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

Elephants and camels

The forecast looks 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 60 litres of fluid in one session and can go without water for eight days. Elephants need to swallow more than 260 litres each day, 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

Water has plenty of important functions in the body, but its role in temperature regulation during exercise is vital. 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 needed just to keep our bodies warm enough comes from the conversion of the chemical energy of foods into useful work. 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 charge 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.