How Much Do I Need To Drink?
It's hot out there, so how do you stave off dehydration and what happens if you don't?
Posted: 2 June 2009
by Jon
It's summer - at last - and with the heat comes a real danger
of dehydration, so how do you know whether you're dehydrated and
how much water should you be drinking when you're on the hill?
Why does it matter?
There are tests that show as little as five per-cent dehydration
can decrease your physical performance by as much as 30 per-cent.
That's the last thing you need when you're on the hill working hard.
As things get worse, dehydration massively increases the risk of
heat illness at which point you're you're talking headaches, cramps,
fatigue, vomiting, coma and death...
You can lose 2% of your bodyweight through dehydration in as
little as one hour, in 2-3 hours that could be 4-6% and after four
hours or so, 7-8% at which point you're at serious risk of
potentially fatal heat stroke.
In between heat exhaustion will massively reduce your ability to
function with fatigue, nausea and high temperatures kicking in.
| As little as 5% dehydration could lead to a 30% reduction in performance. Any more increases the risk of heat exhaustion, heat stroke and, eventually, death. |
Prehydrate or dry...
Your thirst mechanism lags behind your actual hydration level, so
by the time you're feeling thirsty, you're already dehydrated -
there's a time delay if you like. Keep an eye on your urine, it
should ideally be clear or at worse, a light straw colour.
Remember the maxim: 'A good mountaineer, always pees clear.' Okay
it's trite but it's a good guide.
Your body will function best if you're already hydrated before you
set off, so drink little and often over the 24 hours before your
trip. Not only will you start hydrated, but it improves your chances
of staying topped up as the body absorbs fluid most effieciently from
a comfortably full gut.
| By the time you feel thirsty, it's too late - you're already dehydrated, so top up before you head out. |
How Much Fluid Do You Need?
On a hot day, you can lose between 500ml and 2000ml - yep, two
litres - per hour when working hard. Ideally you want to replace as
much as that as possible. We'd suggest an absolute minimum of
500ml per hour and ideally more like one litre per hour as
a starting point, more on really hot days or if you're working hard -
that may mean you need to top up your supplies during the day, but
the pay off is worth it.
| Ideally you should aim to drink around one litre of water every hour, more in very hot conditions or if you're working particularly hard. |
What Should You Drink?
Water's an excellent start, but you can also use sports drinks
which include energy and, in some cases, replacement electrolytes and
minerals which you're also losing through sweat.
If you do use a sports drink, choose one based on glucose polymers
- maltodextrins are the things to go for - and in hot weather mix the
drink more dilutely than the instructions suggest.
Over-concentrated drinks containing too high a concentration of
carbohydrate will slow down fluid absorption. Finally avoid
sugar-heavy soft drinks for the same reason and diuretics like tea
and coffee.
| If you use an energy and electrolyte replacement drink in hot conditions, mix it more dilute than the instructions to speed up absorption. |
How Should You Drink It
Little and often is the most effective way of absorbing fluid
efficiently, rather than just sluicing a litre at a time down your
throat, in which case the fluid which can't be absorbed will be peed
out rapidly.
The easiest way of achieving that is with a hydration
system that you can sip from on the move. As a bonus a single
hydration bladder could hold up to three litres of fluid, far more
than any single bottle. Virtually all modern packs have the facility
to mount a bladder and tube conveniently.
| Drinking little and often - every 15 minutes say - means fluid is absorbed faster. Hydration systems are ideal for this. |
Cold Water Absorbed Faster
Finally, some research shows that your body absorbs cooler water
faster, so Camelbak suggests that before a hot day on the hill, you
half fill your bladder with water then freeze it. The big chunk of
ice will melt slower than ice cubes and your water will stay cold
longer.
| Pre-freeze your bladder before heading out as cool water is absorbed faster than warm stuff. |
More Information
See the links to previous OM health and fitness articles listed below. For gear advice see our review system or ask in the OM forums for friendly input from other OM members.
Discuss this story
this seems to contain a number of conflicting points and is based upon a body weight of 100kg (2% being 2l possible lost per hour). what medical evidence is there to support, or condemn, avoiding tea and coffee. why do you get a cup of tea when you give a pint of blood? or nice sweet squash and a biscuit? to deteriorate performance by 30% you need to sweat 5l - wouldn't someone actually notice this? unless you're in the sahara perhaps or does it get this hot in the lake district? drink a litre an hour. fine for an hour an a half then what do you do with the ice as suggested by camelback? clear urine. a good base indicator but can be other colours depending upon what you're drinking and eating.
