The Prophet (PBUH) never mixed fish and milk, milk and sour foods or dishes, two laxative dishes, two heavy dishes, two liquid foods or two hot dishes, or two cold dishes, two sticky dishes, two constipating types of foods that produced the same condition. Also, he did not join between two types of food that caused opposite effects, such as constipating and diarrhea, or easily digested with heavy foods broiled with cooked foods, dry with fresh foods, eggs and milk or meat and mil . Also, he neither ate a food when it is rather hot, nor old food that was heated for him the next day. Further, he did not eat any salty or rotten foods, such as pickles. All these types of foods are harmful and cause the health to change for the worst.
The Prophet also used to neutralize the harm of some foods by their opposites, as much as he could. When the food is hot, he neutralizes its effect with a cold food, and when it is dry, he neutralizes it with wet foods, such as eating ripe dates with cucumbers and dried dates with butter. He also used to drink the water of soaked dates to neutralize the effect of heavy foods. Further, the Prophet (PBUH) used to eat dinner even if it only consisted of a few dates.
Abu Nu'aym mentions that the Prophet (PBUH) used to discourage sleeping just after eating food because it will harden the heart. The doctors also advise the people to walk even a few steps after dinner, because sleeping just after dinner is very harmful. Muslim doctors add that one could also pray after eating his dinner, so that the food resides in the bottom of the stomach where it will be easily digested.
The Prophet (PBUH) did not drink [water or milk] with his food, because the drink would spoil the food, especially when the water is hot or cold. It is not preferred that one drinks water after sports activity, when tired. after sexual intercourse, before and after eating food and after eating fruit. Also, it is not preferred that one drinks water after taking a bath and after sleeping. All these activities will harm the health and one should not think about the benefits he might gain from drinking with the food.