You survive for days and weeks without food, but you will last only hours without water in extreme heat. You don’t have to be stranded in a desert to become dehydrated, either. Just digging the vegetable patch, or working and exercising on a hot day can result in excess fluid loss.

Dehydration is defined as the loss of water and/or electrolytes from the body without adequate replenishment. Everyone loses more than one litre of fluid per day in urine and faeces, their breath, and in mild sweat, despite not lifting a finger. You can lose up to two litres of fluid per hour if you work hard and sweat profusely. You drain your body even further if you vomit or get an attack of  diarrhoea. If you lose between 5% and 10% of your body fluid, you have mild to moderate dehydration. Once the loss reaches 15% or more, however, severe damage to body organs is likely.

‘I’m thirsty.’ Dehydration’s first warning sign is a craving – thirst – for anything liquid. This mechanism, however, is unreliable. But if the body loses a very high proportion of salts, the craving for liquids diminishes or is not apparent at all. At old age you have also an inhibited appreciation of thirst.

Other warning signs are headaches, dry lips and mouth, loss of concentration, fatigue, and dry and wrinkled skin. The appearance of large amounts of sweat is normally another caution that you should drink something. But this visual check is sometimes misleading. On very dry and hot days, the sweat evaporates so fast that it doesn’t form sweat pearls on the skin. Once dehydration progresses, the body gradually loses weight.

The body releases a specific hormone (vasopressin) when the fluid level becomes too low. The hormone is a chemical signal for the kidneys to preserve water by reducing urine production.

This can go too far. The body’s waste, normally extracted by the kidneys, accumulates in the blood and causes poisoning. If the fluid level continuous to drop despite the effort to preserve water, the kidneys lose their function altogether – kidney failure occurs.

Low fluid levels lead to the failure of the cooling mechanisms; you can’t sweat if you don’t have enough fluid in your body. In addition, if the blood volume decreases, it carries less heat to the skin for dissipation. As a consequence, dehydration causes overheating.

So, you better fill up your reservoirs. But remember coffee, tea and alcohol increase the urine production and dehydrate you even further. Some diuretic medications accelerate the loss of body fluids as well. For the elderly, the combination of heavy sweating, diuretics and an impaired thirst sensation is dangerous.

Always drink more than you ‘think’ you should. If you trust your thirst, you won’t fill up enough, as thirst generally stops when you’ve replenished about two-thirds of the lost fluids. While sweating you lose a large amount of electrolytes, so see ‘Heat Cramps’ for tips on how to top up electrolyte levels.

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