Eating and drinking in a standing position:[1]

Health: There are three things which weaken a person’s body, and one of them is a person who eats or drinks while standing.[2] [This applies especially regarding breakfast, Pas Shacharis.[3]] Accordingly, while one eats, one is to either sit in his place or lean on his left side.[4] [Also in the world of health and medicine there are acclaimed benefits for eating and drinking in a sitting position, versus the possible side effects associated with eating in a standing position.[5]]

Derech Eretz: Aside for the health-related reasons, it is not proper for Torah Scholars to eat or drink while standing, and hence one is to always sit upon drinking the Havdalah wine.[6] This applies also for those that are not Torah Scholars, and applies by all foods and drink, not just by Havdalah.[7]

The custom and practical ruling: Practically, the widespread custom of the world is not to be particular in this matter, especially when having a small snack or quick drink of water or coffee.[8] However, the Arizal was very careful in this to sit even upon drinking water, and certainly by other beverages. Thus, although the widespread custom is to be lenient, and one who is lenient has upon whom to rely, it is proper to be stringent especially when there is no trouble to simply sit down.[9] [Thus, if you are anyways going to sit down you should simply wait until you sit before drinking and eating from your food. Hence, if you’re grabbing yourself a coffee to drink while you work by your desk, rather than already drinking it as you’re walking wait to start drinking it until you’re sitting. It goes without saying that those who are drinking from the kiddush wine should try to grab a chair to sit while drinking it. Likewise, when eating at a buffet and should not eat from his plate in a standing position and should find a place to sit in order to eat. However, when on the run, or if there is no seat available, or when coming to the kitchen for a quick drink, one does not need to be particular to sit.]

________________________________________________

[1] See Sefer Shemiras Haguf Vihanefesh [Lerner] 25:1; Ein Lamo Michshol 1 p. 376

[2] Gittin 70a; Chupas Eliyahu Raba 3, in end of Reishis Chochma; Yifei Laleiv 155:6; Aruch Hashulchan O.C. 179:9; Y.D. 116:15; Kaf Hachaim O.C. 155:25; Y.D. 116:90; Likkutei Maharich 1; See M”B 296:6; Sefer Shemiras Haguf Vihanefesh [Lerner] 25:1; Ein Lamo Michshol 1 p. 376

[3] Yifei Laleiv 155:6; Kaf Hachaim O.C. 155:25

[4] Rambam Deios 4:3; Kitzur SHU”A 32:5

[5] See here: https://www.healthline.com/nutrition/eating-while-standing-up; And “While standing and drinking water, the fluid tends to pass through without any filtration to one’s lower stomach under high pressure. This causes the water impurities to settle in the bladder, and damage the functioning of the kidneys, says Dr Rustgi. It can even cause urinary tract disorders.”

[6] Admur 296:15; M”A 296:4; Mateh Moshe 509; Maharil 8; Miseches Derech Eretz Zuta; Kneses Hagedola 296:3; Elya Raba 296:14; M”B 296:6; Kaf Hachaim 296:42

[7] Elya Raba 296:14; Beir Heiytiv 170:16; Kaf Hachaim ibid; Kitzur Halachos 296 footnote 14

[8] Ben Ish Chaiy Bereishis 2:29; Rav Poalim 2:45 that a) The world is not careful in this b) The Rambam omitted this Talmudic warning, c) Perhaps this is due to Nishtaneh Hativim; Orchos Rabbeinu 3:222 that so was witnessed by Steipler to drink water and coffee while standing, and that perhaps the restriction and danger only applies by a set meal and drink, and not by a mere snack, or water to quench one’s thirst; Shilhi Dekayta 49 in name of Rav Chaim Kanievsky that the custom is to be lenient when traveling; Aderes Tiferes 5:61; Even Yisrael 9:199; Ein Lamo Michshol ibid; See Admur ibid who omits the health reason of Gittin ibid, and implies that it only applies to a Torah scholar

[9] Ben Ish Chaiy ibid; Rav Poalim ibid; See Darkei Chaim Veshalom 278 that he would eat even a snack while sitting