How Much Do I Sweat During Strenuous 5k & Duathlon Exercise – & So What?

Drops of sweat
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I’ve always known I sweat quite a bit and generally attribute that to trying hard. It’s natural right?

I’ve also seen that sweat produced in a gym and thought, “Well it’s quite a bit, but if I bottled it all up it probably wouldn’t be that much.” I’ve sort of lived by that mantra for 20 odd years. Of course I do hydrate and take hydration seriously I just never thought it worthwhile quantifying it for me.

Well I’ve just finished a strenuous turbo session which went on for about 50 minutes but if you knock off all the warm ups, cool downs and rest intervals the strenuous bits were 30 minutes. Well I weighed myself before and afterwards and was amazed that I’d lost exactly 1kg. Which must be 99% water. A litre of water weighs near enough 1kg. So I sweated 1 litre in half an hour.

Well sort of. Being on a turbo does not properly mimic the cooling effect of moving through the air either by running or cycling. Movement of air over the body will create greater evaporation and hence greater cooling meaning that if I were on a real bike I would not sweat so much. Also I could not keep up the high level of exertion for a complete hour (although in a race I bet it wouldn’t be too far off).  Even so a 1kg weight loss for 30 minutes of real effort was quite high.

After a bit of research apparently Ironmen lose 1.1litres/1.1kg per hour when racing and so need to replenish that much. Wow! 7-10 hours that’s a lot of liquid.

I guess in the real world that is probably about what I would lose per hour – I might even lose more as shorter events are more strenuous (other things being equal).

Anyway apparently it’s not a problem for you to lose 2% of your body weight. That’s 1.5kg for me which approximates to about 100 minutes of race time. food for thought.

So I guess for hour long events I can still get away without re-hydrating but for a 2 hour standard duathlon I might have to give some more thought to how much I drink. Might even be the route to resolving my cramping.

& To Answer The Question “How much do I sweat during a ….

1.  A 5k – Probably less than 0.5litres ie a less than a pint

2.  Sprint Duathlon (5:20:2.5) – Probably about 1 litre or 2 pints

3.  Standard Duathlon (10:40:5) – Probably 2 litres or 3.5 pints

  • Fluid replacement and dehydration: how much is enough? (

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