An icy treat before a run in the heat improves performance and lowers body temperature.
Written by: Matt Fitzgerald
Four summers ago I crewed for Dean Karnazes at the Badwater Ultramarathon. For some reason I asked Dean’s other three support crew if I could take the first turn pacing him, while he was still in Death Valley. I encountered no objections. I ran with Dean for 25 miles in 113-degree heat. I then bailed out and let the next pacer take a turn as Dean continued for another 92 miles (having already covered 18 miles before I started pacing him).
It was an interesting experience. We kept Dean going by stopping our two vans after every mile and supplying him with cold drinks, pouring ice water over his head, and giving him ice to put under his hat (which always melted long before he’d covered the next mile).
All of these measures helped Dean, I’m sure. But according to the results of a new study conducted by the University of Chicago’s Jonathan Dugas (with whom I had the pleasure of coauthoring The Runner’s Body, along with Ross Tucker), there’s something else Dean could have done before even starting the race that would have given him an additional boost: eat a slushie.
Granted, Dugas’ experiment was not done in the context of the Badwater Ultramarathon, but it was a fairly extreme test nevertheless for the 10 participants, who were not competitive runners: a treadmill run to exhaustion at ventilatory threshold intensity in a 95°F environment. This test was done twice, with at least five days between the sessions. Before one treadmill run to exhaustion in the heat the participants drank cold (39°F), flavored water. Before the second, otherwise identical session they ate an ice-cold (30°F) slushie of the same flavor and overall volume.
Dugas was curious to see how the slushie would affect the participants’ body temperatures before and throughout the running test, as well as how it would affect their performance. Before they had even stepped onto the treadmill Dugas found that the slushie lowered their core body temperature compared to the cold water. Then things got really interesting. On average, the subjects were able to run approximately 40 minutes before quitting in exhaustion after they had drunk the water. After eating the slushie, they survived for an additional 10 minutes, on average—a massive, 25 percent improvement. Even more interesting, at the time they quit, the participants had slightly higher core body temperatures in the slushie trial than in the cold water trial: 102.8 °F versus 102.3°F.
What do these results mean? First of all they demonstrate that core body temperature is a major limiter of running performance in the heat. They also reveal that reducing core body temperature before an exhaustive run in the heat enhances performance by increasing the amount of time it takes for the core body temperature to climb up to the maximum tolerable level.
There’s more, though. If the effects of the slushie on the subjects were entirely physiological, then they still would have quit running when their body temperature reached 102.3°F, as they did in the cold water trial. And if this had been the case, they might have run only 5 minutes longer instead of 10 minutes. But in actuality the subjects were able to get even hotter inside before raising a white flag.
This particular finding indicates that there was a psychological effect at work alongside the physiological effects. Further indications of this are to be seen in the fact that the subjects gave lower ratings of perceived effort and thermal sensation while running in the slushie trial than in the cold water trial. So it appears that, in addition to cooling them down, the slushie also made the subjects feel cooler and more comfortable, which enabled them to run to the point of reaching a higher core body temperature before they felt too hot and uncomfortable to continue.
There aren’t too many Dean Karnazeses out there. Most runners avoid racing in temperatures of 95°F and above. But if you ever do, be sure to eat a slushie before you start.
Matt Fitzgerald is the author of Iron War: Dave Scott, Mark Allen & The Greatest Race Ever Run (VeloPress 2011) and a Coach and Training Intelligence Specialist for PEAR Sports. Find out more at mattfizgerald.org.