The debate about hydration usually focuses on the question of how much you should drink during exercise. And the trend lately has been to conclude that you probably don't need as much as we used to think — in fact, simply drinking when you're thirsty may be good enough in many contexts. But a key point that's sometimes forgotten is that this assumes you're fully hydrated when you start exercising. If you're already parched when you start, you'll have a much harder time tolerating temporary dehydration during your workout.
This is the issue that Eric O'Neal and his colleagues at the University of North Alabama have been studying for the past few years. At the ACSM conference in May, they presented some new results looking at 24-hour hydration in runners after a dehydrating run, and testing which measurements can tell you if you're getting enough.
The study involved 13 runners who ran in hot conditions until they lost about three percent of their starting weight. They did this on three different occasions, and followed a different rehydration protocol each time: either 3.2, 4.2, or 5.2 litres of fluid over the next 24 hours. The middle option corresponded to replacing about 100 percent of their sweat losses within the first 12 hours after the run, and 200 percent within 24 hours, and it also corresponded most closely to how much the runners would normally have drunk.
At various time points during the 24 hours postrun, the researchers assessed hydration status with:
- body weight
- urine color
- urine specific gravity, a measure of urine concentration
- reported thirst sensation
One of the surprises is that body weight didn't really provide much information. When the subjects drank a lot, they peed more; when they drank less, they peed less. As a result, the researchers conclude that tracking body weight isn't a particularly helpful way of tracking how much you're drinking. To me, this is also a good hint that our bodies are pretty good at keeping things in balance — at least within a certain reasonable range (if you don't drink at all, there's nothing to balance).
The most sensitive marker of fluid consumption during the first 12 hours turned out to be urine specific gravity. This is not too surprising, and it's probably worth noting that the study was funded by a company called Atago, which makes devices to measure urine specific gravity. Apparently the testing is very simple and inexpensive, so perhaps it's worth considering.
Thirst actually emerged as a very sensitive and reliable marker by the end of the 24-hour period. The problem is that differences in thirst didn't start to emerge until the next morning, so you might go to bed not realising that you've hardly drunk anything.
My personal favorite is urine colour. It's far from a perfect measure, and there are lots of things that can affect urine colour — beets and vitamin C are two that endurance athletes might notice. But in general, if you pay attention to subtle changes in your urine colour, it'll give you a warning if you're less hydrated than usual. Heck, USATF and many other sports bodies offer urine colour charts, but I'd be more inclined to watch for changes from your usual colour rather than worrying about the absolute shade (unless you're at number 8 in that chart!).
The differences in urine colour in O'Neal's data are relatively subtle, on the order of one point on the 8-point scale shown in that previous link, so you have to be paying attention. In the end, though, that's probably the most important point anyway: whatever cue you decide to use, you have to be paying attention. The middle hydration level (replace 100 percent of sweat losses within 12 hours, 200 percent with 24 hours) turned out to be adequate to return everyone to normal by the time they were ready to run again the next day. That's not a hard goal to hit, so it seems likely that those who miss it just aren't thinking about drinking between runs. For runners in particular, who don't tend to drink much while they run, that's a mistake.