‘You are sweating half a litre an hour’: what happens to your body when dehydrated – and how to avoid it

As Australia stares down a predicted very hot summer, dehydration presents a serious – but preventable – health risk for many of us.

On day two of the Three Capes walk in Tasmania, which I was looking forward to as the easiest due to being flat, I found myself walking in blazing sun across an expanse of scrub with no shade. Despite drinking regularly from the two litres I was carrying, I started to feel weak, disoriented and desperate.

Eventually I could go no further. It was an unseasonably warm day. I was sweating moderately but not excessively. I became slightly light-headed, and a kind of apathy and despair settled on me, almost like I had lost the will to live. In my tendency for melodrama, I sobbed “just leave me here” to my alarmed husband.

Somehow, we made it to the next camp, where a couple of electrolyte tablets (of which more in a moment) dissolved in water, a meal and a lie down did the trick: by the next day I was as good as new.

But I was left mildly shocked at how little it had taken to drain me of energy or purpose. I thought of colonial explorers like Ludwig Leichhardt, who is believed to have died of thirst on his third, ill-equipped expedition in 1848, from Brisbane to Perth. Presumably such a death would involve slipping into unconsciousness. Maybe not the worst way to go, but I had learned my lesson. What had happened to me was a simple case of dehydration.

The adult human body contains around 60% water. All our cells, including our brain cells, require water to carry out their essential functions. Dehydration impairs that process. A 2018 meta-analysis found that dehydration corresponding to as little as 2% of normal body mass is associated with impairment to attention, executive function and motor coordination.

Dehydration is mysterious because thirst, the obvious barometer of when you may need more fluids, is not always a reliable or accurate indication of how hydrated you are. It tends to kick in when we are already between 1 and 2% dehydrated. (It’s not a design flaw, it’s evolution: otherwise humans would spend their entire time looking for something to drink.)

To compound the problem, water intake is not always the answer: depending on how far gone you are, you may need to replace the salts and minerals known as electrolytes – sodium, calcium, potassium – that you have sweated out. An easy test is to check the colour of your urine: if it is a pale straw colour, you are drinking enough, if it is dark, you aren’t.

Most of us will become aware of dehydration through thirst and excessive sweating, and perhaps a mild sense of dizziness. In more severe cases, failing to drink enough water can cause cramping and lead to increased likelihood of kidney stones.

In 2003 a heatwave in France caused the deaths of 15,000. Most of these deaths were among older people, according to Human Rights Watch. For elderly people, dehydration is more acute and has more complex side-effects including, confusion, compromised spatial awareness, blurred vision, and impaired motor skills. It can cause falls and lead to organ damage.

As my GP explained before I embarked on a longer, more arduous walk in the Flinders Ranges, dehydration can be the result of a diminished thirst signal (which weakens with age, along with a reduction in taste bud sensitivity and saliva production), or a side-effect of other medications (especially laxatives and oral treatments for type 2 diabetes) or simply due to a lower level of fluid in the body as people get older. Infants are another vulnerable group.

What about the rest of us?

Many sporting codes have developed protocols about when it is too hot to compete safely. The Australian Open is played at what is often the hottest time of the year; its extreme heat policy allows for 10-minute breaks and can recommend that matches be halted in exceptional temperatures.

But if there are guidelines for athletes, what about the rest of us? Given that dehydration is entirely preventable, and therefore relatively easy to fix, do we need a public awareness campaign to remind us every summer to watch our fluid intake?

The answer, according to Prof Ollie Jay, is an emphatic yes. Jay occupies a unique position as the only professor of heat health in Australia, at the Heat and Health Research Incubator at the University of Sydney’s Charles Perkins Centre. His multidisciplinary team is studying aspects of heat on pregnancy and maternity as well as urban shading initiatives and has collaborated with NSW Health on a recent update of its online Beat the Heat public health campaign.

He explains that dehydration may be caused by more than one factor and that thirst is not a reliable measure of need. “How much you should drink to prevent it depends on things like your weight, age, general health, level of activity, and variables such as wind, shade and humidity. It’s not as easy as saying men need to drink 10 cups of water and women need to drink eight,” he says, quoting a common formula that he believes is too general to be helpful.

“As a general guide, you can assume that on a hot day, depending on what you are doing, you are sweating half a litre an hour and should be drinking 250ml of water every hour.