According to Dr. Charles Smith, of the Family and Preventative Medicine Department at UAMS, there is no medical evidence to support the myth.
Your mom probably told you that you would get severe cramps and drown if you decided to go for a swim right after eating. The theory behind the myth was that there is an increased blood flow to the stomach and intestines to absorb the nutrients. This means that there is less blood to deliver oxygen and to remove waste products from exercising muscles. In reality, you have more than enough oxygen to supply your stomach and muscles.
If you are swimming for recreation purposes, eating right before entering the water should not pose any problems. If you are going to be in the water for exercise, it makes sense to wait at least an hour to allow most of the food in your stomach to pass through. With any strenuous exercise the diversion of blood flow to the stomach for digestion may temporarily decrease flow to the muscles and may result in some cramping. Moreover, it is difficult to exercise strenuously with a full stomach.
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