Posted: 02/06/2009 13:12
How much do I need to drink? Well usually about 4 pints and then I feel a little drunk and stop. A real lightweight aren't I and not in the backpacking way. Hey! A new category, UL drinking! Anyone else out there a UL drinker. I must qualify this, I used to manage a 15 pint night, but I left Uni and slowed down. I personally have a record of over 9 litres in one 5-6 hour walk yet still being heavily dehydrated. I mean over 24 hours without passing water followed by... well it was not clear or straw coloured. You can get a medical chart showing the colour of urine with the indication of your hydration level based on the colour. It also allows you to know a little about your health too I think. Colour range from clear to almost an alarming orangey-red colour. Not a very nice colour chart and not one for your bedroom.
Posted: 02/06/2009 14:27
You've been eating too much asparagus, Ttg. I could very easily sweat out 5 litres!!
Posted: 02/06/2009 14:33
"I personally have a record of over 9 litres in one 5-6 hour walk yet still being heavily dehydrated. I mean over 24 hours without passing water followed by... well it was not clear or straw coloured." 
Blimey, that must have been a pretty intensive 5-6 hour walk.
Posted: 02/06/2009 14:35
As for getting that hot in the Lake District last Friday on Place Fell I recorded a temperature of 29 degrees in the sun and out of the wind. Taking the anemometer out of direct sun saw the temperature drop to about 27 degrees with windchill knocking it down to about 23. The coldest I recorded was 17.5 in the full wind right on the summit. I drank about 1.9 litres and it wasn't enough. Fortunately Side Farm sold cold drinks and ice cream. Replenishing isn't always easy and I saw one family filling up their .6l Sigg bottles from a run-off just above the path running round above Ullswater. I even resorted to soaking my cap in a stream and putting that on my head. Not as effective as I'd have liked.
Posted: 02/06/2009 15:42
This article reads like an add for hydration systems. After having a quick look round I found this page on camelbak's website. I'll leave you to workout where all the information came from. If you get past the fact that if don't drink water you'll die, what do you need to know. Well say if your walking in the uk. If you listen to your body and drink when your thirsty you'll be fine. If you want to know more have a read of the following:- Myths and also another poiint of viewRight, I'm going to be a bit one sided here for a bit of balance. Do you 'need' a hydration system? Well, no. If you really can't stop for a drink there's loads of water bottles with pull tops or even straws and almost as many side pockets, chest pouches and wrist loops to deposit them in. So what are the problems with hydration systems? 1. People don't tend to notice they running out of water. 2. They often leak, especially when placed in the back of a vehicle. 3. They need careful maintenance. Turn your back for 5 minutes and they'll turn green and walk away. 4. When you put your pack down, the bite-valves will find a way to drop into something unspeakable. If you want a useful tip, it's don't run out of water. To that end water purification is a good idea. Something like a 'drinksafe-systems' or Chlorine Dioxide liquid treatment. There are still lots of streams in the british uplands which you can drink straight from, but it takes judgement and if you get it wrong the results won't be pleasant! Oh and sorry for the rant...
Posted: 02/06/2009 17:59
so if it's a bit warm and you're out for a walk how much more water do you drink than you normally would, say at home, during the same day? have you measured it kate? a gym would be the perfect place to try to calculate your required fluid intake.
Posted: 02/06/2009 18:17
> If you listen to your body and drink when your thirsty you'll be fine. People's thirst perception varies (scientific studies can only ever be population samples). Mine is very poor and late, and I know this from many years experience, so I need to be careful to keep drinking in hot weather. I know that my performance drops if I don't, and that it takes a while to recover. I won't die, but I won't be feeling too good. As for volumes, consider this anecdote from Dr Arnie Baker, author of Bicycling Medicine: "I once went on a desert trip with my wife and rode 100 miles. Both of us needed to replace almost one-third of our weight in fluids that day. I drank 45 pounds - almost 5 gallons (US)..." That's 20 litres...
Posted: 02/06/2009 18:53
> People's thirst perception varies That's not to say that there isn't a lot of nonsense talked about hydration, especially the '8 glasses a day rule'. If it's hot and you're sweating a lot, you'll need to drink more. If it's cold and raining, you won't need to drink as much. If you're hallucinating and your tongue is sticking to the roof of your mouth, you probably need to drink more...
