When you eat, blood moves to surround the intestinal tract ready to absorb the nutrients from the meal. This reduces the blood available to the extremities, such as the arms and legs.
Blood carries oxygen and energy to muscular tissues and removes waste products for recycling. When blood flow is sufficient, muscles can continue to work in relative comfort. Where muscles work too hard, or blood flow is reduced, waste products build up creating acid. This causes painful cramps – the ubiquitous “stitch”.
Since blood flow is reduced to active muscles after eating, the risk of cramps increases. On land this is no great problem. Taking deep breaths and allowing oxygenated blood to flow to the site of the cramp soon clears any pain. In the water, it is possible that a person may drown before the cramp clears. This is particularly true in very cold water, where blood flow may be withdrawn from peripheral tissues to maintain core temperature (see previous post), further reducing the amount of blood available to swimming muscles.
However you do need to be very active to deplete available blood resources. Swimming a race is risky, but lazing around in a backyard pool is probably fine. Though if you are worried, waiting an hour after eating before swimming should negate the slight increase in risk.
To see the results of eating on muscle function, you can test yourself by going for a run before and after eating a large meal. You will probably notice muscular cramps or a stitch forming faster after eating.
Mind you, eating a heavy meal can also make you nice and sleepy. Which for me means that I don’t feel like either swimming or running straight after lunch.
Mao CC, Jacobson ED. (1970) Intestinal absorption and blood flow. Am J Clin Nutr. 23(6):820-3.
Spring B, Chiodo J, Harden M, Bourgeois MJ, Mason JD, Lutherer L. (1989) Psychobiological effects of carbohydrates. J Clin Psychiatry. 50 Suppl:27-33.