Urban myth or scientific fact: does drinking hot drinks on a scorching summer's day really cool you down? Well, scientists do say sweating is the best way to stay cool.
It's 40 degrees Celcius and oppressively humid. You pull yourself up from your sweat-drenched chair and head to the kitchen … to brew yourself a nice 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 apparently sip hot tea to stay cool in the warmer months. Are they crazy, or can a hot drink actually cool you down?
In some circumstances it might, scientists say, because it could trigger a level of sweating that may more than compensate for the added heat of the drink.
But it would depend on a lot of things, including the temperature of your hot drink, how much you consume, and the temperature and humidity of your immediate environment.
Drinking a hot drink on a hot day might seem like a strange choice, but it's likely to cause only a very tiny blip in your core body temperature, says Professor Robin McAllen a neuroscientist at the Florey Institute of Neuroscience and Mental Health in Victoria.
That's because the amount of hot fluid in say a cup of tea is relatively small compared to the amount of fluid in an adult's body, McAllen says. (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 our body uses to cool us down.
The University of Sydney's Dr 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: the sweat needs to be able to evaporate to produce a cooling effect.
If it doesn't, and the extra sweat just drips on the ground, then you are 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 by having a hot drink. You'd be better off sticking to a cold one.
" If you drink a cold drink that's colder than your body, you'll shunt some heat into that fluid to warm it up," Jay explains.
"You lose heat to the fluid, and that's good because it increases the overall amount of heat you lose.
"The trouble is, it is compensated [for] by us reducing the amount we sweat onto the skin surface," he says.
It turns out when you consume a hot drink, you produce much more sweat relative to the small amount of heat added to the body.
"If all of that sweat can evaporate, then I am better off with a hot drink," says Jay.
Icy cold can make us hotter
Jay and his colleagues have just completed a study, not yet published, looking at slushies, drinks made of finely crushed ice.Interestingly, they found that drinking an icy drink can make you hotter than a drink that's 37 degrees C, our normal body temperature.
This is because the icy drink is so cold it may shut down the 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," says Jay.
"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?
So what sort of drink should you reach for on a hot day?A hot drink is okay as long as the extra sweat it causes can evaporate.
"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," Jay suggests.
But the best advice is to drink fluids at a temperature that's most palatable to you, says Jay.
That's because most of us don't drink enough when it's hot, yet we need to if we are to avoid dehydration and ultimately the onset of heat-related illness or 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," says Jay.
Sources: http://www.pubfacts.com/detail/, http://sydney.edu.au/health-sciences/, http://www.abc.net.au/