A cool swim on a hot day is a classic summer experience. But just as classic is what happens as you exit the water: Before you can even begin to dry yourself off, you realize that you're the hungriest you've ever been. You're famished. You're ravenous. It feels like your entire body is made of stomachs, and all of them are completely empty.
If swimming has this effect on you, you're not alone; the post-swimming munchies are real, and backed up by science. Here's why you're more likely to be extra hungry after a swim.
EXERCISE IS AN APPETITE-BOOSTER
Swimming, like any other physical activity, burns calories—which can, in turn, cause the body to increase its production of ghrelin, a hunger-causing hormone, to prompt you to eat back what you've burned. One 1999 study showed that exercise makes us particularly likely to crave salty foods, which might explain the desire to chow down on snacks like chips after a swim (and may also explain why people who only exercise in order to lose weight are generally less successful than those who change their diets). While intense exercise can also have a temporary dampening effect on appetite while your body cools down, most people don't swim hard enough to experience it, causing hunger to kick in practically as soon as you've emerged from the water.
THE LAWS OF THERMODYNAMICS
Swimming isn't just a workout for your lungs, heart, and muscles; it also makes your body do the extra work of maintaining a balmy 98.6°F temperature while you're immersed in water that's usually at least 20 degrees cooler than that. It's a similar principle to the one that causes you to burn more calories just by being outside in the winter: Research suggests that your body will activate its brown fat stores when the temperature drops, revving up your metabolic rate in order to stay warm. And where a hot, sweaty workout can cause the aforementioned appetite-suppressing effect, a body immersed in water doesn't need to increase blood flow to the skin (and away from the digestive system) in order to regulate heat. For your stomach, it's business as usual—and that means hunger.
THE COLDER THE WATER, THE GREATER THE CRAVING
Multiple studies, including one from the University of Florida, have shown a correlation between swimming in cold water and coming out famished (despite the difference in calories burned between a cold-water swim and a warm-water one being negligible). In short, the colder you feel when coming out of the pool, the more likely you are to make a beeline for the snack bar.
Of course, the effect that swimming has on your appetite has nothing to do with its effectiveness as a form of exercise—and on that front, it's one of the best there is, especially for people in search of a low-impact way to stay fit. But if you're watching your diet and don't want to overindulge, you can take the edge off the post-swim munchies by taking a quick, brisk walk to raise your body temperature before you chow down.