Posted: 02/06/2009 18:58
Not quite yet cp.  > People's thirst perception varies True what I said was a generalisation. I think what really gets me is the suggestion that people are do something 'wrong' or 'dangerous' by just drinking when they thirsty or eating when they are hungry. Again this is a generalisation about healthy people not trying to run across desert in summer. I guess what's interesting is the psychology when people don't listen to their bodies. Maybe they don't want to slow the group or they'll just get to the top before they'll stop... And as for sweat. Yeah the body will sweat what it needs to, even if that means gallons. Also worth a read
Posted: 02/06/2009 19:28
> Not quite yet cp. That's weird; I received the notification at 18:57. A ghost in the machine... > I think what really gets me is the suggestion that people are do something 'wrong' or 'dangerous' by just drinking when they thirsty or eating when they are hungry. Yeah; provided you know your body and how it works, just do what you know works for you. I think a lot of this stuff comes from the growing trend to be 'performance athletes' all the time, and the associated striving for optimal nutrition and hydration. What's wrong with a bottle of pop and a banana butty?
Posted: 02/06/2009 19:48
"What's wrong with a banana butty?" with marmite.
Posted: 02/06/2009 19:55
"a bottle of pop and a banana butty? " you can probably buy a powdered very expensive version of that called "sports performance drink". may be the marmite in the powder that makes you run...
Posted: 02/06/2009 23:19
I've suffered from some pretty severe dehydration and a side-effect was permanent damage. However, some of the recommended amounts of stuff to drink are ludicrous! Hyponatremia is a serious risk with some of them - you're simply flushing all the salts out of your body if you just keep drinking water. I'd say just be sensible and don't set out with no water if walking all day in the roasting heat, and not to chug down litres if you're barely sweating. One overlooked side-effect of being dehydrated is the inability to sleep, and can really screw you up if you don't sleep for a few days in a row. The tea one has confused me for a long time: my octagenarian grandparents drink literally nothing but tea. How come they haven't shrivelled completely? There's not a lot of moisture in food, so if tea dehydrates, what on earth is going on.
Posted: 03/06/2009 10:16
Hi Kate,interesting. I wonder if I have hyperhydrosis? Has this been clinically diagnosed? Have you noticed it getting worse as you get older? I'm fine on descents and level walking but once I start climbing I start to sweat buckets. I pretty much have to use a head sweatband or headwear to prevent my eyes being stung by the streams of sweat pouring from my brow. So I slow down. However it does seem to be getting worse (as I get older). According to my Mum it does get worse as you get older. However she was talking about body temperature. She and I tend to have hot bodies which might account for the excessive sweating once I start to exercise. Which is why I asked if you had a clinical diagnosis and if so what led your doctor to this diagnosis?
Posted: 03/06/2009 10:38
So what are the problems with hydration systems? 1. People don't tend to notice they running out of water. 2. They often leak, especially when placed in the back of a vehicle. 3. They need careful maintenance. Turn your back for 5 minutes and they'll turn green and walk away. 4. When you put your pack down, the bite-valves will find a way to drop into something unspeakable.
1 - True you can't see what is in the bladder when it is in the pack but you can't see what is in the water bottle neither when it is in your pack. When drinking from a water bottle you can tell how empty it is but chances are you only bother registering it when it is nearly empty just about the time you realise the bladder is getting harder to suck out water as the water level gets close to the level of the outlet.
2 - They often leak - Camelback and Platypus are prone to leaking yes but Source ones are infinately better. I remember a time when we used those 6 sided plastic bottles from gear shops. They leaked all the time and only when the Lexan nalgene bottles came along did they become better. It has taken a while but source now produce more reliable bladder systems. 3 - Need careful maintenance, I guess you have never had tainted taste in bottle before from the last flavoured drink or a nasty taste when the plastic bottle has had water left in it for a long time. Also, I guess you have never had a source bladder which really does keep down the growths. I have had platypus and camelback before that went pretty nasty. I now have had a source widepac for longer than any other bladder system and have not seen any growth or taint to it despite never looking after it. I have left water in it for 3 months before now without any growth developing. Source have IMHO opinion have got the growth problem sorted. 4 - Funny how bite valves always seem to seek out dirt and sheep Sh*t. Probably why most now supply with covers for them. Hmmm! Is that another problem with bladders solved? Water requirement does vary I can do a 7 hour walk with only a few sips of water and other times I have emptied a full 3 litre bladder and still feel thirsty. I find that I never need the toilet on a walk but at the same time the first time I drink after a walk, say at the pub at the end, I go to the toilet shortly afterwards and am not dehydrated. It really needs people to have common sense. I used to only ever take 1 or 2 litres (1 in winter and 2 in summer) That used to leave me with massive headaches in the evening. I now have a 3 litre bladder, 2 x 1litre siggs and 1 x 1litre nalgene. Depending on the conditions I now take any number of these. That leaves me with from 1 litre (or less if not full) through to 6 litres. I also use common sense and fill up if necessary from suitable mountain streams or springs. There are many in the Lakes to choose from that have served me well and without any problems. One is a nice spring just up from the path from Esk Pike towards Wasdale way. From Esk Pike path you head slightly up hill at the point the stream crosses the wide path. It comes out from some rocks. Clear and cold in any conditions (except excessive cold or drought I guess).
