It’s that time of year — hot and humid mid-July in New York City! Fall marathon training is underway and it is as important as ever to maintain hydration and electrolyte balance. In this hot environment when a runner is sweating heavily, fluid losses can exceed 1 liter per hour. At this rate, it is unlikely that hydration strategies before and during activity will provide enough fluids, carbohydrates, and electrolytes to restore the body to homeostasis.
A study examined the effects of coconut water as a rehydration drink. Blood samples, urine samples, and body weight were obtained from 8 male participants prior to activity. The participants then exercised in a hot environment at 60% VO2max for 90 minutes until 2.5-3.0% body weight was lost. Blood samples and body weight were taken again immediately after exercise as well as 30 minutes later. The participants were then give 1 of 3 rehydration beverages: coconut water, plain water, or a carbohydrate-electrolyte beverage, and drank the volume equivalent to 120% fluid volume loss over the course of 2 hours.
This study used a “rehydration index” to show how well a drink re-established fluid balance— taking account all variables including blood volume, electrolytes, and plasma glucose concentration. Blood volume improved the most with coconut water, and plasma glucose concentration improved with both the coconut water and the carbohydrate-electrolyte beverage, but less nausea and stomach upset were reported with the coconut water. Two of the three measured electrolytes (chloride and potassium) improved most with coconut water—sodium content improved most with the carbohydrate-electrolyte beverage.
So we have some excellent news for the coconut lovers! Coconut water is a great option for full body rehydration post exercise to restore fluid and electrolyte balance! While it provided slightly less sodium replenishment than the carbohydrate-electrolyte beverage, it surpassed the carb-electrolyte drink in all other categories.
Cathlin Fitzgerald, PT, DPT, CSCS, CAFS
-Saat M Singh R Sirisinghe RG Nawawi M. "Rehydration after exercise with fresh young coconut water, carbohydrate-electrolyte beverage and plain water". Journal of Physiological Anthropology and Applied Human Science. 2002; 21(2): 93-104.