That was probably an old wives’ tale in the first place.
“I don’t think it was ever valid” to tell people to wait an hour to swim, said William Evans, director of the nutrition, metabolism and exercise laboratory at the University of Arkansas for Medical Sciences. People used to think that perhaps all the blood flowing to the gut for digestion would cause muscles to cramp up, he added. “But that doesn’t really happen. People can run long distances on a full stomach.
“Obviously, it’s more important if you thought muscles would cramp up in the water. But there’s nothing to that. What determines how much blood muscles get is how much oxygen the muscle needs. There is this perfect system whereby slight lowering of oxygen drives an increase in blood flow. So it never happens that if you eat, then jump into the water, that your muscles will cramp.”
On the other hand, if you eat a lot and your stomach is very distended, the effort of swimming and breathing hard might make you throw up, and you could wind up inhaling stomach contents into your lungs, noted Dr. Ronenn Roubenoff, a professor of nutrition at Tufts University. If you aspirate a large volume, that could block air from getting into the lungs; a smaller volume might simply make you choke a bit, but this could make you panic and have trouble swimming. A small volume of aspirated stomach contents could give you pneumonia, but this would not cause immediate problems while swimming.
In other words, if you feel uncomfortably full, wait a bit before swimming. But there’s nothing magic about waiting an hour. And if you only eat moderately, swimming right away is fine.