"Don't go swimming for an hour after you eat" is actually a good piece of advice. If you do hop into the pool or the ocean right after you eat, you could develop cramps and risk drowning. Your body will always work to take care of its energy needs and that conflicting needs can cause problems. When you exercise, your sympathetic nervous system, a part of the automatic or autonomic nervous system (brain stem, spinal cord) stimulates the nerves to your heart and blood vessels.