It’s a hot day. You feel like you’re melting. You want to be cooler. How do you do it?
Well the answer’s obvious isn’t it? You reach for an ice cold drink to cool you down. Hot plus cold, equals slightly less hot, surely? That’s basic science.
But not so fast, because you remember overhearing that one time someone said that, actually, drinking a hot drink actually cools you down more. That’s why the Brits got into tea when they were lording it over India back in the days of the Raj, right? But that doesn’t sound right does it, how can something hot cool you down?
Well, the truth is ever-so-slightly more complicated that you’d expect.
(And if you’re too hot and bothered to read the science and just want to know what to do, then SCROLL TO THE END for a simple guide)
To start with, the overall effect of the temperature of a drink – whether hot or cold – on your overall core body temperature is very small. This is because the amount of fluid you’re consuming (say, half a litre) is small compared to the amount of fluid in an adult body (around 40 litres) – that’s according to Professor Robin McAllen, a neuroscientist at the Florey Institute of Neuroscience and Mental Health in Victoria.
A major study on the subject cited by half the internet came in 2012, and was published in Acta Physiologica, co-authored by Thermoregulatory Physiology expert Dr Ollie Jay of the University of Sydney with the catchy title ‘Body heat storage during physical activity is lower with hot fluid ingestion under conditions that permit full evaporation’.
In essence, it explains that, under certain specific circumstances, a hot drink can indeed cool you down. This is because it activates temperature sensors in the body that trigger sweating. Sweating is a highly effective mechanism for cooling the body down, so triggering more sweat is useful – and the potential heat-reducing potential of the sweat produced is higher than the extra temperature gained from the hot drink. However, in order to cool us down, the sweat needs to be able to evaporate from the skin (the energy it takes from the skin to change state reduces the temperature of the body) – so if that sweat is simply dripping off you, then it will not cool you down as it takes no energy away from you.
Conversely, drinking a cold drink can reduce sweating, and thus you lose the potential to reduce your temperature via this mechanism.
“If you drink a cold drink that’s colder than your body, you’ll shunt some heat into that fluid to warm it up. 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.”
A slush puppy, or ice, acts in a similar way. According to Jay, “With the slushy, because the stimulation for reducing sweating is so strong, we actually seem to over-compensate. 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.”
However – if you have a slushy and then exercise, it can be effective in reducing your temperature, since the body is having to use energy to help it change state from a solid (ice) to a liquid (water).
“If all of that sweat [triggered by a hot drink] can evaporate, then I am better off with a hot drink. 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.”
But, of course, most of us don’t want to sweat. And if you’re not already sweating, then a cold drink cannot reduce your sweating, as it hasn’t begun, so will lower your core temperature in an expected, albeit slight, way. Additionally, in order to trigger the extra ‘heat sweats’, if you weren’t already sweating, you’d need to drink enough hot liquid to get you sweating, thus raising your core temperature to begin with – crucially, the experiment was conducted during physical activity, which suggests that the subjects were already sweating when they chose whether to drink cool or hot drinks.
BUT HOLD ON A SECOND: one thing I had heard mentioned lots of times is that drinking a hot drink would lead to vasodilation – that is, your blood vessels widening, and being pushed to the surface of the skin, in order to facilitate heat loss from the body to the surrounding environment. Was that true or not?
Well, from what I can find, no one really knows. My best guess would be that, in hot weather, your body is already vasodilating to its maximum amount to cool you down – it can’t really be ‘provoked’ into doing it any more by a hot drink.
And what about your choice of drink? Well, it seems that, ‘at intoxicating levels’ alcohol is a vasodilator – but at ‘even higher levels’ it becomes a vasoconstrictor. So that’s not very useful. Meanwhile, it seems that caffeine constricts the blood vessels, suggesting that the line that a tea or coffee aids vasodilation directly is false. So we can probably rule this one out.