When discussing various forbidden practices which Jews must avoid, the Torah states "לא תאכלו על הדם," which literally means "you may not eat over the blood" (ויקרא י"ט:כ"ו). The Ramban (שם) explains that this refers to a practice associated with witchcraft or sorcery, as indicated by the context in which this phrase is found in the Posuk (שם), and he proceeds to elaborate upon the particular occult practice, relating to foretelling the future, which involved eating "over the blood" that we are required to stay away from. The Rashbam (שם) suggests that the Posuk (שם) is referring to a practice observed by certain non-Jewish people to eat at a gravesite for the purpose of witchcraft; the Chizkuni (שם) writes similarly that there was a non-Jewish practice to eat at the grave of a murdered person, so that he would not be able to take revenge against anyone, which we Jews must avoid. The Radak, commenting on a Posuk in Shemuel Aleph (י"ד:ל"ב בד"ה ויאכל), writes that certain demon worshippers used to eat around the blood of a slaughtered animal as part of their ritual, and this is what the Torah prohibits in the Posuk in our Parsha (ויקרא שם), as is evident from the context in which this commandment appears.
Although it is clear that the intent of this Posuk (שם) is to prohibit some type of eating activity relating to blood and occult practices, as documented by the above Meforshim, Rashi (שם בד"ה לא תאכלו) notes that the Gemara in Sanhedrin (דף ס"ג.) derives numerous other ideas (five, to be precise) from this phrase "לא תאכלו על הדם" which are not really related to each other, but which all have something to do with eating, and, perhaps less directly, with blood. The implication of the words of Rashi, in commenting on a Gemara in Chulin (דף קכ"א. בד"ה השוחט ובעמוד ב' שם בד"ה וממתין), however, is that at least one of these derivations is in the category of an אסמכתא, meaning that the Torah, in using the phrase לא תאכלו על הדם in the Posuk in our Parsha (שם), is perhaps merely hinting at these other ideas, at least some of which are actually laws that are required only MideRabbanan, and thus the true meaning of the Posuk (שם) is as understood more literally by the above cited Meforshim. The same may presumably be said of an additional idea derived from this phrase by Rabbeinu Bechaya, in his commentary on the Posuk in our Parsha (שם), which is not mentioned by the Gemara in Sanhedrin (שם). It would appear, though, at least from the simple reading of that Gemara (שם), that the phrase is in fact teaching all of these other laws directly as well, notwithstanding the more literal, contextual interpretation of the phrase presented above, and that at least some of them are MideOraisa. This certainly seems to be the position of the Rambam (פרק א' מהל' שחיטה הלכה ב') and the Sefer HaChinuch (מצוה רמ"ח); the Pri Megadim (יורה דעה סימן כ"ז במשבצות זהב ס"ק א') outlines the dispute about this matter.
The Gemara in Berachos (דף י:) derives yet another law from the phraseלא תאכלו על הדם , and that is that one should not eat before davening, explaining the phrase, somewhat literally, to mean that one should not eat before praying concerning his blood, meaning, of course, concerning his life. The Gemara שם() then quotes a Posuk in Melachim Aleph (י"ד:ט') which speaks of the wicked king Yerovam Ben Nevat and describes him as having thrown Hashem behind his back, that is, as having rejected Hashem; based on a play on words, the Gemara (שם) suggests, as explained by the Maharsha (חדושי אגדות שם בד"ה אל), that it was Yerovam's גאוה, meaning, his self-centered haughtiness, that led him to reject Hashem, and then says that one who eats and drinks before davening is behaving in a similar fashion because he is acknowledging Hashem only after taking care of his own needs, which is also a sign of גאוה. The Rambam, in his Sefer HaMitzvos (שורש ט'), includes this idea that one may not taste anything before davening among the different laws learned from the phrase לא תאכלו על הדם, implying that he considers this restriction to be MideOraisa, although in his Peirush HaMishnayos in Makkos (פרק ג' משנה א'), where he also lists the prohibitions derived from the phrase לא תאכלו על הדם, he does not mention it at all. The Sefer HaChinuch cited above )שם( also states that this rule that one may not taste anything before davening is among those derived from the phrase לא תאכלו על הדם, and the Minchas Chinuch (שם אות ה') writes that this prohibition apparently is in force MideOraisa; according to Rabbeinu Yonah in Berachos (דף ה. בדפי הרי"ף בד"ה לא תאכלו), however, the phrase really is only an אסמכתא, providing a hint to this rule in the Torah, but the rule itself is not MideOraisa, and this also seems to be the position of the Meiri in Berachos (בית הבחירה שם בד"ה האוכל), and of the Beis Yosef, in his commentary on the Tur (אורח חיים סימן פ"ט בד"ה ומ"ש בשם). The Minchas Chinuch (שם) suggests that this question may relate to the general discussion about whether or not davening itself is mandated MideOraisa; if the Mitzvah of Tefillah is itself only MideRabbanan, then there obviously cannot be a prohibition MideOraisa to eat before Tefillah. According to the Rambam (פרק א' מהל' תפלה הלכה א'), however, and others who hold like him that daily Tefillah is required MideOraisa, based on the Gemara in Taanis (דף ב.), there is room to say that this prohibition is also MideOraisa.
