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Answer: Researchers don't know exactly why this occurs, but they suspect that it's because the cooler temperature of the water causes your body to lose heat and the blood vessels in your skin to constrict. This prevents the release of certain hormones that suppress appetite during and soon after exercise, says Rudy Dressendorfer, PhD, an adjunct professor at the University of Alberta's Human Performance Laboratory in Edmonton.
One study conducted by Dressendorfer and his colleagues found that people who exercised in colder water consumed 44 percent more calories following their workout than those who expended the same amount of energy in warmer water. But you don't have to fall prey to after-swim hunger. Dressendorfer says that you can reduce it somewhat by taking a brisk 15-minute walk to warm up your body immediately after you get out of the pool.
Originally published in Fitness magazine, February 2006.