The Not So Simple Science of Hydration

ImageYou would think that hydration requirements would be pretty straight forward.  Your body needs water, so it’s good for you right?  Well, yes, and it’s not that simple.  While sports dietitians have long known about the dangers of hyponatremia (low blood sodium levels) due to drinking too much water, they don’t always agree on how much water you need when exercising, what the optimal carbohydrate concentration in a sports drinks is (not to mention when to start using one), if coconut water is a superior hydrator, or even when and how much electrolytes to include. 

I myself am in the process of trying to figure out my hydration strategy for my next race, the Mt Evans Ascent.  Being at altitude means I’ll have increased fluid needs from increased respiration, both from being at altitude and because I will be running uphill.  However, if I drink too much water I may experience that uncomfortable sloshing feeling.  Also, if I drink too many carbs I run the risk of GI distress, particularly stomach cramps and nausea, something that has on occasion been an issue for me. Below are some general hydration tips, and how I’m planning to implement, or ignore, them.  The bottom line is that it’s important to personalize your own nutrition and hydration strategies and to practice them before race day!

  • The main goal of consuming water during exercise is to replace water lost as sweat and prevent dehydration.  General recommendations are to drink 6-12 ounces every 15-20 minutes (18-48 oz every hour) and 16 oz of water for every pound lost after exercise.  The reality is that water needs vary from person to person, and some people can get away with drinking less water during exercise than others without sacrificing performance.  While it was originally thought that water weight loss as little as 2% during exercise could hinder performance, some researchers are not saying that is not the case.  It also depends on if you are running in the heat or not-you can lose more water weight in the cold and not sacrifice performance.  To calculate your personal sweat rate you need to weigh yourself naked before and immediately after a 1 hour run.  The difference, in ounces, is how much water you need to drink during your run.  So for instance if I weighed 2 pounds less after my 1 hour run I would need to drink 32 oz/hour (16 ounces in a pound). Of course this test assumes you didn’t drink anything during your run. If so, you have to factor that in.  For short runs it’s probably not necessary to bring water, as long as you are not starting dehydrated and make sure to drink water afterwards. 


  • Conventional (sports nutrition) wisdom has called for sports drinks with a 6-8% carbohydrate concentration (meaning 6-8 grams of carbohydrate per 100mL) for exercise greater than 60 minutes.  Anything higher than that is too concentrated for your stomach to handle during exercise and may result in GI issues and actually hurt your hydration.  There is some data that suggests that even 6-8% is too much for optimal hydration.  From this data have come products such as Skratch Labs and Osmo, which have a 3-4% carbohydrate concentration.  Everyone’s stomach is different, so some people may be able to tolerate a 6-8% concentration drink such as Gatorade, others might not.  I personally have a very hard time tolerating products such as Gatorade, which falls in the 6-8% range.  Check out the link at the bottom for an article on why too high of a concentration may actually dehydrate you.  Listen to your gut (pun intended) and practice drinking different amount of fluids with different carb concentrations to see what makes you feel your best (and fastest).


  • Ah how the coconut is enjoying its 15 minutes of fame. Coconut water, coconut milk, and coconut oil are all enjoying some good press these days it seems.  Coconut water is higher in potassium than pretty much any sports drink, and is marketed as being the perfect hydrator.  However, sodium is the main electrolyte lost during exercise, not potassium, and coconut water provides very little sodium.  So if you are a heavy sweater or a salty sweater who prefers to use coconut water during exercise you’ll have to make sure to get your sodium in some other way, like salt tabs.  There is no good data to prove that coconut water hydrates better than plain water, so don’t feel like you have to use it if you don’t like it.  I had major GI distress the one time I drank it before a run, and almost had to stop running for fear of throwing up. I’m not sure what that was related to, because I have never had a problem with coconut water after exercise and do sometimes use it as a post run beverage because I like the flavor. More evidence for practicing your sports nutrition strategy before race day!


  • When does one start needing electrolytes?  Generally for workouts lasting longer than 60 minutes.  This is also the time frame where you may need a sports drink of some kind as well.  If you are doing an intense workout or tend to be a heavy sweater you may want to start including them sooner though.   Use your judgment.  I generally don’t take a carbohydrate or electrolyte supplement for any flat run less than 8 miles (which would take me a bit over an hour). For a hill workout I may take electrolytes sooner, but not necessarily carbs.  The recommendation is for 250-500 mg Sodium per hour, which is hard to get in most sports drinks without drinking way more fluid than necessary. This is where electrolyte replacement tabs may come in handy.

My plan:  For the Mt Evans Ascent I plan on wearing a small water belt that contains two 8 oz bottles.  I’m thinking in one bottle I will have plain water and in the other I will put a half tab of Gu Brew electrolyte tabs (1 tab has 10 calories, 1 gram of carbohydrate, 320 mg Sodium, 55 mg Potassium), but I may decide to do electrolyte in both (game time decision!). I plan on bringing 2 more tabs to use to refill the bottle, ½ tab at a time.  I plan on using gels for carbohydrates instead of a sports drink. Since I’ll be running close to 3 hours, I will need anywhere from 54-144 oz of fluids during the race.  Sadly, I have not calculated my own sweat rate (note to self: calculate sweat rate), so I’ll have to estimate. My goal is to drink both of the water bottles (so 16 oz) per hour, which is a little under the general recommendations, but I want to avoid that sloshing feeling.  Especially since I also worry a lot about drinking too much during the race and having to stop at the bathroom!  If it’s cold I may have a hard time with even 16 oz per hour.  Since I tend to drink on the mid-lower side of water requirements, I make sure to really focus on proper hydration after!



For an interesting read on Osmo and hydration:

More interesting info (if you’re a sports nutrition nerd that is):


Disclaimer: I have no relationship with any of the above mentioned products



One thought on “The Not So Simple Science of Hydration

  1. Pingback: Electrolyte Drinks - Summer Exercise Help

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