The day before a race, many of us (including myself) like to stay close by our water bottles. We make a point of drinking a bit more than normal, and, as a corollary, peeing a bit more than normal. But is this necessary? Does pre-exercise hydration need to start so early? Is it too late if, like Jerry Seinfeld, you wake up in the morning and feel so thirsty you swear you must have already run a marathon?
A study from the University of Guelph investigated just how quickly the body can rehydrate. Athletes were instructed to avoid fluid and arrive at the lab in a dehydrated state. They then drank two 300-mL boluses of fluid within 15 minutes, and waited 45 minutes more. Hydration status was assessed by measuring urine specific gravity, or the “concentration of stuff” in a urine sample (~More stuff = more dehydrated). After just 45 minutes, all the athletes had returned to a hydrated state.
Of course, you may not want to drink 600mL in a span of only 15 minutes before your next race – unless you have some sort of “express pass” to the front of the porta potty line – but the point is that your body is able to hydrate itself pretty quickly. The recommendation we give in the lab is to gradually drink about 400-800mL of fluid in the 1-2 hours before exercise.
You also probably don’t have a refractometer on hand to measure urine specific gravity, but you can get a decent idea by looking at your pee. Get up about an hour before your next morning run and go pee. Your pee will probably be yellow to dark yellow, and chances are you’ll feel a bit thirsty. Now gradually drink a glass or two of water over the next 45 minutes or so, and go to the bathroom again before you leave for your run. Your pee should be a lighter shade of yellow, or almost clear (Feel free to do this and report your findings in the comments section! No photos please.).
No matter what distance you are running, proper pre-exercise hydration is a must. About one third of the athletes that come into our lab arrive at least mildly dehydrated. A 2010 meta-analysis found that mild pre-exercise dehydration was associated with lower maximal aerobic capacity and impaired endurance performance.
Don’t care about aerobic capacity or performance? Even if you run for pure enjoyment, think about this: In a study investigating dehydration and perceived effort, athletes exercised at the same moderate intensity on 4 separate occasions, each time at a different level of dehydration. The more dehydrated they were, the harder they perceived the exercise to be, even though the actual intensity was the same! Being hydrated before exercise allows for maximum enjoyment!
For athletes tackling longer distances like a half marathon or marathon, making sure you’re hydrated before you toe the line means less chance of dehydration becoming a problem during your event. For a marathon you may want to go one step further and develop a hydration outline through practice in training runs, especially if you have a very high sweat rate or if conditions on race day are hot. But hydration during exercise is a topic for another day…
Above all else, remember: Pay attention to your body! Use recommendations and guidelines in combination with personal experience and your own feelings. And remember that more is not always better. I work with a lot of athletes and some actually come into the lab over-hydrated. They usually complain about feeling bloated or full, but feel that as long as they keep drinking and ensure they don’t get dehydrated, they’ll be fine. As with most things related to the body, a balance needs to be struck. You don’t want to start a race feeling absolutely parched, but you also don’t want to miss the start because you’re in the porta potty!
Carb loading before the big race: Who needs to do it? How much should you eat? When should you eat it? And maybe some more Seinfeld references!
Logan-Sprenger HM. Unpublished data, 2010.
Gigou PY et al. Meta-Analysis of the effects of pre-exercise hypohydration on endurance performance, lactate threshold and VO2max. Med Sci Sports Exerc 2010;42:S254
Moran DS et al. Evaluation of different levels of hydration using a new physiological strain index. Am J Physiol 1998;275:R854-60