Eating and drinking before Davening:

A. Before Alos:[1]

• Letter of law-within 30 minutes before Alos: It is forbidden to begin a meal starting 30 minutes prior to Alos. However, snacking is allowed, which includes eating an unlimited amount of fruits, and up to 55 g of bread and alcoholic beverages.
• Letter of law-prior to 30 minutes before Alos: It is permitted to eat a meal prior to 30 minutes before Alos.
• Kabbalistic ruling and final custom: Some Poskim rule it is a grave prohibition according to Kabballa to eat, prior to dawn, after awakening, throughout the entire year. This prohibition applies starting from midnight and onwards, after awakening from a standard sleep of 60 breaths. Practically, the custom of Sephardic Jewry is to be stringent. However, Ashkenazi Jewry is lenient and so is the Chabad custom. 
• Cases of exception according to Kabbalah: If one did not sleep that night for at least 60 breaths, then according to all, he may eat up until dawn. Likewise, some Poskim rule that if one plans to return to sleep for a second time before Alos, then this prohibition does not apply. Likewise, if one is weak or sick he may eat prior to Alos even according to the Zohar.

B. After Alos:[2]

• Prohibition of any eating: Beginning from Alos, it becomes Rabbinically forbidden for one to eat any food, even a mere snack, prior to Davening Shemoneh Esrei.
• Water: It is permitted to drink water prior to Davening. This allowance applies even on Shabbos and Yom Tov, prior to making Kiddush.
• Coffee and tea:[3] It is permitted to drink prior to Davening plain coffee and tea without sugar or milk. Furthermore, the custom is to allow drinking it even with sugar and milk, and those who do so have upon whom to rely if they do so with intent to increase their concentration during the prayer. Nonetheless, one who can suffice without sugar and milk is to be stringent.
• One who began eating prior to Alos: One who began eating a meal prior to Alos must stop his meal when Alos arrives.

C. Cases of exception:[4]

• Eating for medical purposes: It is permitted to eat foods and have drinks prior to Davening for medical purposes. This allowance applies to all forms of foods and beverages, even if they are foods that are sometimes eaten for mere pleasure [i.e. are relevant to haughtiness].
• Eating and drinking to satiate hunger and thirst and for concentration in prayer-Chabad custom: Despite the above allowance, it is forbidden to eat or drink prior to Davening for the purpose of satiating one’s thirst or hunger, unless one is not able to have proper concentration in his prayer until he eats or drinks. This allowance applies even in today’s times when in general people do not have much proper intent during prayer, nevertheless, if he chooses to eat and drink for this purpose of strengthening his concentration, then he may do so. [Practically, the Chabad custom is to encourage eating and drinking prior to prayer for purposes of increasing concentration. It is however forbidden for one to eat simply because he is hungry or thirsty, without having intent to do so to strengthen his concentration of prayer, as stated above.]
• Shema before eating:[5] One is to read the morning Shema prior to eating prior to prayer, even when eating for Kavana purposes. In such a case one should say it with intent that it is under condition that if he does not manage to say it on time later on, then he intends to be Yotzei with this reading, and if he does manage to say it on time later on after immersion, then he intends to not be Yotzei with this Kerias Shema.
• Tefillin before Shema and eating:[6] If one is eating before Davening, then he is not to say the morning Shema until he puts on Tefillin. Nevertheless, the custom amongst most Chassidim is not to wear Tefillin when saying Shema prior to Davening, even after sunrise. However, some Chassidim are accustomed to wearing Tefillin upon reciting Shema prior to Davening , and so was the directive of the Rebbe Rashab and Rebbe Rayatz. This is not the widespread Chabad custom.
• One who is weak and cannot wait until the Minyan finishes to begin eating:[7] One who is weak, and it is thus difficult for him to delay eating until the congregation finishes prayer, may pray privately in his home without a Minyan. [This delay of eating however is only if he has enough strength to properly pray privately, otherwise it is better for him to eat before davening and then go Daven with a Minyan.]
  • It is forbidden however for him to Daven in Shul prior to the Minyan. Furthermore, once he has arrived in Shul it is forbidden for him to leave even to return back home to pray in private.

D. May a child below Bar Mitzvah eat before Davening Shacharis [i.e. Hamotzi breakfast]:[8]

• Prior to the age of Bar/Bas Mitzvah, a child may eat prior to Davening without restriction, and may therefore eat a meal with bread and eat treats and the like and it is forbidden for a parent to oppress his child with these restrictions. Nonetheless, from age 12 and onwards for a boy, it is proper for parents to begin encouraging their child to resolve on his own not to eat unnecessary foods before Shacharis. Foods that are needed to be eaten for health purposes, and extra concentration of prayer, may be eaten even after bar Mitzvah.

___________________________________________

[1] See Admur 89:5; 70:5; Kuntrus Achron 70:2; Rama 581:2; M”A 89:14; Zohar 2:215; Beir Heiytiv 581:12; Shaareiy Teshuvah 581:12; M”B 581:21; Kaf Hachaim 581:69; 89:28-31; Ketzos Hashulchan 11:3; Igeres Hateshuvah chapter 3; Igros Hakodesh 3:75

[2] See Admur 89:5-6 and 70:5; Michaber 89:3; Tur 89:3; Rebbe Eliezer Ben Yaakov in Brachos 10b; Ketzos Hashulchan 11:2

[3] Ketzos Hashulchan 11:2

[4] See Admur 89:5-6 and 70:5; Michaber 89:3

[5] See Admur Kuntrus Achron 70:2; Admur 46:9 in parentheses

[6] See Admur 58:4-5; 66:11; Siddur Admur [Letter 22]; M”A 66:12; Degul Merivava and Rav Akiva Eiger, brought in M”B 46:33; Kaf Hachaim 25:27; Os Chaim Veshalom 25:6; Kuntrus Eitz Chaim 25; Igros Kodesh of Rebbe Rayatz letter 3302; Sichas Shelach 1952; Rebbe in Toras Menachem 28^th Sivan 1952, vol. 6 p. 5, printed in Shulchan Menachem 1:125-130

[7] See Admur 90:11

[8] See Admur 106:3; M”A 106:3 and 269:3; M”B 106:5; Piskeiy Teshuvos 89:19; See also Admur 269:3; 343:7;471:10; 472:23; Tzemach Tzedek O.C. Safek Limi 4