You often see pictures of elite marathoners pouring their drinks over their heads. Actually, scratch that: it’s not just elites. Lots of people pour their drinks over their heads on hot days. It feels good (unless you accidentally grab the Gatorade).

But is it useful? Or is it always better to take the time to drink your drink?

In a light-hearted editorial in the journal Temperature, Nathan Morris and Ollie Jay of the University of Sydney’s Thermal Ergonomics Laboratory run some numbers on this question. The results are surprising.

Some basic numbers: If you drink 250 mL of water (about a cup) at just above freezing (1 C, 34 F), you’ll get rid of 39 kJ of heat, or about 9 calories. (To be more specific, kilojoules and calories in this context are a measure of the energy dissipated as heat; the heat is produced as a byproduct of muscular exertion, which is powered by the energy from food. However, you can't short-circuit the process by simply pouring water over your head and hoping this will burn a lot of calories to make you slim—though there is a bit of very thin evidence that keeping the thermostat at a setting that forces your body to burn some energy to stay thermally comfortable might influence long-term weight balance.)

If you drink a slushie that’s half water, half ice, you’ll get the extra benefit of the energy that’s required to melt the ice inside your body. A 250-mL slushie will rid you of 81 kJ of heat (19 calories).

If you instead pour that cup of water on your head, and—this is the tricky part—ensure that it spreads around your body surface so that it all evaporates rather than dripping to the ground, you’ll get rid of a staggering 607 kJ (145 calories). Evaporation is a fantastic way of dissipating heat.

Of course, it’s hard to dump a bottle of water over your head without spilling any. But even if you spill 85 percent of the water, Morris and Jay point out, you still get more heat loss than from drinking the slushie.

There are some other caveats. For example, the water has to evaporate. If it’s a muggy, humid day where your sweat is already dripping to the ground, you won’t be able to evaporate any more water (though pouring cool water on your head may still feel good!). The best situation is when it’s dry and/or breezy enough that your sweat is evaporating: the ideal, they say, would be “cycling in the desert.”

The best option, in the end, is probably “all of the above”: Drink some, pour some on your head. That’s what a lot of the athletes at the U.S. Olympic Marathon Trials on a hot, dry day in Los Angeles in February did. Sometimes science confirms what the body is already telling us.

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