Sweating The Details

I’ve been doing a lot of work this year trying to figure out how much I sweat in different weather conditions, and on occasion I’ve dragooned a few other Runner’s World editors into my “study.” Yesterday, 10 of us ran in 100-degree conditions, and measured our sweat rates.

Here’s the background: Many scientific experts have now decided that runners should drink according to their “thirst.” They acknowledge that this will leave us 1-2-3 percent dehydrated, but say there’s no strong evidence that this modest dehydration has a negative health or performance effect. And it at least will help us avoid overhydration, called hyponatremia. Others advise that we should run for an hour, weigh ourselves, and then figure out how much we need to drink to cover the sweat loss.

My thought is that runners do virtually everything “by the mile,” so we ought to figure our sweat rate per mile. In a recent Runner’s World, I proposed in a Table that many of us sweat a little more than 3 ounces per mile per 100 pounds (hence 6 ounces if you weigh 200 pounds) at 50 degrees, and more than that as the temperature rises. At 100 degrees, I had figured that sweat rates rise by 50 percent over 50 degrees, hence 4.5 ounces per mile if you weigh 100 pounds and 9 ounces/mile if you weigh 200 pounds.

Yesterday, it was 100 degrees and my sweat rate of 8 ounces/mile was the lowest in our group. Others went up to 15 ounces per mile. (We measured our pre- and post-run weight on quite accurate digital scales, good to the nearest .2 lbs.)

I conclude a number of things from this test. First, in 100 degree weather, no runner can possibly replace his sweat. For example, I was running 8mph yesterday, so I would have to drink 64 ounces, a half gallon, every hour to replace my sweat. And others in our group would have had to drink almost twice as much.

Second, it follows from this and lots of other things that running in 100-degree heat is not a good thing to do. At least not for long periods of time. We covered only 4.5 miles in our test run. In the heat, you should run shorter and slower.

Last, it seems that sweat rates climb really steeply at the high temperatures. Running at 80 degrees is tough and uncomfortable. But running at 90 and 90+ degrees is a whole, different, and ugly experience.