There is a grain of truth to it, however. And it would be good advice to the person who actually "swims" hard at the swimming pool. Most of us just splash around or float on air mattresses, and having a full stomach won't hurt us one bit.
We found this discussed in the "Family Medicine Column," by John C. Wolf, D.O., an associate professor of family medicine at Ohio University College of Osteopathic Medicine.
"Avoiding swimming for at least one hour after eating is much like the advice 'don't run with scissors.' Both are good general safety precautions but not 'hard and fast' rules," Wolf writes. "Swimming after eating can increase one's risk of drowning and, therefore, should be avoided. To put it in proper perspective, though, this risk is actually quite small.
"I'll try to explain why this is so. When we eat a meal there is a dramatic shift of blood flow into the abdomen to sustain the increased muscular activity of the digestive tract.
"As the demand for blood in the gut increases, less is available for carrying oxygen to the arms and legs. The muscles in these limbs require this oxygen to support the increased activity necessary during exercise like swimming.
"It is possible to swim with sufficient vigor that the muscles require more oxygen than the circulation can provide at the moment. Fortunately, the muscles can continue to work in this situation for a while, but their metabolism is shifted to an anaerobic (without oxygen) mode.
"The anaerobic activity causes the metabolic byproduct lactic acid to accumulate in the muscles and in the blood. These changes, along with reduced oxygen level and other metabolic changes, can cause muscle cramps.
"Despite the heavy metabolic demands of running a marathon, runners don't eat a five-course meal just before a race. Guess why. Yes, the same reason one shouldn't eat a large meal and then go swimming decreased performance and increased risk of muscle cramps.
"Eating a light meal and playing in the water is perfectly safe. Situations requiring vigorous activity should be avoided after eating. For a novice swimmer, this may be simply trying to stay afloat; for a competitive athlete, it may be swimming in a race.
"In general, the larger a meal, the longer the delay before safe swimming. Since most people tend to let their desire to play in the water override their good judgment, the old adage is still the best to follow: Don't swim for one hour after a meal. And, for the record, I don't advise running with scissors or playing with matches, either!"