Calculating water loss

[wjr]

With an ultra planned for next year (65m), and a build up plan starting with the Rosedale 30m this coming weekend, I decided it was time to get a little more serious about hydration.

So, today I set out to calculate weight loss through sweat. Over 12.4 miles I lost 5.5 pounds. That seems to equate to 200ml per mile of fluid intake required.

I've either miscalulated or have previously significantly underestimated my hydration needs.

Does this rate seem about right to anyone else who has worked this out?

[Derby Tup]

What sort of terrain is the 65miler over? Are you running (most of) it or will it be a walk / run? What time of year is it?

tbh it all sounds a bit technical to me. However 200mls / miles sounds a lot, especially if you're running in winter. That equates to 6litres (c10pints!) over 30miles

How much have you drunk doing the Yorkshireman? 30 miles isn't much further

[wjr]

Hi DT. On the Yorkshireman I had around 2+ litres. But neither of those two runs ended terribly well. The goal next year (June) is The Wall run (mostly off road along Hadrian's Wall). Over this distance I suspect there'll be some walking, though strategy yet to me worked out as this distance is certainly new territory

[Derby Tup]

iirc you've set off a bit sharpish on the two YORMs you've done and 'blown-up' a bit. Pacing becomes more and more important for further you try to run. I'd suggest you perhaps set off steady on the 30miler and listen to your body re: drinking

I'd plan to run / walk the Wall, especially if its a big step-up in distance when you do it. For me finishing is everything!

[Mike T]

With an ultra planned for next year (65m), and a build up plan starting with the Rosedale 30m this coming weekend, I decided it was time to get a little more serious about hydration.

So, today I set out to calculate weight loss through sweat. Over 12.4 miles I lost 5.5 pounds. That seems to equate to 200ml per mile of fluid intake required.

I've either miscalulated or have previously significantly underestimated my hydration needs.

Does this rate seem about right to anyone else who has worked this out?

Remember weight loss is not the same thing as being dehydrated - using fat/carbs generates water, and using glycogen liberates water - you can lose 2% or so of body weight and not be at all dehydrated - indeed if you do not lose weight during a long run you have drunk too much. This is useful:
http://www.westhighlandwayrace.org/info/medical.htm

[wjr]

Certainly true for the first DT. The second was better, but I hadn't changed my diet in the few days leading up so whilst pacing was much better I ran out of steam. I'm running the ultras with my brother and a pal, so with three heads at least one should be thinking clearly at any point in time, thus hopefully ensuring common sense prevails all the way through!

[wjr]

Thanks Mike, a useful read.

[Splatcher]

It's easy to get too technical & forget your body's got a good way of telling you to drink more - you get thirsty. But...I find that thirst doesn't cut in properly when I'm running, in that I don't notice I need a drink. However once I start drinking, I can tell how much I need, & will drink a lot when I need it. So, remember to drink often enough, & have enough available. I try to make sure I drink at least every hour, usually more often than that. Thirty miles over the hills usually means I'll get through about 1.5 litres, as long as the weather's not so hot I'm sweating a lot. Then all calculations in advance are a waste of time (imho). Just keep drinking, as long as you feel thirsty. And don't drink higher volumes just because you think you ought to. If you need to stop to wee you've drunk too much - or at least that's a good rule of thumb for runs up to 12 hours in my experience. I've only done a couple of longer runs so can't really comment on them. Weighing a few pounds less after an ultra is to be expected, it's not necessarily a sign that you're so dehydrated it's affected your performance.