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Hydration & Summer UK Hillwalking

It’s hot, you are planning a long day out in the hills – so how much fluid should you take/carry with you? Find out: Why dehydration can slow you down. How your body keeps cool. How much water to drink. How to purify water on the go. How to carry your water.

Elephants and camels

Adults lose nearly three litres of water every day just sitting around

The forecast remains good, your rucksack is bulging with food … what could possibly spoil a long day walking in the hills? How about running out of fluid? Camels can drink over 114 litres of fluid in one sitting and can go without water for eight days. Elephants need to swallow more than 200 litres each day and take’s in 15 litres  per slurp, but us humans need a mere three and half litres a day to maintain fluid balance. Adults lose nearly three litres of water every day just sitting around; a half a litre in sweating, one litre in breathing out and about one and a half litres in urine. Head for the hills or do any sort of exercise and you need even more.


Water has plenty of important functions in the body, and one of these function’s is temperature regulation during exercise. Although your skin temperature can vary a lot, core body temperature is kept within narrow limits and in a healthy adult this is 37°C. A lot of the heat is needed just to keep our bodies warm enough and comes from the conversion of the chemical energy of foods. This process is very inefficient and 75-80 per cent of the available energy in foods is lost as heat and this amounts to about one calorie per minute.

This is fine if you are sitting around doing very little, but as soon as you start to move across the hills more energy is produced to fuel this exercise and the excess heat production goes up to around 20 calories per minute. After a couple of hours this can add up to a massive amount of heat which, if not removed, would mean a well-cooked (and dead) individual. A rise in core temperature of as little as 5°C above the normal 37°C means serious heat exhaustion. Incredibly, this point would be reached after only about 15 minutes of hard exercise if the heat is not removed. The fact that we can keep going for hours with only a 2-3°C rise in body temperature means that the heat produced is lost almost as fast as it is produced.

Cooling mechanism

This is achieved via the evaporation of sweat from the surface of the skin. Evaporation of one litre of water from the skin will get rid of 580 calories of heat from the body – equivalent to the heat required to produce 40 cups of steaming tea! So sweating is vital during an endurance event to prevent severe overheating – but at a cost. The loss of large amounts of sweat brings the threat of dehydration. 

You may be someone who sweats a lot to the extent that it drips from your skin. Any fluid that drips from your skin is wasted, since cooling depends on the evaporation of sweat and increases the risk of dehydration. With less fluid in your bloodstream, your heart has to pump harder, but ultimately blood flow to your skin (for cooling) and muscles (to provide them with oxygen and nutrients) is reduced. Research has also shown that the rate of perceived exertion (how hard you think your body is working) is affected by dehydration. The more you drink the easier the exercise feels.

Drinking strategy

Aim to drink before you set off, especially in hot weather. Then drink around 150-200ml every 10-15 minutes during exercise. In very hot conditions you may not be able to keep pace with your fluid losses. The maximum rate of water absorption during exercise is 800ml/hour whereas your sweat rate may be as high as 2000ml/hour.

The best way to check if you are drinking enough is by the volume and colour of your urine (a tricky measurement when out on the hills!). Small amounts of deep yellow pee means you need to drink more. Headaches, stomach cramps, digestive problems, side aches, diarrhoea and nausea can all be related to simply not drinking enough during exercise in hot weather.

Now the tricky part
How much water do I carry and what in? Once you have worked out your work rate and fluid intake from the drinking strategy above you can then calculate your water load to take, this water load may restrict your activities. Having a method of purifying water on the go is a simple solution that seems to be the norm across Europe but in Britain seems to be shunned. There are simple lightweight water purifying options open to those that walk, wild camp in the UK.


  • Kills off all water-borne pathogenic organisms.
  • The boiling process needs to be maintained for 5 minutes which can be time consuming.
  • The best general solution for family situations.
  • Only requires a pan and a heating implement.

Water Filters:

Carrying a ultra-lightweight water filter such as the Care Plus® (65grms) Personal Water Purification Filter with Straw, is a filter of choice for our training teams. This device has no moving parts and will filter up to 375,000 litres of water, now that is a lot of days in the hills. You can even use the Care Plus® Water Filter as an in-line filter with your Hydration Bladder & Drinking tube…..Remember NO water filter will remove Viruses, so a disinfectant needs to be used alongside your filtration process. In independent test results this device will filter our Bacteria’ such as E-coli & Salmonella. Protozoa’s such as Giardia & Cryptosporidium and……

Water Disinfection:

Virus FREE drinking water. All water taken from an open source should be disinfected after filtering if drinking and must form part of the cleaning/filtration process to ensure you are drinking the cleanest water possible. NO water filter removes viruses, but disinfection will deactivate any viruses present after filtration. Carrying a small 30ml bottle of water purification disinfectant such as Hadex is ideal in the trekking situation and ensures you are…..

Hydration bladders:

Our training teams, use a range of bladder bags/devices including the small 1 Litre bag supplied with the Care Plus filter, ideal for short training sessions or when delivering first aid training locally to our centres. For a reliable source of drinking water that can be drunk or used for first aid purposes, they filter, then disinfect and include the filter to their bladder system to add a second filter run, this is especially good process if the water is from a suspected heavily contaminated source.  If you required to carry more than one litre then what not consider using a larger 3 Litre bag such as the Coyote Hydra System with bite valve, this bag is made from heavy duty products and is more robust than other commercial products found in store…..read more.

About Remote First Aid

Remote First Aid has been delivering outdoor first aid courses at its bases in Snowdonia, The Wye Valley, The Peak District & Aviemore Scotland for over 20 years and is a leading provider and syllabus setter in this filed of expertise. Why not join a course where your trainers all come from a Mountain Rescue & NHS frontline setting brining you the vary best training one can attend. Check out our 8 hour one-day course for those enjoying the outdoors here.

Why not check out our YouTube channel on the Filter systems here.


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