A stomach ache after a run is not something anyone enjoys, but it is a symptom you can use to teach yourself better nutrition and hydration before and after a workout. As you run, most of the body's attention is focused on the muscles that are being used and are in immediate need of fuel, and this does not include the stomach. Consequently, the blood flow to your stomach and digestive system is reduced to a maintenance level, and any digestion of food/fuel is slowed dramatically. Caution should be taken on the types and amounts of your consumption of food and liquid both before and after your run, allowing you to understand why your stomach hurts and how to eliminate the problem.
Anne Valente in her August 2008 article, "Running On A Sensitive Stomach," concludes that the act of running produces a good amount of impact per foot strike; consequently, the mere up and down jostling of the stomach can produce an unpleasant feeling in the stomach and possible cramping. This is something that may take time for the individual runner to get used to but can be helped if the runner takes some precautions about what he drinks and eats prior to and after each run.
Food and Liquid Consumption
Exercise physiologist Kitty Consolo, PhD, is cited in Valente's article as suggesting that changes in your diet before a run may need to be made. Avoid heavier, greasier foods and especially dairy and high fiber foods. If you do consume these types of food, make sure to do so many hours ahead of your run (allow up to 24 hours). This will allow ample time for digestion, so food is not sitting undigested in a stomach that's being jostled during your run.
As for liquids, make sure to drink plenty of water at least an hour before your run. Not only can your main running muscles cramp from lack of hydration, but so can your stomach.
Runners, especially beginners, may find that their stomach pain after a run will diminish and even disappear as their fitness level increases. In their clinical study "Gastrointestinal Problems In Distance Running," in the International SportMed Journal, Dr. Stephen M. Simons and Dr. Gregory G Shaskan report that the greater the fitness level of the runner, the fewer the gastrointestinal tract problems. In other words, your G.I. tract gets used to the lessened blood flow for digestion as you run more. The stomach starts to understand how to function more effectively in this state, causing less stomach aches.
In dealing with the stomach pain after your run, look quickly to the proper nutrition for recovery. Use a recovery drink that combines the right ratio of protein and carbohydrates that will help your insulin response and consequently the replenishment of glycogen or fuel to your muscles and stomach. Dr. Edmund Burke's book "Optimal Muscle Recovery" recommends a ratio of 4:1 carbohydrates to protein for post-run stomach and muscle recovery. If too much protein is used, it can slow the rate of transfer from the stomach into the bloodstream and delay re-hydration, meaning slower recovery from cramping.
Combination of Factors
A combination or series of correct techniques can help reduce and stop your stomach pain after a run. First, check your nutrition and hydration before and after your run. Don't eat too much or the wrong things too soon before your run. Make sure to avoid dairy, high fiber and greasy, heavy foods, preferably up to 24 hours before hand. Stay hydrated with water or sports drinks to help promote your body's overall functioning, especially that of your stomach and digestive system, during your run. Be patient, and expect the stomach pain to lessen or go away completely as your fitness level increases. Experiment with running at different times of day, changing your eating habits and varying your running pace, to see exactly what helps reduce the stomach pain after your run, because every person reacts differently. Also, try running on the softer surfaces of a dirt trail or path in your local park, as this will reduce the impact of your foot strikes and jostling of your stomach.