black&white #37: Does drinking tea really cool you down on a hot day?

3 min steep

Heatwaves in Britain are becoming a regular occurrence, with record temperatures recorded across the UK earlier this year – and scientists predicting that plus 40 °C temperatures are becoming increasingly likely. Unsurprisingly, the public has sought new ways to stay cool in such extreme heat, with fan sales up 1300%. But what about the conventional wisdom that drinking tea – or any hot drink – can actually cool you down on a scorching day? Is it a myth? Or is there some truth in the statement?

In countries with significantly hotter climates than the UK, tea-drinking has been part of their culture for centuries. The Bedouin, arabic-speaking nomadic peoples of the North African and Middle Eastern deserts, have a long tradition of drinking so-called Bedouin tea – a strong, sweet black tea. In India, you’d be far more likely to find people drinking steaming cups of aromatic chai than an iced tea or coffee, as you would hot mint tea in Morocco.

So what does the science have to say about this? In 2012, the first of a series of papers was published to compare the effect of hot and cold drinks on body temperature during light exercise – volunteers were asked to cycle at a relatively slow pace for 75 minutes in around 24°C heat, 23% humidity, and while drinking water at either 1.5˚C, 10˚C, 37˚C or 50˚C. The change in core temperature – which usually sits somewhere between 36.5-38.5°C – was slightly greater when 50˚C water was consumed compared to 1.5˚C and 10˚C water.

Crucially however, when the researchers considered the effect of drink temperature on body heat storage – a better indicator of total body temperature and thermal stress – the results were quite different. After drinking warm or hot drinks, the overall body heat storage was actually lower following exercise than after drinking cooler drinks.

This appears to be because of sweat. When you drink hot drinks, you sweat more – and sweating is one of the key ways your body modulates temperature and maintains heat balance. Put simply, the heat lost from sweat outweighs the internal heat gained from drinking a hot drink. In more scientific terms, excess sweat is evaporated from the skin surface, increasing heat loss from evaporation and reducing your overall body heat storage.

However, there are some caveats. The study was conducted under hot, dry conditions that allowed sweat to completely evaporate. This was achieved by a good airflow and keeping humidity low. If the conditions were hot and humid – and you weren’t wearing the right clothes – sweat evaporation wouldn’t happen. In this case, drinking cold drinks would probably be better, minimising inefficient sweat loss and improving hydration. In another study in the same series, the researchers compared the effects of people consuming 37˚C fluid or ice (such as a slushy-type drink) during exercise. Again, this showed that heat loss was less effective after ingesting ice than after drinking a body-temperature fluid. 

In essence, it’s all about the conditions you’re in. If it’s hot, dry and breezy, a hot drink might actually cool you down. If it's humid and muggy, probably best to stick to cold drinks.