There’s an interesting discussion about fluid loss while running on Marathon Talk this week. (At 50:20 in the podcast.) Dr. Mark Hetherington explains how weight lost during a long run doesn’t indicate the amount of fluid you should be taking in during the run. He uses a 60 kg female runner as an example. If she weighs 57 kg after running a 3 hour marathon, only 1.5 kg is actually water lost from the vasculature. 0.5 kg is the weight of glycogen and fat that is used up during the run. And 0.75 kg comes from water that wasn’t in the vasculature to begin with. It was stored in the muscles and is released as the glycogen is burned. (1 gram of glycogen releases 2.5 grams of water.) The last 0.25 kg loss comes from water that, again, didn’t start out in the vasculature. Water is created as glucose is oxidized. So, basically, if this runner drank 3 kg of water during her 3 hour run, she’d be taking on much more than she needed. The risk there is hyponatremia. Furthermore, Hetherington goes on to say that if she didn’t drink anything during that time, she would only have dropped 2.5% of her body weight, which isn’t significant as far as dehydration or performance goes. He does point out that he’s talking about a fit runner on a cool day. SO, what does this mean for the ultramarathoner running in the summertime? Sweat Science has a recent piece entitled “Drinking only to thirst (no more, no less) improves performance” that seems to go a long, and very simple way, towards answering that question. Great, now I can spend the day not feeling badly about forgetting to weigh myself before all those long runs. I’ll use to the time to feel badly about locking the cat in the garage all night.
PS. There’s also a fun interview with Ellie Greenwood on that podcast.