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A friend of mine asked a experienced long distance hiker, how to drink during such a hike. The answer was, drink 0.1l water per hour and then you are fine.
Is this correct and can anyone add some facts to this answer? Can I stay hydrated over several days with a this given amount of water?
What are factors to drink more water and how much more e.g. (wind, activity, temperature, ...)?
Side question:Are there studies on this topic?
For example, on Mount Everest, the average person needs to drink 4-5L of water each day just so that their body can function properly. You lose water through your breath, perspiration, urine and bowel movements. If you're a big guy that's out of shape, breathing heavy and sweating lots while hiking through the Western Cwm, then you're going to need a lot more water than the skinny little Sherpa that's leading the way.
There may be situations where 100ml per hour is plenty, I can definitely survive on less than that depending on what I'm doing, I managed to go 48hrs straight without any food and water at all while trekking out in the summer heat once, but my body definitely wasn't functioning normally.
It's important to understand your own body and how much water YOU need for certain activities.
This comes with experience, your friend may be able to operate on only 100ml/hour, but that doesn't mean everyone else can. Learn to recognize the beginning stages of dehydration (fatigue, headache, yellow urine) and drink enough to stay hydrated. If you find yourself in a situation where you need to budget water, then do it the same way you would budget fuel on a long drive: go slow, don't over work the engine, keep it cool, and take the easy route.
Dress - In hot hot environments, you will actually retain water better by wearing long sleeve shirts and pants made out of light breathable materials. Also cover your head and neck. There are reasons why you don't see anyone in the Sahara riding camels wearing shorts and t-shirts, and the sun is only one of them. Hiking in shorts and t-shirts will keep you cooler, but so will hiking with a spritz bottle of ice water and a portable fan... Hiking in sweat-dampened clothing may not feel like it keeps you quite as cool, but keeping the sun off your skin will heat your body less, and you won't lose water into the air through evaporation quite as fast.
Breathing - Close your mouth. Breathe through your nose. You're sinuses preform a function beyond producing excess amounts of mucous when you get a sniffle. At the very least exhale through your nose. Leaving your mouth wide open while you hike will dehydrate you just as fast a sweating. Move at a pace that your respiratory system can keep up with.
Toileting - Hold it (if you can). Sometimes when people get moving, other things get moving too. You need to give your large intestine enough time for it to reclaim as much water as it can from your stool. Relieving yourself multiple times will also relieve you of extra water in your stool. In more desperate situations, try to soak up as much of that precious moisture as you can before discharging your bowels.
Recycle... - In extremely desperate situations:
This seems a bit low to me, but there are lots of other factors to consider. The main ones are temperature and exertion/walking speed. Different people also definitely need different amounts of water. One of my friends was nicknamed desert-man as he drank approximately 4x as much as everyone else.
If you are in the UK or a similarly cool climate, then 100ml per hour is not entirely unreasonable (although still on the low side). But if you are somewhere hot, then it definitely wouldn't be enough.
Another major factor is what do you drink while not walking. If you drink 100ml per hour for an 8 hour walk then drink 2L while setting up camp that is very different from continuing to drink 100ml per hour.
I found this interesting article on the topic of cold weather and hydration.
http://www.unh.edu/news/news_releases/2005/january/sk_050128cold.html
In cold weather you lose significant moisture just by breathing the dry air. Even in 100% humidity ( very rare in winter) the cold air can suck moisture from your lungs since it warms up in the lungs and can then absorb more moisture.
IMHO, 0.1l per hour would be the minimum to replace just the moisture lost by breathing in cold conditions. Even if you are super efficient in managing your layers to avoid excess sweating ( very difficult for most people), you will still need more to keep your hydration levels in balance.
0.1l per hour could work if you supplement it with "topping up" at meals. (i.e. when you've got the stove out and are melting water drink as much as possible. ) This matches some advice I've seen from some outdoor adventure programs.
http://www.alpineskills.com/bcsm_sbsv.html
I really feel like there is some piece missing in your friend's advice.
No, 100 ml per hour is way too little in many circumstances. That would mean only 1 l over a 10 hour hike. Anyone that's been on a 10 hour hike, even not in particularly hot or dry weather, can tell you that's not nearly enough.
For hiking in hot desert conditions, 1 l per hour (10 times your suggestion) is more like it. I have done significant hiking in the deserts of Arizona, New Mexico, and Utah, and I can tell you the 1 l per hour is a much more reasonable figure. I usually bring a gallon and turn around when it gets to half a gallon. Sometimes that has been in less than 2 hours. Sure, I could have survived with a bit less than that, but I drank whenever I felt thirsty.
However, trying to survive in such conditions with only 1 l for 10 hours is downright dangerous. After only a few 10s of minutes you'd feel a little thirsty. After one hour, even with drinking 100 ml, you'd definitely feel thirsty. It wouldn't be dangerous yet, but just being uncomfortable is not a good thing. It keeps your mind off of other things you should be paying attention to and can interfere with rational decision making. Besides, you're probably out there or the enjoyment, and hiking around the hot desert when you're thirsty is not enjoyable.
After two hours, even with 200 ml total to drink, you're going to be very thirsty. This still isn't by itself dangerous, but it's starting to get there. Soon you'll feel yourself dragging. Your ability to hike at normal speed will go down.
If you don't turn around NOW, you'll be in serious trouble. Even if you do turn around now, you've got another 2 hours at least to get back, although probably more since you'll be dragging. After 4 hours with only 400 ml, you'll be very thirsty and the dehydration will have some effects on your body. If you make it back to the car after 4 hours and 400 ml, you drink a decent amount then, rest in the car in the shade for a while, you should be OK.
However, if you didn't turn around after 2 hours, you're headed for trouble. You are already somewhat dehydrated, although not enough for long term harm yet. But staying out there with only 100 ml per hour in the hot desert will only degrade your condition. After another hour or two, you probably won't be able to go much farther. Getting out on your own is no longer a option. You are now too weak for that. Your body tries to conserve the water it has, and reduces sweating. This can lead to heat stroke, especially if you can't get out of the sun or your mind is too foggy to figure out how.
This is the beginning of the end. Even if you find shade and maybe not succumb to heat stroke, your kidneys will start shutting down and waste products start building up in your blood. A little while longer, and the kidneys will start to become irreversibly damaged. At the same time, your blood volume goes down, and your heart has a harder time pumping what's left. If you're lucky, you pass out before your heart gives up and you die of cardio-pulminary arrest. In a few days, the coyotes tear thru your clothes and pick the large chunks of meat off your bones, then the turkey vultures pick off the rest, and the rodents gnaw on the remaining bones for the calcium.
Some day someone will find the few larger bones that haven't been scattered, and you've become a statistic. If they discover your pitiful 1 l water bottle, you may even be mentioned in future lectures where the audience is shown pictures of your remains and told not to do what that moron did.