The Rambam (פרק ו' שם הלכה ד') rules that it is forbidden for a person to even taste anything starting from daybreak (מאחר שיעלה עמוד השחר) until he has davened Shacharis; the Mishnah Berurah (או"ח שם ס"ק כ"ז), referring to a ruling of the Ramo elsewhere (שם סימן תרנ"ב סעיף ב'), indicates that this prohibition actually goes into effect starting at about half an hour before daybreak, which means that it is forbidden for one to begin to eat something within half an hour of daybreak. If, however, one began to eat at an earlier hour, when it was permissible to do so, there is then a question as to whether or not he must stop once daybreak arrives. According to the Tur (שם סימן פ"ט), it is the position of the Rambam that one need not stop eating at daybreak if he began earlier, but he notes that his father the Rosh quoted those who held that one must indeed stop at daybreak in such a case. The Beis Yosef (שם בד"ה ואם) points out that there seems to have been an error in the text of the Rambam which the Tur used, because the Rambam in the editions that we have says nothing about this, and he then suggests that the reference of the Tur (שם) should perhaps be to the Rashba, who does indeed state ( חדושי לברכות שם בד"ה כל הרשב"א) that one who begins eating at an earlier time need not stop when the time for davening arrives. The Aruch HaShulchan (שם סעיף כ"ו), however, suggests that the Tur (שם) is correct in referring to the Rambam because of a ruling of the Rambam elsewhere (פרק ב' מהל' קריאת שמע הלכה ו', ועיין בהשגת הראב"ד שם), which the Aruch HaShulchan himself elaborates upon earlier (שם סימן ע' סעיף ח'); he thus concludes (סימן פ"ט שם) that this is in fact the majority viewpoint.
The Shulchan Aruch (או"ח שם סעיף ה') quotes both positions, mentioning first the one according to which one must indeed stop eating at daybreak after having started earlier. The Magen Avraham (שם ס"ק י"ד) notes that the Shulchan Aruch later (שם סימן רל"ב סעיף ב') indicates that although one may not begin to eat in the afternoon prior to davening Minchah once it is close to the time when Minchah may be davened, if one began to eat, he need not stop when Minchah time arrives, but he explains that Shacharis is different because the prohibition is based on the above cited Posuk in our Parsha (שם); since it is rooted in the Torah, the prohibition is stricter. The Rosh in Berachos (פרק א' סימן י') presents this idea as well. The same idea is also mentioned by the Beis Yosef (שם בד"ה וא"א) and by the Perishah, commenting on the Tur (שם אות ז'), as well as by the Taz (שם ס"ק ג'), the Mishnah Berurah (שם ס"ק כ"ח), the Kaf HaChaim (שם אות מ"ב), and others. The Magen Avraham (שם ס"ק ט"ו) then indicates that the other view, that one need not stop eating at daybreak, is based on the idea that Tefillah is required only MideRabbanan to begin with, and we thus do not have to be so strict. According to the Mishnah Berurah (שם ס"ק כ"ט) and the Kaf HaChaim (שם אות מ"ג), however, the consensus of the Poskim is that the first viewpoint is correct, and one who started eating at an earlier time when it was permissible to do so is thus required to stop eating at daybreak. It should be noted, though, that the Magen Avraham (שם ס"ק י"ד) and the Kaf HaChaim (שם) quote from the Zohar and from the Mekuballim that even if one gets up in the middle of the night, after having slept, as the Pri Megadim points out (באשל אברהם שם ס"ק י"ד), and it is still long before daybreak, it is nonetheless improper to eat before davening Shacharis. The Mishnah Berurah (שם ס"ק כ"ח) writes that it is proper to be stringent and follow that viewpoint, if possible; he adds, though, citing the Machatzis HaShekel (שם ס"ק י"ד) and the Chayei Adam (כלל ט"ז סעיף א'), that if one gets up very early to learn and needs to eat something in order to feel better, he may certainly do so.
