However, fat and protein require a much longer time, approximately five to seven hours, to digest and absorb. The size of the meal also can influence the overall time needed for digestion and absorption. Large meals may require many hours to clear the gastrointestinal tract, whereas, smaller meals may be digested in just a couple of hours. Nervousness often associated with athletic events also can impair normal digestion and absorption of food. An average-size adult will have about five quarts of blood circulating throughout the body.
Children have less, the amount depending on the size of the child.
Following ingestion of a meal, blood will be diverted from areas of the body with low needs to the stomach and intestinal area. This extra blood helps the processes of digestion and absorption of the food that has been eaten. During exercise, large amounts of blood are diverted to the working muscles and to the skin for sweat production and cooling. In this process blood is actually shunted away from the gastrointestinal tract. Thus, digestion and absorption of food can be impaired during exercise because the digestive system receives less blood instead of more.