Posted: 03/06/2009 15:56
Hi Major Cynic
I've not had a formal clinical diagnosis - I inherited the condition from my father who was a doctor, albeit a haematologist, who diagnosed me. As far as I'm aware it's not got any worse with age (I remember removing the surface polish of a table during my PhD viva through the sweat coming off my forearms onto a cotton blouse, when I was about 26 - warm day + anxiety = major sweat!). It does get a lot worse during physical exercise, but even walking gently can bring on a sweat if it's remotely warm outside. I spend much of the summer feeling slightly damp, even when sitting still in my office doing nothing but browsing on OM. But though discomforting and sometimes embarrassing, it's not going to kill me, and I've just learned to live with it (avoid wearing certain colours, ensure you take on board enough liquids - and salts, reassuring the gym staff that I'm not going to expire after 5 minutes on the exercise bike). In my case - and my dad's - the hyperhidrosis is / was combined with an inability to keep warm during the winter when being inactive. I've never actually discussed this with a specialist (not sure what the medical equivalent of a central heating specialist would be called), but Dad and I felt the conditions were linked as a kind of malfunctioning heating syndrome!
Posted: 03/06/2009 16:15
"There's not a lot of moisture in food" yes there is. food is mostly water. if you believe the you should drink 2 litres of water a day (8 cups) which is touted by anyone who believes such things then you and they have been misled by the touters who conveniently leave out the last part of the statement which is "which is mostly obtained from the food you eat" nearly as bad as the "contains antioxidants/destroys those nasty harmful free radicals" and "detox" fools. this conveniently ignores what's on the next page in the textbook that the nasty free radicals are what the body uses to fight infection and detox - well nature made a big boo boo in giving us a liver and kidneys. "but you can't see what is in the water bottle neither when it is in your pack" why on earth are you carrying it in your pack? you can't see anything when it's in there. you do see when you take it out of your pack to take a drink. i carry it in a side pocket so i don't even have to spend those precious seconds stopping to take a drink. neither is right or wrong. you need something to carry water in. you don't need a bladder to do so.
Posted: 03/06/2009 17:45
ttg it's true, as I said I was being one sided. But the whole point is that spending £25 for a plastic bag with a Catheter is not going to stop dehydration. People can and do just fine with a few PET bottles.  As for you points:- > bother registering it when it is nearly empty  I think this might depend on who you are... if you got water in more then one bottle it really should spell it out to you. > Source ones are infinitely better. Between makes, mileage may vary. But still be weary about compressing these things. > tainted taste in bottle before I've never seen a bottle grow algae before, but have seen this in both source and platypus tubes (not Camelback but doubt they're better)(if you look after it shouldn't be a problem) > most now supply with covers Proves a point really. Solves a problem if you remember to use it... (to be fair you could have that problem with pull-top bottles fix to the out side of a bag)
Posted: 03/06/2009 17:49
Source are cheaper about £17 -19 for a 3l I think. If you work out how many times it is used before replaced then I am sure it would account for a lot of 56 pence 500ml water bottles bought from the supermarket. Afterall these don't last long and often get thrown after a trip or walk. Not environmentally friendly even if recycled (reduce, reuse, recycle in order of preference). PET bottle burst easier than bladders. Sigg do not burst easier than bladders but these lose their inner coating and Mine all have a tainted taste (kind of metallic). Surprised about seeing algae in source, really not been my experience nor anyone I've known with them neither. Platypus and Camelback have all had feathers of growths in their tubes with me before now. I agree with you that people have to buy the container, fill it, carry it and use the contents to stay hydrated. No system works if someone does not use it through the course of the activity. You don't even need one system if the streams where you are going are clear and frequent. I used to scoop water up by hand from streams in the Lakes as a kid and I am only in my mid 30s. Never been ill in my life from stream water in the Lakes. Indeed filled up last weekend from a stream that had run out of a tarn too. Considered a big no-no from those mag writers who seem to promote water purifiers on behalf of those companies. Drink if you want to drink I think is one solution. Just make sure you are able to do that at the time you want to by whatever means makes you happy. BTW is it just me or have we already had this debate about 2-3 times in the past year alone? I thought we had tied it down to a compromise, a tie if you like. No system is better than the other it is just a case of do whatever you want that works. My solution is a bladder or a bottle together with a pint in the pub at the end of the walk. Talking of which I think I will leave work now to go to such an establishment. Anyone joining me?
