Did your parents ever warn you not to go swimming right after you ate? It always seemed like oddly specific advice from the people who fed us bologna sandwiches and kicked us outside for unsupervised play all summer. But at the local pool, everybody’s mom and dad became an expert in the dangers of swimming and undigested food.
Thing is, they were right. Digestion and exercise are like opposing teams on the football field— each competes for control of physiological resources, namely energy and blood flow. When we’re digesting a meal, blood flow to the stomach and intestines increases. When we exercise, blood flow to our muscles increases. Since we’re not constantly increasing our total blood volume, this means that our body must prioritize one process over the other. If I’ve just eaten Thanksgiving dinner, my body focuses on breaking down all that turkey and sweet potato pie. (This is also why we get sleepy after a big meal.) But if I’m sweating through a BODYPUMP class, the focus is on my working muscles. Trying to do both at the same time can lead to cramping, nausea and other GI distress.
This is most likely where the one-hour-after-eating swimming rule started. If you get a sore stomach in aerobics class, you can just stop, but if you’re out in the deep end of the pool, it might be too difficult to swim to the edge when your stomach starts cramping. And drowning due to bologna-sandwich intake is just no way to go.
But it’s not all spy vs. spy—after all, without digestion, we wouldn’t get the energy we need for exercise. And the truth is, exercise has many positive effects on our digestion. What’s surprising is how many of them we can get from a simple after-dinner walk.