Ingesting fluid before and during exercise minimizes the detrimental effects of dehydration on cardiovascular dynamics, temperature regulation, and exercise performance. Adding carbohydrate to an oral rehydration solution provides additional glucose energy for exercise. Adding electrolyes to the rehydration beverage maintains the thirst mechanism and reduces the risk of hyponatremia.
Concern centers on the dual observations that (1) a large fluid volume intake impairs carbohydrate uptake; while (2) a concentrated sugar solution impairs fluid replenishment.
Consuming 400-600 ml of fluid immediately before exercise, followed by regular fluid ingestion during exercise (250 ml every 15 mins) optimizes gastric emptying by maintaining relatively large volumes in the stomach.
Gastric emptying slows when ingested fluids contain a high concentration of particles in solution (osmolality) or possess high caloric content. Fewer particles facilitate water movement from the stomach for intestinal absorption. Too high osmolality you will get pulling of water into the intestine by osmosis and you will get stomach/bowel cramps. The ideal oral rehydration solution to maintain fluid balance during exercise and heat stress contains between 5-8% carbohydrates (divide the carbohydrate content in gram by fluid volume and multiply by 100). Adding a moderate amount of sodium to fluid stabilizes plasma sodium concentrations, which benefits the ultraendurance athlete at risk of hyponatremia. Added sodium in the rehydration beverage also reduces urine production and sustains the sodium-dependent osmotic drive to drink.
So next time you can check your rehydration drink be it 100 plus, Gatorade and see how they measure up to what I have just written :-).
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