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I usually run for an hour in hot weather every Saturday (I'm in the tropics, so it's usually 30 degrees or above, with around 90% humidity). I weigh myself before and afterwards, and usually I'll lose about a kilo in weight in that time - which I assume is mostly water - I sweat like a pig.

(Currently I'll drink about 600 ml of water while running, plus another 400 ml when I'm done, so I guess I'm actually losing about 2 kilos through dehydration during that hour. I weigh about 75 kg, so that's roughly 2.5% of my body weight that I've lost in that time.)

The people I run with don't drink anything while running. They're possibly better habituated to the environment than I am (mostly locals, while I've only been out here for a couple of years) - what could I do to reduce my fluid intake while I'm running?

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30 degrees Centigrade = 86 degrees Fahrenheit (Just FYI) –  JohnP Feb 26 at 14:27

2 Answers 2

I wouldn't worry about reducing my water intake during a run in hot weather. Just because your fellow runners don't drink anything doesn't mean it is a good idea for you (or them). Particularly under the conditions you are describing I would recommend you keep drinking water. It will help you perform better.

As to the weight loss during the run: are you using a medical scale? If not, do no trust the numbers. If you want to track that, weigh several times. To give you an example of what a non-medical scale can do against you, I tracked my weight every morning when awaking for nearly a year.

I found that often stepping off then on would vary by half to a full kilo, sometimes more. If I moved the scale a few inches I could get the same effect. I could weigh, go back to bed for twenty minutes, then weigh again and suddenly "gain" weight - to the tune of 2% or more of my bodyweight.

Ultimately, the first barometer to use is how you feel. If you cut back on the water does your performance suffer? How does your recovery change, if at all? In every group I've been in (in and out of the military) where significant physical activity took place in hot conditions (dry or humid), those who hydrated performed and recovered better.

I would go with recording your metrics, adding subjective such as how you felt during and after the run for a baseline of two weeks. Then increase by 50% and repeat for two weeks. Then go 50% below baseline for another two weeks. Then compare how each phase worked for or against you. From there make changes and track the results until you arrive at what seems to be optimal for you.

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Why would you want to cut your water intake? The sweating is your body attempting to shed heat and cool itself. If you deliberately cut your water intake, you are short circuiting your cooling efforts.

Also, you are not only losing water weight. You are also using up glycogen, which has a weight to it. An hour run is long enough you will see a significant change in your weight due to both water and glycogen use.

As you get more adapted to the climate, your need for water may diminish as a natural consequence, but don't try to force the issue. You may just end up introducing heat exhaustion/stroke instead.

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Thanks. The less water you have to drink, the less you have to carry. Particularly in the place I live, where there isn't much access to drinkable water while out running, any way to reduce your requirement would save weight and allow me to cover a greater range. I can see there's potential for negative effects, but I'm interested to know if there are ways to accelerate the adaptive process. –  JamesF Mar 3 at 15:56
    
@JamesF - I wouldn't worry about it. You're talking about a pound or two of water, that's not going to impact "your range" as much as not having the water will negatively impact both length and quality of your run. I suggest a camelback pack, that's what I use on long runs/rides. –  JohnP Mar 3 at 16:44
    
@JamesF having had a bad experience of not carrying any water during hot weather and trying to run a half marathon, I can tell you that now I'd rather carry a camelback with 2litres of water than going without. The extra weight is good for training and it will enable you to go further. Also don't forget to either mix in some electrolytes or take a salt tablet with you. –  petrfaitl Mar 8 at 23:44
    
I ran a half marathon with a FuelBelt on (four bottles; two with water, two with Gatorade). I PRed that race, if that makes a difference. The weight of small water bottles is negligible, and I'd rather trade in a few seconds' worth of time for not collapsing from dehydration. –  WeirdFish Mar 11 at 13:09

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