Sunday, August 11, 2013

Phun Physiology: Does a hot drink cool better than a cold one?

The Cup of Tea, ca. 1879. Mary Cassatt (American, 1844–1926)
 Oil on canvas (Wikimedia public domain)

Now that the dog days of summer are chasing my wheels, I decided to research an astonishing claim I recently read on a cycling listserv about keeping cool. I’ve included links to references for the interested reader.

My search led me to not one but two popular-press articles published a year ago asserting that a cup of hot tea could cool cyclists better than a cold beverage. If there is any merit to the claim, then American cyclists ought to institute afternoon tea not just for propriety, but also for safety and performance.

A team of researchers at the University of Ottawa made the remarkable discovery that drinking a hot beverage triggers a sweat response which is more than able to compensate for the warming effect of the hot beverage.

The finding begs the question of whether turning on the sweat spigots leads to greater cooling in all cases. The lead researcher, Ollie Jay, notes that the cooling effect of drinking a hot beverage does not hold in all situations. He states:

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.

The tell-tale sign of producing too much sweat? When beads of sweat drop to the ground. At this point, evaporative cooling is not occurring. And there is no advantage to increased sweating. Jay states, “if the sweat’s not going to assist in evaporation, go for a cold drink.” Jay notes,

[I]f you’re in a humid locale—for example, anywhere on the East Coast—don’t try drinking hot water. But on a hot day in the desert, a cup of hot tea might actually be the trick to help cool you down.

Other studies support the intuitive notion of drinking cold beverages to stay cool. One study by Lee and Shirreffs (2007), for example, reports that at 25.4C [77.7F] and a relative humidity of 61%, exercising cyclists who drank cold beverages had significantly lower skin temperatures and lower pulse rates than cyclists who drank warm fluids.

Yet another study by Burdon et al. (2010) conducted at 70% relative humidity and 28C [82.4F] reveals that

Consumption of cold beverages during prolonged exercise in the heat improves body temperature measures and performance.

This team of researchers postulated,

Beverages consumed by athletes exercising in the heat should perhaps be cold for performance and safety reasons.

In sum, it is probably safe to assume that the heat and humidity of North Carolina summers warrant cold drinks all around, especially if you are working hard! 

What’s your cup of tea?

Let’s ride!


Unknown said...

Dean, I love these conversations - I remember your dissertation on cramps many years ago.

I have a chiller at work that we bought hoping to make 25F fluid with it, but some of the cold piping was not insulated and it sweated like crazy. The state change from vapor to liquid and then dripping away to the ground is a huge loss of efficiency.

In this case the water dripping off was a big part of it. The dripping water is a big part of the loss - if it just stayed in place the efficiency would not drop nearly as much.

In think with sweating, the loss of dripping sweat is still a contribution to cooling - you just can't get any more cooling from the state change of evaporation.

You might hypothesize, from an evolutionary POV, that we would stop sweating (or at least stop sweating beyond what can be carried away by evaporation) whenever the humidity is too high for the rate of cooling needed. Clearly dehydration is a real problem. It seems like dripping off a lot of sweat is more advantageous then conserving it - maybe because it does have a limited, but useful cooling effect.

lynnef said...

Clearly they didn't include the "I get hot flashes just LOOKING at a hot drink in the heat" participant group.

dean furbish said...

@Unknown: You make valid points: 1) the reduced cooling efficiency of sweating due to not being able to capture the phase change of evaporative cooling in high humidity; 2) the fact that there is some cooling benefit of increased sweating in high humidity, although doing so raises the risk of dehydration.

Thinking out loud with you for a moment, although I'm aware that athletes can acclimate to the heat and also that well-conditioned athletes sweat less --- I'm wondering if there is such a thing as "humidity acclimation." I guess I've just created another library assignment for myself!

dean furbish said...

@lynnef: Love the science humor. 'Cause it could be true!

If there is a cognitive aspect to gastric juice production involving the hypothalamus because we are thinking of food, why can't there be a cognitive trigger in some cases for hot flashes when we think of hot drinks, since we believe that the hypothalamus is involved in those situations as well?