How long do you have to wait after eating to shower?
Digestion requires good blood flow and digestion generates heat. If one takes a warm shower, does exercise or goes for a swim, the blood needs to be diverted to the skin surface for cooling purposes. This interferes with digestive process and you can end up having a stomach ache, can faint or even have a heart failure if you are weak.
Though most people would see no ill effects having showered right after a meal, but weak or old people can have a different reaction. So a healthy habit is to wait for about 30 mins after a meal before taking a shower.
The reason someone would faint would be because there is not enough blood flowing to the brain. Digestion requires a high percentage of blood and when you take a warm shower the blood vessels dilate in order to cool down the body by releasing the heat from blood to the skin. More blood flows through those vessels and since so much blood is being used elsewhere the brain may be lacking blood and the bodys reaction to this is to faint because fainting will keep the body level and the blood will flow evenly. This may not affect a lot of people but people with health concerns and also people with bad circulation may have problems.