Wait two to four hours after eating a large meal, and avoid fatty foods and fiber on the day of a big workout. These foods are particularly difficult to digest and can cause irritation, diarrhea, indigestion and -- of course -- pain. Stick to small portions of simple carbs and light protein, which will give you quick fuel without the digestive aches.
Drink 4 to 6 ounces of water every 15 to 20 minutes and nix caffeine, soda and dairy milk at least 4 hours prior to your workout. Caffeine and the lactose in dairy milk both stimulate the GI tract. The carbonation in soda can cause painful bloat. Also avoid sipping through a straw, which can cause you to swallow air and bloat.
Stand with feet shoulder width apart and your hands together over your head to perform dynamic torso twists. Bend your upper body to the right side, and then the left to stretch your obliques, a common area for runner's side stitch. With your arms in front of you, gently twist your upper body to the right and to the left. Perform your stretches for three to five minutes.
Slow your pace to ease stomach cramps, especially while running and swimming. Letting your heart rate slow slightly will help direct more blood to your gut, which can reduce cramping. If you're swimming, tread water slowly for a few minutes or switch to a backstroke, which will relieve some of the pressure on your stomach.
Practice good posture as you run, lift weights and bike. Slouching creates excess pressure on your belly, which can cause stomach acid to reflux into the esophagus.
Practice good breathing techniques, especially when you run, swim and lift weights. While running, try inhaling for three steps and exhaling for two. To avoid swallowing water and causing stomach bloat while swimming, turn your head to the side to inhale, not forward, and exhale into the water as you stroke. As you strength train, inhale when your muscles are relaxed -- say, as you lower from a situp -- and exhale while your muscles are contracted. This will help prevent pressure from building up on your digestive system.
Consider using an over-the-counter acid reducer to calm your stomach before a big workout. Acid reducers coat the stomach and esophagus to lessen indigestion and reflux.
Perform lower back and abdominal exercises two to three times a week to strengthen your stomach muscles and improve your posture. Simple crunches and variations such as reverse crunches and bicycle crunches can strengthen your upper and lower abs and your lower back.