Drinks Fountain

Hot drinks cool you faster – fact or fiction?

In Science Fun by Morgan ByeLeave a Comment

Imagine you’re sat on a tropical beach. The sort of beach they only have in magazines and nature documentaries. The sun beating down on paradise. The humidity and cooling sea breeze is just enough to make anything other than another cocktail unbearable. Rather than decided to top up your tan a little further you decide that you really must keep up your marathon training. After all, those unfortunate orphans cannot raise the money to save their orphanage without your fundraising efforts.

You go for your run down the perfect sands with the waves lapping at your feet, you break into a perfect glisten of sweat – you know, that perfect Hollywood shine only seen on the rippling biceps of stars in action movies.

Getting back to your chalet, the butler poses you a conundrum.

“Would sir, care for a cup of tea or something colder perhaps?”

Cold Coke on the beach

Grabbing something cold to drink

Instinctively, in times of dehydration – particularly from heat or exercise – we reach for something cold. In fact when you think back to almost any soft drink or beer commercial there is always a reference to the “ice-cold refreshment…

A cold drink sure is great! You can literally feel the cold fluid spread through your chest as the liquid cools your stomach.

But as many of us will attest, cold drinks feel like they can go straight through.

Afternoon tea

My mother on the other hand, all her life, has been an advocate of a hot beverage. Obviously, being British, her drink of choice is tea. Proper tea, no fruity infusion stuff, black, English tea, stewed until the liquid resembles something approaching crude oil. Then and only then she “scares” it with only the smallest of drops of milk.

My mother resolutely drinks tea, all year, anywhere in the world. When I took her the shores of the Dead Sea in Israel – one of the driest places in the world – she couldn’t wait to get back for a cuppa.

Dead Sea

Her argument is that a hot drink cannot be drunk fast. A hot drink starts with a testing sip of only a few drops. After a little blowing and a bit of time a second sip might be attempted. The fact that the drink is too hot to drink aids in regulating the rate in which you drink. It stops you chugging that bad-boy down in one.

Here comes the science

The scientist in me has always scoffed at my mother’s logic. Water is water, right? A given volume of water is more or less the same at any temperature you can be putting it in your body. Any drink, hot or cold, doesn’t maintain its temperature for very long as the liquid would rapidly equalise to body temperature. After all a high school teacher once told me that everything is held in the stomach for an hour before it is allowed to pass on.

Maximum absorption of water into the body of course requires a few tricks. To get water into the body means that it has to pass a few membranes and to do so is much easier under favourable conditions. What you need is a bit of sugar (ideally a simple sugar like glucose found in common sugar you’d put in your tea) and a bit of salt. Doing so makes the water you are trying to take in much closer to that of the stuff in your body, and means less work.

By the way, if you are ever ill with food poisoning and need to rehydrate fast you want about 6 teaspoons of sugar with 1 teaspoon of salt in half a litre (or a pint in Blighty). Stir like crazy and force it down. It is definitely not the tastiest thing going, but it gets the job done – trust me.

The same trick is used by every “energy” drink manufacturer on the planet. They market the stuff as “extreme hydration”, but a doctor would all it an isotonic saline solution also known as “a drip“. The joke being that no professional athlete would ever put the stuff near their body, without a huge sponsorship deal.

What about that caffeine?

Of course, most hot drinks like coffee (and tea to a lesser extent) have high levels of caffeine. Energy drinks have really high levels of caffeine, some as much as 3 espressos.

Caffeine is actually classified as a psychoactive drug that stimulates both the central nervous system and metabolism. This stimulant effect is great for reducing fatigue (if you are actually using energy drinks with exercise) and with making us feel less drowsy.

While it is true that caffeine does increase urine output – it stimulates an increase in water and salt excretion – the increased output is vastly outweighed by the volume of water in a typical cup of coffee. Regular coffee drinkers usually develop a tolerance to the caffeine signals and don’t actually pee any more or less.

Researchers like a challenge

A group at the University of Ottawa, Canada actually tested the age old myth of summertime. Their experiments, published in Acta Physiologica, took 9 guys and had them cycle for 75 minutes with moderate effort. They were given water every 15 minutes at either 1.5, 10, 37 or 50 °C.

The poor guys had their body temperature measured at 8 points across the body, including two internal measurements, one up the bum, the other in the lower throat using a probe stuffed up their nose.

If the extra sweat just drips on the ground, then you’re better off drinking a cold drink.Dr. Ollie Jay

The researchers found that a hot drink actually stimulated the sweat response and the additional sweating more than compensated for the heat in the drink. The cold drinks did the opposite. A cold drink stopped the sweating and actually kept the body hotter than the cooling power of the drink.

The results are actually only very slight and they come with a major caveat. The sweat must fully evaporate to get the desirable cooling effect. If you are exercising too hard, wearing too many clothes, are in a humid environment or basically anything that causes the sweat to bead and drop on the floor rather than evaporate… then you’re just better off with a cold drink.

The careful balancing act

The human body is a constant balancing act. Scientists call it homeostasis, but essentially keeping things “normal” is really important to staying alive and the body spends a lot of time and energy making sure. Often, too much of something is just as dangerous as not enough. This applies to almost everything, including oxygen and even water. You might die from dehydration, but drowning doesn’t look any fun either.

A hot drink, whether your poison of choice is tea, coffee or choco with extra cream and marshmallows, is always too hot to drink at any speed. Even when a hot drink is drinkable, we have a societal reaction to enjoy the drink. This pacing allows for a much better quantification of the intake. A small intake, followed by a pause allows the body to feedback whether more is necessary.

Coffee

A room temperature drink can be drunk quickly without consideration. In fact, you might neck half a litre of water so fast that you body doesn’t have time to register whether anything actually happened. How often do we still feel thirsty after the first glass of water and go ahead and chug a second.

Now you have a litre of fluid sat in your stomach, and let me put it this way – the average male bladder is half a litre (women’s are 50 % bigger, I’ll let you decide what that means).

But how does it feel?

From a feeling perspective. Well… that’s all it is, a feeling. If you are my mum, you are going to stay with your tea, whilst many are going to stay with an ice-cold coke.

On a hot day, then boy does it feel nice to have anything that makes you feel a bit cooler. Standing in front of an AC unit feels pretty nice. Things like cold drinks and ice cream have the benefit of cooling the body a little (and only temporally) from the inside. If you drink a slushie with a lot of ice, the energy required for turning the ice into water (a “phase change” in science-speak) will cool you even more.

When the winter days are closing in and the bitter winds howl around you. On those days, wrapping your hands around that hot cup, feels like the drink is warming you from the inside.

Humanity is a species that craves experiences. Playing with our body temperature is a great way to do that. But does it make a difference to how much water you have in your body? No. Not in any significant way.

Your body knows what it likes, and it is going to keep you drinking and peeing all day until you find perfection. Just watch out for brain freeze!

About the Author

Morgan Bye

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Morgan Bye is a British science writer, out of Vancouver, Canada, dedicated to helping scientists better communicate their science. He has a Masters in Biochemistry, a PhD in Biophysical Chemistry and even spent time as a scientist at an Israeli research institute.