Don't swim right after you eat if you are in terrible shape, but if you're reasonably fit, don't exercise too intensely in the beginning, and stop if you feel belly pain, you're better off eating.
When your stomach fills with food, its muscles contract and require large amounts of blood. When you exercise vigorously, your heart pumps large amounts of blood to your skeletal muscles. If your heart is not strong enough to pump blood to both your stomach and your skeletal muscles, blood is shunted from your stomach muscles, the muscles lack oxygen, lactic acid builds up in muscles and they start to hurt. Most people can exercise after eating without suffering cramps because their hearts are strong enough to pump blood to both their exercising and stomach muscles.
If you are going to exercise for more than an hour, you have to eat or your muscles and liver will run out of sugar. Your liver must constantly release sugar from its cells into your bloodstream, and there is only enough sugar in your liver to last about an hour when you exercise vigorously. So, eating before exercising can help you to exercise longer.
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