Question

Explain the mechanisms for how electrolytes and fluid are maintained in balance during exercise (including but not limited to endocrine regulation of this). What are some of the negative consequences on performance in a variety of exercise intensities and durations? What are the evidence-based strategies that can be used to mitigate the detriment to performance?

Answer 1

Answer :

Fluid and electrolytes balance :

Fluid supplementation is necessary for exercise in which fluid losses must be offset by intake to avoid the negative effects of hypohydration on health and performance. Several aspects of gastrointestinal function have been studied to gain information concerning the assimilation of ingested fluids to maintain fluid balance during exercise. Research results with regards to gastric emptying and secretion, intestinal absorption and secretion, and aspects of fluid retention, including urine production and plasma volume changes, can be utilised to formulate an appropriate fluid supplementation regimen. Increasing the volume of ingestate and decreasing the carbohydrate concentration promote gastric emptying of fluids. By maintaining a low osmolality secretion is reduced, thus leading to a greater rate of net fluid absorption. Adding sodium and carbohydrate (up to approximately 7%) increases the net intestinal absorption rate. Increasing carbohydrate concentration above this level begins to have a deleterious effect on intestinal absorption of fluid. Sodium also promotes retention of ingested fluids and leads to an increased plasma volume response during rehydration. The primary goal of supplementation should be considered, fluid vs carbohydrate provision, and the beverage composition altered accordingly. Beverage composition to maximise fluid provision will not maximise carbohydrate availability.

Prolonged exercise leads to a progressive water and electrolyte loss from the body as sweat is secreted to promote heat loss. The rate of sweating depends on many factors and is increased in proportion to the work rate and the environmental temperature and humidity. Sweat rate is highly variable between individuals, and can exceed 21 h-1 for prolonged periods. Since it is established that dehydration will impair exercise capacity and can pose a risk to health, the intake of fluid during exercise to offset sweat loss is important. Fluid intake is also aimed at providing a source of substrate, usually in the form of carbohydrate. The availability of ingested fluids may be limited by gastric emptying or by intestinal absorption. Gastric emptying of liquids is slowed by the addition of carbohydrate in proportion to the carbohydrate concentration and osmolality of the solution. With increasing glucose concentration, the rate of fluid delivery to the small intestine is decreased, but the rate of glucose delivery is increased. Water absorption in the small intestine is a passive process and is stimulated by the active absorption of glucose and sodium. The optimum fluid for rehydration during exercise depends on many factors, particularly the intensity and duration of the exercise, the environmental conditions, and the individual physiology of the athlete. There is no advantage to fluid intake during exercise of less than 30 min duration. The composition of fluids to be used will depend on the relative needs to replace water and to provide substrate. Where rehydration is a priority the solution should contain some glucose and sodium and should not exceed isotonicity: this will require the glucose concentration to be low (20-309 g l-1) or the substitution of glucose polymers, and the sodium content to be high (perhaps as much as 60 mmol l-1). Where substrate provision is more important, a more concentrated solution, incorporating large amounts of glucose polymers in concentrations of 150-200 g l-1, is to be preferred. To minimize the limitation imposed by the rate of gastric emptying, the volume of fluid in the stomach should be kept as high as is comfortable by frequent ingestion of small amounts of fluid. Addition of sodium, and perhaps also of potassium, may be important for rehydration after exercise.

Negative consequences on performance in a variety of exercise intensities and durations:

extreme breathlessness

breathing problems such as wheezing or coughing

chest pain or pressure

extreme perspiration

dizziness or fainting

severe muscle pain or cramps

nausea

severe pain in any muscles or joints

extreme and long-lasting fatigue after exercising.