Posted by Dave on July 19, 2011 | 17 Comments
Few topics fill runners with more dread than trying to figure out what and how much to drink for a workout. Should you consume an energy drink? If so, what kind? Or is plain water fine? Should you drink beforehand? Can you drink too much? When the weather warms up in the summer, the questions only multiply: Is it dangerous to train in heat? What about the humidity?
Unfortunately there are few solid answers. There’s no doubt that drinking too little over an extended period of time can be dangerous, but in fact it may be even more dangerous to drink too much. Tim Noakes reported that in 1985, of the 17 runners hospitalized after the 56-mile Comrades Marathon in South Africa, 9 of them were overhydrated: their blood sodium levels were dangerously low. In the 1987 race, three runners nearly died of the condition, and some runners in the U.S. have indeed died of overhydration.
You can get a sense of how much water you’re losing during a run by weighing yourself with no clothes on before and after a run and subtracting: 1 pound=16 ounces of water; 1 kilo=1 liter. It’s best to do it over several days because the amount you sweat can vary from day to day. Over three days where the temperature was in the low 70s and the weather was extremely humid, I lost between 3 and 4.8 pounds in a 1-hour run, averaging 4 pounds. If all of the weight loss was attributable to sweat, that would be nearly a half-gallon! Not all of that weight loss is necessarily sweat, though. You also can lose weight by metabolizing carbohydrates, but Noakes says scientists disagree on precisely how much is lost in this manner. Let’s suppose that 75 percent of my weight loss was indeed sweat: That’s about three pounds or 1.5 liters. Should I drink the full 1.5 liters of water during my run? If I run a marathon in 3.5 hours, should I try to drink 1.5 liters per hour, a total of 5.25 liters?
Unfortunately (or perhaps fortunately) the body can’t absorb water that quickly during a workout. Noakes estimates that the average runner can absorb somewhere between 400 and 800 milliliters per hour. If you are a heavy sweater and consume water faster than that, particularly during an extremely long race such as a full Ironman triathlon or ultramarathon, you risk bloating and severe discomfort. If you tried to consume 1.5 liters per hour during a 10-hour race, you conceivably could have drunk 10 liters more than your body can possibly use!
If you don’t sweat much and still drink a lot, then the problem is potentially worse; this is where potentially fatal overhydration comes into play. Women, because they tend to sweat less than men, are at more danger for overhydration. Slower runners also tend to sweat less and have more opportunities to consume liquids during a race or workout, so they should be careful not to blindly drink as much as possible.
The consensus among many researchers including nutritionist Matt Fitzgerald is that runners should simply drink when they are thirsty. I’m not sure this advice is ideal, particularly for beginning runners. It can be difficult to tell how thirsty you truly are when you are near exhaustion. If, like me, you sweat a lot, you’re going to be losing much more in sweat than you can possibly absorb by drinking during a long race or workout in the heat. That’s not necessarily devastating: Most runners finish races with some level of dehydration, and there isn’t much evidence suggesting it impairs their performance. Indeed, race winners tend to be more dehydrated than other competitors.
But just as beginning runners have more potential to over-hydrate because they are on the course longer than advanced runners, so too is it possible that they may become extremely dehydrated. It takes me almost twice as long as an elite runner to finish a marathon, and it takes me longer to run between each aid station. If I am offered 6 ounces of water at each station, every two miles, that adds up to about 22 ounces per hour at my 8-minute-mile pace. That’s about 650 milliliters, or somewhere in the middle of the 400-800 ml range my body can absorb. If I skip just one station, I’ve only had 16 ounces, or 470 ml. I might be drinking only a little over half the water my body can actually absorb, which is still well short of the 1.5 liters per hour I may be losing on a warm day. For me, it rarely makes sense to skip a water station. For others who sweat less during a run, different advice might apply.
What this means for you: Drinking more water than you sweat/urinate away per hour during a workout can be dangerous. Knowing how much you sweat can help you decide how much to drink, but you likely won’t benefit from drinking more than 800 ml per hour. While thirst is a good guide, beginning runners might want to establish a regular drinking pattern.
A tip for measuring your sweat loss: You don’t have to forgo drinking during a run to measure sweat loss. Just weigh yourself with your water bottle before and after the workout. Your spouse may be confused when he or she sees you naked on the scale with a water bottle, but at least you won’t be thirsty.