Posted: 03/06/2009 18:06
> £17 -19 If PET bottles don't last for you, you can always buy a thicker plastic bottle or just plastic bag without for Catheter and buy me a beer with the change > You don't even need one system if the streams where you are going are clear and frequent. Very true. But needs jugement and sometimes local knowledge.
Posted: 03/06/2009 18:31
tea and coffee contain caffine which is a diaretic which dehydrates you . because you add water to make a drink it compensates for the caffine. so it will hydrate you but not as well as water , which will not hydrate you as well as a drink containing a small amout of sugar as this passes through the gut faster . so alot of liquid with a small amout of sugar , sounds like beer to me .
Posted: 03/06/2009 18:54
I see an interesting experiment coming up, blasting over a few mountains with nothing but beer to stay hydrated  ...........come to think of it that probably wouldn't be much fun 
Posted: 03/06/2009 19:07
"tea and coffee contain caffine which is a diaretic which dehydrates you " caffeine is a very mild diuretic in the concentration in teas and coffee. IT DOES NOT DEHYDRATE YOU. that is total and utter bs formulated by associating the effect of caffeine with an idea that because you pee more you are losing more fluid. WRONG! so very, very WRONG! caffeine relaxes the bladder control muscle. hence you pee more often. you do not pee appreciably more fluid than if your bladder control was normal as you'd just have more urine to get rid of at the time. so there is little difference in the TOTAL amount of pee. taking it further, as "sports" drinks are touted to improve your performance and other such nonsense on very dubious grounds (30% more than plain water although they never have a test that measures it against plain water with sugar in it) but caffeine has been proven to improve performance albeit in high doses. makes you pee rather frequently though. much sport science investigation takes place which is geared to pro athletes and others of similar persuasion and their conclusions do not translate to normal people. sport drink. eat some fruit instead. much healthier.
Posted: 03/06/2009 19:20
> BTW is it just me or have we already had this debate about 2-3 times in the past year alone? Bottles vs bladders is almost as prevalent and tedious as Paramo vs Gore-tex... Then, of course, there are the Bottle Wars; those who claim that you should be using an 'optimally-engineered hydration bottle', rather than any old PET bottle... or that drinking from a recycled PET bottle will kill you with nasty chemicals... or that polycarb bottles have lethal amounts of Bisphenol-A, or that...blah, blah, blah...
Posted: 03/06/2009 19:22
> sport drink. eat some fruit instead. much healthier. Dr Baker who I mentioned earlier suggests that diluting fruit juice 50/50 is as good as, if not better than 'sports drinks'... Or making your own: 1 cup unsweetened fruit juice
3/4 teaspoon salt
3/4 cup sugar
2.25 quarts water (US measures: quart = 4 cups = 950ml) ps. a costermonger is so named because they were originally known for selling a variety of apple called a 'costard'. These were sold for their clean water content, rather than (or as well as) food. The old equivalent of bottled water...
Posted: 03/06/2009 19:31
"Then, of course, there are the Bottle Wars" and the stove wars......
Posted: 03/06/2009 19:51
God knows how much you need, but on the 100m over Bank Hol - I guess that's a good test - I was very strict with myself, it was a hot day and very sweaty till evening. I made myself drink a 500ml bottle between every checkpoint (avarage every 7 miles, 15 CP's in total), with weak High5 energy drink, but not every bottle, too sickly after a while. At each checkpoint made sure I had another couple of glasses of water straight down with grub and refilled bottle. Virtually kept this up overnight (maybe slightly less in the cool) and into next hot day for 26 hours. No ill effects. Some seemed to drink very little, I think they suffered (near 25% drop out overall, and 50% of fastest runners dropped out). I'm 5'6", about 101/2 stone (stocky) and 57 this year. One guy dehydrated and failed last year, so this year drank and drank in the heat ...... ended up in hospital for a few days for drinking too much and diluting his body salts. Nothing remotely scientific about the above, but I seemed ok on it, hope it helps. I made very sure I was well hydrated during the week before.
Posted: 03/06/2009 20:58
Coffee stimulates smooth muscle therefore it causes premature emptying of the stomach, reduced intestinal transit time and, as has already been mentioned, urinary frequency but it does not cause dehydration! Coffee can exacerbate already existing conditions of stomach, bowel and heart but generally its consumption in reasonable quantities and frequency is beneficial in many ways.
Posted: 04/06/2009 01:34
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