Halakhah zu Bereschit 18:5

It is a general rule of healthy practice that before eating, you should engage in some strenuous activity, whether walking or working, so that your body is warmed up, and then you should eat. As the verse says, “By the sweat of your brow you shall eat bread,”³Bereishit 3:19. and “She does not eat the bread of laziness.”⁴Mishlei 31:27. You should loosen your belt before eating. This is hinted at in the verse אקחה פת לחם (“I will take a morsel of bread”⁵Bereishit 18:5.): the word אקחה in reverse is an acronym for התר חגורה קודם אכילה (“loosen the belt before eating”), and פת לחם is an acronym for פן תבוא לידי חולי מעיים (“lest you cause yourself digestive disorders”). While eating, you should sit in your place or recline on your left side. After eating, don’t move about excessively, because that will cause the food to descend from the stomach before it has been fully digested, which can be harmful; rather one should walk a little and rest. But you should not go on a long walk or engage in strenuous activity after the meal. Nor should you sleep till at least two hours have elapsed after the meal, so that the fumes will not ascend to the head and cause damage. Likewise, bathing, bloodletting and sexual intercourse are not beneficial after a meal.