A couple of years ago research from the University of Ottawa's Thermal Ergonomics Lab about how drink temperature affects sweat rate came out. You know those stories about how people in India sip hot tea to stay cool during the hot months? It turns out they're not crazy: when you do the measurements carefully with modern thermometry and sweat measurement, you find that hot drinks really do trigger elevated sweating that more than compensates for the added heat of the drink.
Still, it's not clear exactly how or why this happened. The same group, led by Ollie Jay (who's now at the University of Sydney) has just published a new paper in the Journal of Applied Physiology that takes a closer look at exactly what triggers the sweat changes, with interesting results. They did two separate experiments; in the first, subjects cycled at a moderate pace for 75 minutes while drinking fluid at three different temperatures: 1.5 C (cold), 37 C (body temperature), and 50 C (warm). They drank a small dose of 3.2 ml per kg of body weight before and at three points during the exercise. Meanwhile, their temperature at various points in the body (skin, rectal, and ear canal) was measured, along with local sweat rate on the forehead, upper-back, and forearm.
It's pretty clear and unambiguous: the warm drink causes an increase in sweating, and the cold drink causes a decrease in sweating. Surprisingly, though, the temperature measurements at all three sites were identical across trials – so sweat rate was changing even though the the drinks weren't causing a measurable change in body temperature.
The second experiment is even cooler. The protocol was similar, except with just two drink temperatures (1.5 C and 50 C) – and instead of swallowing, the subjects either swished the drink around in their mouth for 15 seconds per dose then spat it out, or had it delivered directly to their stomach via a nasogastric tube! The point was to figure out whether the temperature sensors responding to the drink temperature are located in the mouth or the stomach. Again, the results were unambiguous: swishing the drink in the mouth didn't cause any change in sweat rate, while delivering the drink to the stomach caused a suppression (for cold drink) or elevation (for hot drink) of sweat rate.
This experiment is mostly interesting for what it tells us about how the human body works – essentially they've discovered an entirely new set of thermoreceptors, likely in the stomach. Can we draw any practical lessons from this? Well, one key to remember is that sweat is a double-edged sword: it helps cool you down, but it also dehydrates you. At relatively low levels of heat production, more sweat is generally a good thing, because it evaporates instantly and helps keep you cool. At higher levels of heat production – i.e. exercising hard on a hot, muggy summer day – you're already sweating so much that you've maxed out your evaporative capabilities and are just dripping the excess sweat to the ground instead of evaporating it, which does nothing for you.
So, as the authors of the paper point out, drinking colder drinks when you're really overheating and oversweating should reduce your sweat rate without compromising your cooling power. In the relatively moderate conditions of the study (23.7 C room temperature, 32% humidity), the subjects sweated about 100 ml less when they drank the cold drink compared to the body-temperature drink, with no difference in temperature. In hotter and more humid conditions, the savings might be considerably more. As for the utility of sipping hot tea, it makes sense if you're sitting under a shady tree watching a cricket match (and thus not dripping sweat), but don't expect to see it on the drinks table at any races.
(Oh, and one final note: the drinks in this study were carefully chosen to avoid causing a big change in body temperature (i.e. small amount, relatively modest temperature difference). But that doesn't mean it's not possible to actually affect core temperature, which is why drinking ice drinks is a different question entirely. The researchers cite some interesting studies from the early 1970s by Nadel et al. in which subjects were given either ice cream or hot pudding. In that case, the hot pudding supplied a heat load that was seven times larger than the warm drink in this trial, and ice cream caused a heat deficit five times greater than the cold drink. As a result, the subjects did indeed show changes in sweat rate, but they also showed changes in core temperature, which made it impossible to be sure what caused the sweat changes.)