THE STUDY
Less heat stored with hot water
Nine men cycled for 75 minutes drinking water at 1.5, 10, 37 or 50 °C. Body heat storage was lowest with 50 °C water, because they sweated more.
See the numbers
WHAT THE RESEARCH FOUND
Hot tea, cooler you?
Only if sweat dries.
A hot drink can cool you down, in the right conditions.
A University of Ottawa lab study found that warm water made people sweat disproportionately more. When that sweat could evaporate, the extra cooling outweighed the heat in the drink. On a humid day, or in heavy clothing, the advice flips: choose a cold drink.
50 °C water drunk in the lab study
Check the conditions- Sweat cools as it dries
- Humid air stops it
- UK advice: cold drinks
- The free cooling system is evaporation
THE SHORT ANSWER
It works by
making you sweat.
The drink adds a little heat.
The extra sweat can take away more.
THE CATCH
Sweat has to evaporate
The lead researcher said that on a very hot and humid day, in a lot of clothing, or when sweat just drips off, a hot drink is a bad idea. Go for a cold one.
Check the conditions
UK ADVICE
What the NHS says
In a heatwave the NHS says to have regular cold drinks and to avoid alcohol, caffeine and hot drinks. Its advice is written for everyone, in typical UK conditions.
Heat advice
WHAT UK GUIDANCE SAYS
In a heatwave,
keep it simple.
Official UK advice is written for everyone, including people most at risk from heat. It favours cool drinks and cooling the skin.
Drink regularly through the day
UKHSA says to drink enough that your pee is a pale straw colour, especially if you are active.
Choose water or diluted squash
Water, diluted squash and lower-fat milks are good choices. Limit sugary drinks and alcohol, which dehydrates.
Have cold drinks, not hot ones
The NHS heatwave page says to have regular cold drinks and avoid alcohol, caffeine and hot drinks.
Cool your skin
Take a cool shower, or put cool water on your skin or clothes. Carry a refillable bottle of water when you go out.
TWO NUMBERS WORTH KNOWING
A small study,
a clear condition.
One figure on the drink,
one on the dry air that made it work.
50°C
Water temperature that left the least heat stored
Nine healthy men drank four doses of 3.2 mL per kg of body weight, at 1.5, 10, 37 or 50 °C, while cycling for 75 minutes. Heat storage measured by calorimetry was lower with 50 °C water than with any of the others.
THE DRINK · UNIVERSITY OF OTTAWA
≈ 23%
Relative humidity during the tests
The room was about 23.6 °C and 23% relative humidity, with steady air movement, chosen so that sweat could evaporate fully. The results only apply directly to healthy men aged 18 to 39.
THE ROOM · UNIVERSITY OF OTTAWA
The extra sweating seemed to come from warmth sensors in the throat and stomach rather than a rise in core temperature, the study suggests.
A QUICK CONDITIONS CHECK
Would a hot drink
help right now?
Pick a situation. The bar shows how well it matches the conditions in which the study found a hot drink lowered body heat.
Based on Bain and Jay's University of Ottawa study (2012) and the researcher's comments to Smithsonian magazine. This is general information, not personal medical advice.
WHY SWEAT COOLS
Free cooling,
if the air allows.
Sweat cools you as it evaporates, carrying heat away from the skin. The study found that warm water caused a disproportionate increase in sweating, which more than made up for the small amount of heat in the drink.
Humid air, heavy clothes and dripping sweat all stop that evaporation. Then the hot drink's heat simply adds to the body, which is why the researcher said to go for a cold drink when sweat cannot evaporate.
Questions
Questions people ask
Does hot tea really cool you down?
It can, in narrow conditions. In a University of Ottawa study, drinking 50 °C water lowered body heat storage during exercise, but only in dry air where the extra sweat could evaporate fully.
Why would a hot drink make you sweat more?
The researchers think warmth sensors in the throat, mouth or stomach trigger extra sweating. Core temperature did not have to rise for this to happen.
Is a cold drink better in a UK heatwave?
UK guidance says so. The NHS advises regular cold drinks and avoiding hot drinks, alcohol and caffeine, and UKHSA suggests drinking cool water to cool down.
Does the study apply to everyone?
No. It tested nine healthy men aged 18 to 39 cycling in a dry room. People at higher risk from heat, such as older people and young children, should follow official heat advice.
Is humid weather the problem?
Yes. Humid air slows evaporation, so extra sweat stays on the skin or drips off. The researcher said a hot drink is then a bad idea because it still adds heat.
Sources
- University of Ottawa (Bain, 2012): Body heat storage, sweating and skin blood flow following cold and warm water ingestion during exercise
- Smithsonian magazine: A hot drink on a hot day can cool you down
- NHS: Heatwave, how to cope in hot weather
- GOV.UK (UKHSA): Beat the heat, staying safe in hot weather
Updated