The forecast looks good, your rucksack is bulging with food ... what could possibly spoil your day on the rocks? How about running out of fluid. Camels can go without water for eight days, but they do drink around over 60 litres of fluid in one session beforehand. Elephants need to swallow 250 litres each day, but us humans need less than four litres a day to maintain fluid balance.
Even on a rest day you'll use 500ml in sweating, one litre in breathing out and a little over one and half litres in urine
Adults lose around two and half litres of water every day just sitting around; 500ml in sweating, one litre in breathing out and a little over one and half litres in urine. Do any sort of exercise and you need even more fluid.
The Human Thermostat
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, deep body temperature is kept within narrow limits and, in a healthy adult 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 amounts to about one calorie per minute. This is fine if we are sitting around, but as soon as you start to exert yourself on the rock or climbing wall, more energy is produced to fuel this exercise but 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 one 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 is 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 must be lost almost as fast as it is produced.
This is achieved via the evaporation of sweat poured onto the skin surface. Evaporation of 1 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. With inadequate fluid supplies in your body the blood flow to your skin (for cooling) and muscles (to provide them with oxygen and nutrients) will be 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.
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!)
Aim to drink before you set off, especially in hot weather, and I'd suggest aiming to drink 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 (approx. three and a half pints).
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.