If you're in good shape, your mother probably lied when she told you not to go swimming after you eat. On the other hand, if you are in lousy shape, she was probably right. When food reaches your stomach, a valve at the end of the stomach called the pyloric sphincter, closes to keep food in the stomach. Your stomach is a muscular balloon that is forced to contract and squeeze food into liquid soup before the pyloric valve can open to allow food to pass into the intestines. So food inside the stomach forces the stomach muscles to contract, and contracting stomach muscles require your heart to pump a huge amount of blood to supply them with oxygen. When you exercise vigorously, your heart pumps blood to your skeletal muscles. If you are in lousy shape, your heart may not be strong enough to pump large amounts of blood to both your stomach and exercising skeletal muscles at the same time, so the arteries leading to your stomach muscles close and your contracting stomach muscles do not get enough blood to meet their oxygen needs. Lactic acid builds up in the stomach muscles and they start to hurt.
If you are in reasonable shape, your heart should be strong enough to pump blood to both your exercising and stomach muscles at the same time. If you are going to exercise for more than an hour, you need to eat or your muscles and liver will run out of sugar. Your brain gets almost all its energy from sugar in your bloodstream, but there is only enough sugar in your bloodstream to last three minutes, so your liver has to release sugar constantly from its cells into your bloodstream. But 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. If you do not eat before you exercise for a long time, your liver can run out of its stored sugar called glycogen, your blood sugar level can drop, your brain then will lack its source of energy and you will feel weak and tired. Eat just before you exercise for more that an hour, and when you plan to exercise more than two hours, you should eat and drink frequently during your exercise.
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