As for exactly what one may not consume before davening Shacharis, it would appear from the Rambam (הל' תפלה שם) that it is prohibited to taste anything at all, even water; this is certainly the understanding of the Rambam (שם) presented by the Aruch HaShulchan (שם סעיף כ"ג), as well as by the Pri Chodosh (שם ס"ק ג'), and the Rambam earlier (פרק ה' שם הלכה ב') implies this himself as well. The Kol Bo (סימן י') and the Avudraham (סדר שחרית של חול ופירושה, ברכות השחר, עמוד ל"ב) and others quote that this is also the opinion of the Rif, although Rav Ovadyah Yosef (שו"ת יביע אומר חלק ד' חאו"ח סימן י"א אות ה') documents that it was actually Rabbeinu Peretz, not the Rif, who authored this opinion; the Behag (הל' ברכות פרק ה', דף ה:) also seems to hold this way, as does the Sefer HaChinuch cited above (שם). In any case, though, it is the minority opinion, as most authorities allow one to drink water before davening in the morning, and Rav Ovadyah Yosef (שם אות ו') asserts that the Rambam may actually hold this way as well. The Raavyah in Berachos (סימן ל') writes that the only type of drink which one may not have before davening Shacharis is one which has the capacity to really satiate a person, and this is derived from the fact that the aforementioned Gemara in Berachos (שם) accuses a person who eats or drinks before davening of displaying inappropriate גאוה, that is, pride or haughtiness, and thus only the consumption of the type of item which demonstrates such a state of mind is forbidden. Water, however, does not fit into this category and is thus permitted; he then brings a proof to this idea from a Gemara in Eiruvin (דף ס"ה.). This position is cited by the aforementioned Rosh in Berachos (שם), by the Mordechai in Berachos (סימן כ"ג, דף מ"ו. בדפי הרי"ף) and by the Hagahos Maimoniyos on the Rambam (הל' תפלה שם אות ד'), among others; it is also presented by the Tur (או"ח שם) and is codified by the Shulchan Aruch (שם סעיף ג'). This appears, therefore, to be the more widely accepted practice; the Mishnah Berurah (שם ס"ק כ"ב) and the Kaf HaChaim (שם אות ל"א) both add that one may also drink coffee or tea before davening Shacharis because they too are not items whose consumption demonstrates גאוה. The Kaf HaChaim (שם אות ל"ד) writes, however, that it is admirable to follow the stricter view and avoid even water after daybreak before davening, and the Mishnah Berurah (שם) points out that it would be inappropriate for one who is drinking tea at that time to do so with a group of friends, an idea also mentioned by the Kaf HaChaim (שם אות ל"א).
There is then some further discussion as to what one may add to one's drink; the Pri Chodosh cited above (שם) says that one can not add sugar because that certainly is a sign of inappropriate גאוה, and this idea is posited as well by the Radvaz (שו"ת הרדב"ז סימן אלף ר"ט, חלק ד' סימן רל"ח). It is worth noting that the Ohr Zarua (חלק א' סימן ק"ח) and other Rishonim specifically mention honey as being unacceptable to consume before davening, because it is representative of a sense of גאוה, and the Mishnah Berurah in a different context (שם סימן רע"ב ס"ק כ"א) compares sugar to honey, as does the Sdei Chemed (פאת השדה, מערכת הדל"ת כללים, כלל מ"א בד"ה וכתב עוד). The Be'er Heitev (שם סימן פ"ט ס"ק י"א) also rules that to add sugar to a drink is not proper, and the Chayei Adam quoted above (שם) concurs; the Kaf HaChaim (שם) quotes many others who agree to this, but then suggests that someone who is always accustomed to adding sugar to his drink may do so because he can not drink in any other way, and this is true especially for someone who believes that he will not be able to daven properly without having his drink with sugar. The Aruch HaShulchan (שם) writes that he knows of no reason to outlaw using sugar, and then suggests that perhaps adding sugar directly to the drink is forbidden, but keeping some sugar (cubes) in one's mouth while drinking the drink would certainly be allowed, but he seems to indicate that this is not necessary. The Mishnah Berurah (שם ס"ק כ"ב) notes that people are generally lenient about using sugar in a drink before davening, although he too quotes this idea of putting sugar (cubes) in one's mouth while drinking, indicating that this is allowed because it is not a sign of גאוה, and he actually concludes that otherwise, adding sugar is forbidden before davening unless it is for a weak or sickly person. The Aruch HaShulchan (שם) also discusses the question of adding milk to one's drink before davening, saying that he thinks it is forbidden because milk is very filling, but he admits that people are generally lenient about this, using the milk simply to provide a little more flavor. Rav Ovadyah Yosef (שם אות י"ב) concludes that one may certainly be lenient about using sugar in one's drink, and he appears to be lenient as well about using milk (שם סימן י"ב אות י"ג, י"ד), apparently because such is the norm, and thus adding these items is not a demonstration of גאוה. It should be noted thatthe Mishnah Berurah states in his Biur Halacha (שם בד"ה ולא לאכול) that even one who must eat or drink before davening should try to say at least Kerias Shema beforehand, if possible.