Fluid or hydration status is extremely important in endurance sports like the marathon, getting it wrong can have disastrous consequences. As such hydration is a balancing act, not taking in enough fluids will result in dehydration and taking in too much fluid will result in Exercise-Associated Hyponatremia (EAH). EAH can be a life-threatening scenario where an athlete or individual takes on more fluid than they are losing, causing a dilution and subsequent reduction in sodium levels within the body resulting in fatigue, nausea, vomiting, dizziness and alterations in consciousness.
How Fluid Should Be Replaced
It is uniformly agreed that fluid replacement during exercise is important to prevent excess fluid loss (dehydration) and to avoid body weight loss of >2% and excessive changes in electrolyte balance which can compromise performance. How that fluid is replaced during exercise is currently of great debate with researchers unable to come to a consensus on which approach is best, drinking to a plan of 600-800ml per hour, drinking to thirst (using the sensation of thirst to determine when to drink) or drinking ad libitum (drinking whenever and in whatever volume). In light of this lack of consensus it would seem reasonable that any rehydration strategy should be flexible taking into consideration the duration of the event, the outside temperature, the effort required, sweat rate, the terrain and gradient etc. It should use thirst as a guide while not straying too far from an intake of 600-800ml per hour, but essentially not drinking more than is being lost through sweat.
Sweat rates are highly variable between individuals with an average sweat rate of approximately 1.35L/hr. There are calculators available that can help determine ones specific sweat rate. Alternatively, a simple way to establish a rough sweat rate is to weigh one’s self prior to and immediately after a 60 minute workout. The weight loss during that period divided by the time (60mins) will provide a rough sweat rate estimate – it is important to be well hydrated before undertaking the workout. The benefit of establishing an individual sweat rate estimate is it aids in understanding how much fluid is lost to sweat per hour of exercise and therefore roughly how much fluid will need to be replaced per hour.
What To Drink
Armed with a sweat rate estimate and a rehydration strategy of drinking to thirst while making sure one isn’t straying too far from the amount of fluid needing to be replaced due to sweat loss, gives you 2 of 3 key components to a solid hydration strategy. The final component is the fluid type that should to be taken in; water, hypertonic (Gatorade), hypotonic (Mizone) or isotonic (Powerade) drinks. In endurance sports the simple answer to this question is all, it is important to use a mixture of water and drinks that contain electrolytes as well as carbohydrates. Lastly, it is important to try different products and combinations during training to see what works best and to also get used to drinking while training, so when it comes to race day it is one less thing you have to think about.