We sweat to cool our bodies down. It is a way to shed excess heat. Water evaporating from our skin provides a cooling effect. In humid conditions, you notice that the water does not evaporate; rather it drips off your skin because the water vapor in the air is heavier. The sweat has nowhere to go.
In addition to water, sweat is composed of electrolytes. This comes from extracellular fluid, derived from our blood that has moved to our sweat glands, making its way out of our bodies through our skin. This extracellular fluid is mostly water plus sodium chloride (NaCl). There are trace amounts of potassium, magnesium, and calcium in our sweat, but these are in negligible amounts.
When we sweat, the body tries to reabsorb some of the sodium (Na) that is lost, and everybody has a different reabsorption rate. This is the main reason that there is not a “one size fits all” formula of how much sodium to replace. It is safe to say, however, that the faster you sweat, the more fluid AND electrolytes you will need to replace through drinking. The question is … how much?
Hyponatremia: More Dangerous than Dehydration
Replacing the lost water is imperative when you are sweating profusely, but it is also important to replace the electrolytes. While it is far more common for an athlete to suffer from dehydration (excessive water loss) than hyponatremia (excessive sodium loss), the latter is actually more serious and sometimes fatal.
HypoNAtremia (notice the “Na” for sodium) is the dilution of blood sodium levels. In athletes, it usually happens because (A) they drink too much water, or (B) they lose fluid through sweat and only replace the water, not the Na. This is where electrolyte supplements come in.
Watch for these warning signs of hyponatremia:
- Dry mouth
- Puffy hands and feet
- Muscle cramping
- Nausea or vomiting
- Weight gain during a workout
Types of Re-Hydration Solutions
Endurance events deplete an athlete’s body of three vital nutrients: Water, Salt, and Carbohydrates. Rehydration solutions contain various amounts of each of these and can be used based on what needs replenishment.
Hypertonic solutions have a higher concentration of solvents (sugar and minerals / electrolytes) than normal blood concentration. They are high calorie and meant to give you quick energy. Coca Cola and Gu are good examples. They are not particularly hydrating but are imperative in retaining energy during long (90+ min) events.
Isotonic sports drinks have electrolytes comparable to blood concentration, plus carbohydrates of about 6-7%. The classic is Gatorade, which was specially formulated to replace electrolytes and carbohydrates lost during short to moderate bouts of exercise.
Hypotonic beverages are made to replace electrolytes and water, but not carbohydrates. They are best for hot, long athletic events. Nuun is a good example. They are great for replacing water and electrolytes, but you need an additional supplement to replace carbohydrates.
Weather will dictate how much water, salt and carbohydrates you need to replenish. In hot and humid weather, when sweat rate is highest, you may need to rely on more hypotonic beverages and in cool wet weather you need much less. It is recommended that you replace carbohydrates (whether in a solution or solid form) for any event that last more than 90 minutes.
There is no one answer to the question, “What is the ideal combination of water/electrolytes to maintain hydration?” However, here are a few guidelines:
- Don’t copy someone else. Their body chemistry may be quite different from yours.
- Start with your instincts. Try going by thirst, but if you have a high sweat rate, it takes effort to hydrate enough.
- Drink 4-8 oz. water every 15 minutes. You may find you need more.
- Add electrolytes on hot & humid days, or if you notice salty sweat as indicated by burning eyes or salt marks on your clothes/skin.
- Pay attention to signs of dehydration or hyponatremia.
- Realize that this varies from person to person, and even day to day.
Cramping: notice that cramping is a symptom of both dehydration & hyponatremia. Salt is just ONE factor in cramping. Cramps can also be caused by muscular fatigue, glycogen depletion, or inadequate pacing. As with most things running-related, look at the big picture, as it may be a combination of things.
*Notes from TrainerRoad’s ‘Ask a Cycling Coach,’ Podcast 221. “Andy Blow talks sweat, hydration and cramping.” Accessed 9/30/19. https://soundcloud.com/trainerroad/hydration-sweat-and-cramping-with-precision-hydrations-andy-blow-ask-a-cycling-coach-podcast-221