The Gemorah in Berachos 10: says that anyone that eats before davening is considered a haughty person. Hashem says about him “after he has been so haughty, then he accepts upon himself the kingship of heaven”. First one must accept upon himself his spiritual responsibilities, then can he worry about his physical well being.The Rambam says that even a small taste of food will fall under this prohibition.
Based on this, the Shulchan Aruch 89:3 says that it is forbidden to eat before davening. The Shulchan Aruch continues to say that to drink water is allowed. The Mishna Berura adds that tea and coffee are also permitted. The rule is that anything which is needed to give you strength to concentrate for davening is not considered haughtiness. Some say that if one needs to eat before davening, you should first say the parsha of “Shema Yisrael”.

Rabbi Tzvi Aryeh Hyman, Mir Yeshiva
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