It's one of the great urban myths of summertime – that drinking a cup of piping hot tea will kickstart your sweat glands and ultimately cool you down more than an ice-cold drink.
Or is it?
Studies over the years have supported the seemingly obvious conclusion that cold drinks lower your core temperature more than hot drinks. But new research from the University of Ottawa suggests that these past studies used a flawed method of determining core temperature – in fact, under certain circumstances, a warm drink may be your best bet for staying cool.
One fact that's not under dispute is the importance of sweating: It's the most effective tool your body has to get rid of excess heat. The recent discovery of temperature-sensing receptors in the mouth, throat and stomach raises the possibility that a relatively small amount of hot liquid could trigger a sweat response without warming the body up.
To test this idea, Ollie Jay and his colleagues at the University of Ottawa's Thermal Ergonomics Laboratory had nine volunteers perform a series of 75-minute cycling bouts at a moderate effort level, while periodically drinking water at 1.5, 10, 37, or 50 C. The results will be published in the journal Acta Physiologica.
Unlike previous studies, Dr. Jay didn't rely on just a rectal thermometer as a proxy for core temperature. Instead, he monitored metabolic rate and deployed eight thermometers on various parts of the body, plus a rectal thermometer and one inserted through the nose down the esophagus. Using an approach called "partitional calorimetry," he was then able to obtain a much more accurate estimate of the total amount of heat entering and leaving the body.
The difference is subtle but crucial, because if you drink a big glass of ice water, you'll have cold liquid sitting in your stomach – right above where you've shoved your rectal thermometer. As a result, the thermometer may register a lower temperature even though the rest of your body hasn't really been cooled.
Sure enough, unlike previous studies, the new study found that drinking hot water triggered a sweat response that more than compensated for the heat of the drink. Cold drinks produced the opposite response, with a reduction in sweat cancelling out the cooling power of the drink.
So is this proof, finally, that drinking a hot drink on a hot day is smart physiology?
"Yes," Dr. Jay says. "Kind of."
The caveat is that your sweat must fully evaporate in order to produce the desired cooling effect. If you're exercising hard, or wearing too many clothes, or in a very humid environment, you may produce sweat more quickly than it can evaporate, in which case it's no longer desirable to ramp up your sweat rate further.
As a guideline, keep an eye on sweat accumulation, Dr. Jay says: "If the extra sweat just drips on the ground, then you're better off drinking a cold drink."
The next question Dr. Jay is tackling is how the body's internal temperature sensors work. He's testing the effects of menthol, which activates the mouth's cooling sensors without changing its temperature, and capsaicin from hot peppers, which activates the mouth's heating sensors. He's also sending water at different temperatures directly to the stomach of his subjects via a nasogastric tube, to test the stomach's sensors without triggering the mouth sensors.
For practical purposes, learning how these sensors work may ultimately prove to be more valuable than detecting subtle changes in body temperature. Both Dr. Jay's recent results and earlier studies agree that the direct heating or cooling power of a small glass of liquid can only produce very small changes in body temperature.
Until researchers like Dr. Jay discover more about these sensors, there is an alternative: If you drink a slushie before exercise, you can produce a measurable drop in core temperature that results in better performance in hot conditions. In this case, it's not just the temperature of the slushie that matters; it's the "phase change" energy required to transform a solid (ice) into a liquid (water), which draws heat from your body.
For now, if you're planning to exercise hard in the heat and you're looking for a boost, that's your best bet.
Alex Hutchinson blogs about research on exercise at sweatscience.com. His latest book is Which Comes First, Cardio or Weights?