From the time of amud hashachar, it is forbidden to eat or drink before praying. The Chachamim support their words (Berachot 10b) on the verse (Leviticus 19:26), “Do not eat on the blood,” which they interpreted as, “Do not eat before praying for your blood.” Further, they teach, “Anyone who eats and drinks first and [only] afterwards prays, Scripture says of him (I Kings 14:9), ‘You have thrown Me behind your body’ (“gavecha,” the word used for “your body,” alludes to “ga’avatecha” – your pride). HaKadosh Baruch Hu said, ‘After this person acts arrogantly he accepts upon himself the yoke of Heaven?!’”
However, water is a permissible drink before prayer because there is no aspect of pride in drinking it. Similarly, one is permitted to eat food and drink beverages that are intended for medication. Because they are being used as medicine, there is no aspect of pride in consuming them (Shulchan Aruch 89:4). For example, someone who is suffering from constipation is permitted to eat prunes before prayer since he is eating them as medicine (see Mishnah Berurah 89:24).
Someone who is so hungry that he cannot concentrate on his prayer is allowed to eat before praying because the law regarding him is similar to that of a sick person who must eat; his eating does not possess any aspect of pride (Shulchan Aruch 89:4; see Mishnah Berurah 26).
A weak person, who is able to pray individually first and eat breakfast afterwards, but cannot delay his breakfast until after the time of prayer in a minyan, should pray individually and eat after prayer. L’chatchilah, after the meal, it is good to go to the minyan in order to hear Kaddish and Kedushah (Bei’ur Halachah 89:3; see further in this chapter, halachah 7).
A minor who has not yet reached the age of bar mitzvah is allowed to eat before prayer, for educating minors not to eat forbidden food applies when the food itself is not kosher. However, when the Chachamim “created a fence” not to eat before prayer, or before Kiddush, they did not obligate minors to abide by that “fence,” since the food itself is not forbidden (Mishnah Berurah 106:5; Yabia Omer, part 4, 12:15; however Kaf HaChaim 106:11 is stringent).