Chapter 12 - The gastrointestinal system

Abstract

This chapter focuses on human studies, beginning with the classical work of William Beaumont (1785-1853) in 1833 on the gastrointestinal (GI) system. Beaumont concluded that moderate exercise was conducive to rapid and healthy digestion; however, the severe and fatiguing exercise retards digestion. In the next century, interest was focused on the exercise influencing hunger pangs, gastric emptying, and gastric secretions. During prolonged exercise, the loss of fluid and electrolytes in sweat, together with the depletion of energy stores in the liver and working skeletal muscles, can produce dehydration, hyperthermia, hyponatremia, hypoglycemia, and glycogen depletion. Any one of these factors could contribute to fatigue and limit physical performance. These effects are exacerbated when exercise is done in the heat. The magnitude of the alterations can be substantial, but they are highly variable among individuals. Sweat losses can range from 1 to 4 L.h-1 and sodium losses from 10 to 120 mEq.l-1 of sweat. Exogenous carbohydrate utilization can exceed 1 g.min-1. The GI system is essential in attenuating these losses and supplying energy, but its capacity to do so, may be compromised by reduced blood flow, changes in hormone and neurotransmitter profiles, and the disruption of intestinal barrier function. The latter impairment could produce circulatory and thermal dysfunction.