לא תאכלו על הדם, “you must not eat the flesh of an animal until it is absolutely dead, or, according to some opinions, in the case of sacrificial meat, before it had been salted to remove any traces of life sustaining blood. A third opinion claims that this is a general prohibition not to eat meat stemming from an animal that has not been ritually slaughtered. This would be supported by Samuel I 14,32: ויעש העם את שלל ויקחו צאן ובקר וישחטו ארצה יאכל העם על הדם, “the people (who had been described in the previous verse as being famished) pounced on the spoils; they took the sheep and the cows and calves and slaughtered them on the ground and they ate with the blood.” [When King Saul heard about this he was terribly upset and chided them for having sinned against G-d. Ed.] An alternate interpretation: the words לא תאכלו על הדם are a variation of the commandment not to practice necromancy. Seeing that the survivors of slain people are charged with avenging their dead, they may feel inclined to sit on the graves of the dead to assuage their dead relatives’ anger at not having been avenged. The dead are apparently credited with some power in this respect. (According to Rash’bam, the Emorites believed this, especially about witches who had been murdered. Seeing that the uncircumcised gentiles do eat blood, the Torah would have addressed this subject at this point also.)
לא תאכלו על הדם, “You shall not eat over the blood;” here too the Torah warns about Jews not indulging in pagan practices which were current at the time the Torah was given. People used to dig a hole and pour blood into it and the demons would congregate around such holes. While in the company of these demons they would consume their meal. Seeing that they fraternized with the demons the demons would reciprocate by revealing future events to the people eating there with them. This was a wide-spread practice amongst the Egyptians and the Israelites had learned from them until they too began to believe in the power of these demons and they would seek out their company. In order to wean the Israelites of such idolatrous practices the Torah forbade eating a meal in the presence of blood (which had not been covered up, buried or poured out).
(ג) אסור לו להתעסק בצרכיו או לילך לדרך עד שיתפלל תפלת י"ח (ויש מקילין לאחר שאמרו מקצת ברכות קודם שאמרו ברוך שאמר וטוב להחמיר בזה) (תרומת הדשן סי' י"ח) ולא לאכול ולא לשתות אבל מים מותר לשתות קודם תפלה בין בחול ובין בשבת ויום טוב וכן אוכלין ומשקין לרפואה מותר:
(3) It is forbidden to busy one's self with his own needs or to set out before praying Shemonah Esrei (and their are those who are lenient that say some blessings before they say Baruch SheAmar, and it is good to be stringent in this - Terumat HaDeshen 18) nor may one eat or drink, but water may be imbibed before prayer, both on a weekday and on Shabbat or Yom Tov, and likewise all food or drink for medicinal purpose is allowed.
(ד) הצמא והרעב הרי הם בכלל החולים אם יש בו יכולת לכוין דעתו יתפלל ואם לאו אם רצה אל יתפלל עד שיאכל וישתה:
(4) One who is thirsty and one who is hungry are considered to be ill. If one has the ability to focus one's mind, one should pray. If not, if one desires, one should not pray until eating or drinking.
הוי עיר הדמים במדרש הוי מקמי קרתא דשופכי דשפכו דמים בגוה ויש לפרש כי הנה אמרו ח"ל לא יאכל אדם עד שיתפלל שנאמר נפש כל בשר דמו בנפשו הוא עי לא תאכלו על הדם פירוש דכתיב כי נמצא כי עיקר האדם הוא על שם הדם שבו ולמה נקרא אד"ם ולא נקרא ד"ם רק נ"א אלא כשמדבק את עצמו בבורא יתברך וממשיך בקרבו אלופ"ו של עולם נתחבר האלף אליו ואז נקרא אד"ם וזהו לא תאכלו על הד"ם ר"ל כשעדיין לא חיבר אליו אלופו של עולם רק אחר התפילה כשחיבר נקרא אד"ם אז מותר לו לאכול ולא קודם דמו לאלופו של עולם ואז כנזכר וזהו הוי עיר הדמים פי שהם נקראים דמי"ם שאינם מחברים אלופו של עולם והנה קשה למדרש והלוא כל העולם קודם תפילה נקראים ד"ם
When my grandmother, Rebbitzin Rivkah, was about eighteen years old, in 5611 (1851), she fell ill. The doctor ordered her to eat as soon as she woke up. However, since she did not want to eat before her morning prayers, she would pray very early in the morning, and have her breakfast after davenen.
When her father-in-law, the Tzemach Tzedek, learned of this, he told her: “A Jew must be healthy and strong. Concerning the mitzvos it is written, Vachai bahem! — ‘One should live in them,’ which means that one must invest the mitzvos with life (chayus — ‘vitality’). And in order to be able to bring chayus into one’s mitzvos, one must be strong and happy.”
He concluded: “You should not [pray] on an empty stomach. Better to eat in order to daven, than to daven in order to eat.” He then blessed her with long life. [She lived till over 80, being] born in 5593 (1832) and passing away on Yud Shvat, 5674 (1914).
My father [the Rebbe Rashab] recounted the above teaching [of the Tzemach Tzedek] to someone at yechidus, and added: “And this should be done joyfully.”