Ibn Sina in his Canon said,
“To enter a bath fasting will render the body extremely dry, and make the person thin and debilitated.
To enter the bath after a heavy meal, on the other hand, will make a person stout, by drawing the humours towards the subcutaneous tissues. Moreover it removes the obstructions by transferring the undigested aliment from the stomach to the tissues.
To enter the bath at the moment when the first digestion has ended and before a sense of hunger returns is beneficial and produces a medium degree of stoutness.”