ABSTRACT
Introduction An athlete can sustain brief periods of physical activity without ingestion of any additional fluid or metabolites. However, the loss of fluid equivalent to 3-4% of body mass is enough to impair human performance, and after 60-90 minutes of vigorous endurance activity, depletion of glycogen reserves in the muscles and liver also limits muscle and brain function. During prolonged periods of physical activity, the primary roles of the stomach and duodenum are thus to allow the absorption of sufficient fluid and minerals to replace sweat losses and to augment depleted muscular and hepatic glycogen reserves. If the stomach and duodenum fail to fulfil these functions, not only is physical performance impaired,1,2 but dehydration can ultimately progress to death from heat stroke,3,4 a catastrophe well recognized among both athletes and workers in deep underground mines. If a prolonged bout of heavy exercise is contemplated, it is thus important to know how to optimize the delivery of fluid and carbohydrate to the circulation of the exerciser. This implies an understanding of how to maximize rates of gastric emptying for various nutrients, factors influencing their subsequent uptake by the circulation, and how these processes are modified when people are engaged in various types and intensities of physical activity. This chapter thus considers the influence of physical activity upon various aspects of gastric and duodenal function, using this background to make some practical recommendations for meeting the nutritional needs of those who engage in prolonged and vigorous endurance activity. We begin with some comments on methodology, and then consider the impact of various patterns of physical activity upon gastric and duodenal function, relating these findings to the optimal choice of beverage for fluid replenishment in various types of sport. Detailed examination of the influence of habitual physical activity upon common gastro-duodenal pathologies, including peptic ulcers and gastro-oesophageal cancers is deferred to the following two chapters.
