Fluid replacement during exercise.

Current evidence indicates that adequate fluid ingestion during exercise enhances athletic performance, prevents a fall in plasma volume, stroke volume, cardiac output and skin blood flow, maintains serum sodium concentrations and serum osmolality, lowers rectal temperature and the perception of effort, and prevents a progressive rise in heart rate. Rates of sweating and urine flow are not influenced by fluid ingestion. The evidence suggests that the maintenance of serum osmolality and serum sodium concentrations at pre-exercise levels is the important determinant of these beneficial effects of fluid ingestion on cardiovascular function and thermoregulation. The provision of glucose in the ingested solution may be necessary to optimize performance; glucose ingestion that enhances fluid and sodium absorption in the small bowel may also present a progressive rise in oxygen consumption during exercise. Sweetened carbohydrate-containing drinks may also increase fluid intake during exercise, thereby minimizing voluntary dehydration. Hence, the optimum solution for ingestion during exercise should provide carbohydrate, probably at rates of about 1 g/min and electrolytes in concentrations that, when drunk at the optimum rate, maintain serum osmolality and plasma volume at pre-exercise levels by replacing exactly that water and electrolyte losses from the extracellular space. At present, the composition of the fluid that will optimize electrolyte and fluid replacement of the extracellular space is not established. Neither are the optimum rates of fluid ingestion during exercise known. At low sweat rates (< 1 liter/hr), it is probable that all of the lost fluid can and should be replaced; rates of fluid ingestion needed to offset higher sweat rates may exceed the maximum intestinal absorptive capacity for water. Furthermore, high rates of fluid intake (> 1 liter/hr) are achieved with difficulty during exercise, especially when running, and are likely to lead to feelings of abdominal discomfort, possibly due to the accumulation of unabsorbed fluid in the small bowel or colon. Practicing to drink regularly during training might reduce the severity and frequency of these symptoms, possibly by increasing intestinal absorptive capacity. Most athletes are "reluctant" drinkers during exercise and do not ingest fluid at rates equal to their rates of fluid loss; hence, they develop progressive (voluntary) dehydration during prolonged exercise. Surprisingly, the level of voluntary dehydration that develops during exercise is relatively independent of the duration or intensity of the activity. The factors that explain these phenomena remain elusive. Fluid consumption during exercise is enhanced by the ingestion of cold, sweet fluids. Simultaneous food consumption also stimulates fluid ingestion.(ABSTRACT TRUNCATED AT 250 WORDS)