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  • Nicola Garrett

Can a hot drink keep you cool in hot weather?

Reaching for a thermos of hot tea at the beach may seem unnatural, but is it?

It's 35 degrees Celsius and oppressively humid. You pull yourself up from your sweat-drenched chair and head to the kitchen… to brew yourself a steaming hot mug of tea. Sounds a bit wrong, doesn't it? Most of us would be more likely to reach into the fridge for a cold drink, but plenty of people in India and elsewhere sip hot tea in warmer months. So can a hot drink actually cool you down? In some circumstances it might, scientists say, because it could trigger a level of sweating that compensates for the added heat of the drink. But it would depend on a lot of things, including the temperature of your drink, how much you consume, and the temperature and humidity of your immediate environment.

Cool theory behind hot drinks In all likelihood, a hot drink on a hot day will cause only a very tiny blip in your core body temperature, says Robin McAllen, from the Florey Institute of Neuroscience and Mental Health in Victoria. That's because the amount of fluid in a cup of tea is relatively small compared to the amount of fluid in an adult's body, Professor McAllen says. This means the same goes for the cooling effect of a cold drink. However, drinking hot tea still activates temperature sensors that trigger sweating, and sweating is a key mechanism your body uses to cool you down. The University of Sydney's Ollie Jay, who researches how the human body responds to heat, says a hot drink can indeed trigger a net cooling effect through excess sweating. But there is a crucial caveat: to produce that cooling effect, the sweat needs to be able to evaporate. If it doesn't, then you're no better off. In other words, if you're exercising hard or in a very hot and humid environment where you're sweating more quickly than it can evaporate, it wouldn't be wise to increase your sweat-rate further, and you'd be better off sticking with cold drinks. "If you drink [something] colder than your body, you'll shunt some heat into that fluid to warm it up," Dr Jay explains. "You lose heat to the fluid, and that's good because it increases the overall amount of heat you lose. "Trouble is, [that heat loss] is compensated for by us reducing the amount we sweat." It turns out that when you consume a hot drink, you produce much more sweat relative to the small amount of heat added to your body. "If all of that sweat can evaporate, then I'm better off with a hot drink," Dr Jay says.

Icy cold can make us hotter In 2016, Dr Jay and his colleagues published a study that looked at the thermal impact of slushies — drinks made of finely crushed ice. Interestingly, they found an icy drink can make you hotter than a drink that's 37C (our normal body temperature). This is because the icy drink is so cold it may shut down your body's sweating mechanism to the extent your body ends up storing more heat. "With the slushy, because the stimulation for reducing sweating is so strong, we actually seem to over-compensate," Dr Jay says. "The reduction in evaporation of sweat from the skin is greater than the extra heat you shunt into the slushy to warm it up inside your body." So, what to drink on a hot day? "If it's hot and you do want to drink a hot drink and you don't mind sweating, then you could drink it with a cold fan blowing on you to help the sweat evaporate," Dr Jay suggests. But the best advice, he says, is to drink fluids at a temperature that's most palatable to you. That's because most of us don't drink enough when it's hot but we need to to avoid dehydration and ultimately, the onset of heat-related illness or an increase in cardiovascular strain. "Not many people are going to want to drink one to two litres of hot fluids, but drinking one to two litres of cold fluids is a lot easier," Dr Jay says.

Article from ABC Life

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