Weight loss and water loss

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.

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16 Responses to Weight loss and water loss

  1. footfeathers says:

    I’ve been a light water drinker for years. Of course, I adjust slightly for temps and effort but typically drink 12-15 ounces per hour in long races. I’ve never considered the science of it; it just feels better.

    Not sure whether you’ve mentioned it in a previous post, but are you doing any downhill training (for WS)?

  2. lizahoward says:

    Not as much downhill training as I’d like. Repeats on shorter hills. We don’t have anything really long and steep here.

  3. olgav100 says:

    I hope you don’t follow this research at WS – you’ll get pulled off even if you feel like a daisy. They could careless what you say about your oxydizing processes. The have a number to work with. Just take some salt so you don’t end up hyponatremic. And don’t overdose either.
    OK, you have your work cut off for now, I’ll step down:)

    • lizahoward says:

      Hi Olga! So you’re saying I should drink water during Western States and leave my copy of the British Journal of Sports Medicine at home? 😉
      Don’t worry I’ve got a good handle on how much water (electrolyte drink) I need to take in. I’ve been practicing. It will be interesting to see if the weighing in process changes with research showing weight gain isn’t predictive or indicative of hyponatremia.

  4. SteveQ says:

    The studies that have been done were done in events of 2-6 hours. I doubt they have any relevance for running Western States, though they might for training runs. Still, you have to plan your hydration for the race itself.

    • lizahoward says:

      Agreed. It’d be much easier if there was some “output” guideline. Something like “Make sure you’re urinating (clear urine) once every 2 hours.”

  5. Tony Maldonado says:

    I agree with foot feather, go with what feels better. Was it Dr. sheehan who said ” we are each an experiment of one.” ?

  6. Clea says:

    Yeah, I listened to that too and I think I probably drink too much. I think when I feel crappy after a long run, it’s because I’ve diluted all my salt so much. Don’t you just LOVE that podcast? I am such a dork, I get so excited when a new one comes out, and I love that they have been so long lately!

  7. Lisa says:

    Good info Liza. I was just wondering how much water to take in during my next 12 mile run. It’s cool here in Portland, actually perfect after a horrendous Winter and Spring. For my 10 miler last week I drank 20 ounces of water. I guess I just drink when the urge arises.
    At what point do you take electrolyte supplements? I bring GU just in case but for now I eat dates for energy and they work. I think even for a 31 miler, if I drank an electrolyte drink before the run and dates and GU during, I should be fine.

    It’s all about doing the long runs to see what works out right??
    Thanks for posting that info!

    p.s. Had an adrenaline pumping day at work. My pt decided to vagal down to a heart rate of ZERO. It was loads of fun as about 20 people were there to help me. Pt did great- thank goodness he was young (44) and otherwise pretty healthy and fit. Do you remember those hospital days???

  8. John says:

    Good discussion. For me, I’ve learned that I need to consider many factors. Distance, effort, weather conditions, and access to water. I’m afraid to only “drink to thirst”. Nearly got into trouble on first trek into GC doing this. At a long distance, there is so much mental work going on, it can be hard to discipline yourself.

    • lizahoward says:

      Agreed. My mind isn’t processing much of anything later in a race. I can’t tell you the number of times I’ve spent 30 minutes before an aid station repeating what I need to do when I get there — only to leave without doing it. (Now I make index cards for my crew with questions to ask me. “Do you want tylenol? Do you need a kleenex?” I don’t know that I’d take in enough fluid during a 100 miler if I didn’t have a plan.

  9. lizahoward says:

    I think you’re exactly right; Each long run is a chance to figure out nutrition and fluid intake. Took me 3 years to get that through my thick head and then another 2 1/2 to actually act on it. Right now I’m practicing my race nutrition each time I run no matter what the distance. I figure I’m training my stomach to get used to processing what I’m putting in too. I use an electrolyte drink in my handheld and take one GU every 45 – 60 minutes. I’ve been adding in some electrolyte tabs on the hour as it’s gotten more hot and humid here. The electrolyte drink has 100 calories and I’m using it to decrease the number of gels I have to take. Seems to be working well.
    I made a patient vagal down to 2o starting an IV (poorly) when I was a paramedic student. Oops.

  10. Johana says:

    Pretty interesting podcast and case here. Any way I never used running as a weight loss method. I use it just to keep by body as healthy and I can so I can manage to run my daily diet programs. Also I like “drinking only to thirst (no more, no less)” and should be a must in any training program that you are using.

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