. Even water comes due to the meal, like we say 'Pas Shacharis and a flask of water overcome the 83 illnesses.'

Rashi (107b DH v'Kiton): The water is for one who has no wine.

Tosfos (Berachos 37b DH Omar): R. David of Mitz used to soak break in water until it lost the form of bread so he could eat to have strength to teach, without need to delay for Birkas ha'Mazon.

Kaf ha'Chayim (OC 152:24): This implies that even so, it is called Pas Shacharis.

Poskim

Shulchan Aruch (OC 155:2): Before going to learn, one may eat bread in the morning, if he normally does so. It is good to do so regularly.

Tur: We say 'like a Talmid eats before going to learn.'

Bi'ur Halachah (DH Yuchal): The Tur connotes that he should eat only a little.

Bach (2, citing Mordechai ha'Aruch): We and our Rebbeyim are not accustomed to eat Pas Shacharis. We heard that the grain must come far from civilization, and never heard roosters or people pulling boats. One who regularly eats it must do so every day. Chachamim refrain, for one cannot fulfill it.

Taz (1): If one is not concerned for this opinion and fulfills Pas Shacharis simply, he does not lose at all.

Magen Avraham (3): The Gemara is unlike the Mordechai. Perhaps it is a mistake based on Shabbos 66b, which says that Pu'ah (a certain plant) helps for a certain sickness and witchcraft, but not if it was exposed to the sun, moon, or rain, or the sound of iron, roosters or footsteps. Rav Nachman bar Yitzchak said that this renders Pusa (i.e. Pu'ah) futile. This was mistakenly understood to refer to Pas Shacharis!

Eliyahu Rabah: Also exposure to the sun, moon, or rain ruins Pu'ah! Also, grain cannot grow without them! That Gemara cannot discuss Pas Shacharis. If anything, perhaps his source is Berachos 56a (which says 'like people say, 'the bread fell in the well and can't be found.''

Chacham Tzvi (15): We do not heed the Mordechai, for all Poskim and many Gemaros hold that any bread suffices. Drinking much wine helps an inflated spleen. Rav Acha said that one with much wine will never have the problem! Rather, he should eat Pas Shacharis regularly (Gitin 69b). This shows that Pas Shacharis is easy to obtain!

Kaf ha'Chayim (22): Mateh Yehudah (or Mateh Yosef) says that the Gemara, Tur, and Shulchan Aruch connote that we discuss regular bread that removes hunger, unlike Mordechai ha'Aruch.

Sha'ar ha'Tziyon (3): Water is for one who does not have other beverages. Rashi connotes like this.

Drishah (2): Rashi explains that it gives wisdom to the foolish, for his mind is settled, for he is not afflicted (by hunger). This enables him to grow wise, learn, teach, explain and defend his words.

Mishnah Berurah (11): It is a Mitzvah to accustom onself to a good Midah and conduct to guard his health in order that he will be strong to serve the Creator.

Kaf ha'Chayim (23): The Gemara discusses bread kneaded without salt. Our bread has salt in it, so eating it without salt is like eating bread with salt. The water should be hot, like nowadays people eat cake with coffee. Divrei Chamudos says that just like one should not eat before praying for his food, he should not eat before learning his fixed quota. We can say that this applies to a fixed meal. It is better to accustom himself to eat a small amount first, so he can learn for a long time. If one cannot have bread, coffee with sugar helps a little.

Kaf ha'Chayim (24, Mishbetzos Zahav 2): The Shi'ur is k'Beitzah. The bread should be from the five grains, for it says "He will bless your bread." A cooked dish from the five grains is not Pas.

Eshel Avraham (Butshats): Any food helps, not only bread, as long as it satisfies him. Even drinking coffee, milk, honey, sugar or wine helps. Even a judge appointed over the Tzibur may eat or drink in front of people, as long as he does not fix a meal.

Kaf ha'Chayim (25): Eating while standing weakens the body. One must be extra careful about this regarding Pas Shacharis.

Shevet ha'Levi (4:16): Undoubtedly, Pas Shacharis is merely a good counsel. It is not a Mitzvah. This is clear from the Shulchan Aruch's words '...if he normally does so. It is good to do so regularly.' The Tur calls it a Mitzvah because it strengthens Avodas Hash-m. The Gemara says that it helps him merit to learn, teach... Surely it is a Mitzvah to bring himself to this, but eating is not a Mitzvah itself. It is like other Mitzvah related conducts. If the Tur meant a real Mitzvah, the Shulchan Aruch would not disagree needlessly!

Shulchan Aruch (157:2): When the fourth hour comes, one should fix his meal. A Chacham who learns waits until the sixth hour, but no longer, for this is like throwing a rock into the pouch if he did not eat anything in the morning.

Magen Avraham (1): On Shabbos and Yom Tov some delay past midday, for we calculate six hours (based on 12 daylight hours) from when one rises.

Mishnah Berurah (2): The six hours are from when one rises.

Kaf ha'Chayim (2): Mateh Yehudah (or Yosef) says that we always count six Zemaniyos hours from dawn.

Mishnah Berurah (4,5): Eating after six hours does not help or harm. L'Chatchilah, one should eat in the sixth hour even if he tasted earlier. Chachamim used to eat a little Pas Shacharis, and fix the meal in the sixth hour.

Kaf ha'Chayim (3): The Beis Yosef requires eating at the beginning of the sixth hour, but many prove that it is until the end of six hours.

Mishneh Halachos (7:25): Pas Shacharis is until the sixth hour. It is a Heter for a Chacham to wait until the sixth hour. Even though one should eat Pas Shacharis due to "guard your souls", since he normally learns first and does not want to interrupt, Chachamim permit him until midday. Surely, if he rose early and learned his quota earlier, he may eat when most people eat.