It’s no surprise that it’s hard to exercise on a hot day. You go slower. An easy workout is gruelling. You have no endurance.

The reasons are also no surprise. Blood is directed to the skin for cooling, which means it is diverted from working muscles. The hotter it is, and the harder the effort, the harder exercise becomes. Eventually, you slow down or stop, unable to go on. Exercise physiologists debate why. It could be that muscles are starved for blood. It could be that the brain gets too hot. It could be that the heart eventually can’t beat fast enough to satisfy all the demands for blood. But even without knowing why, researchers have found they can delay the time to utter exhaustion by getting people to chill before they start.

So companies sell devices, like cooling vests to wear before exercise, or even portable cold baths for prerace immersion. Researchers have tested methods like having athletes swim for an hour in cold water or sit in a cold room or stand in a cold shower. No matter what the method, companies and researchers report a precooling effect. The problem is that none of the methods are easy, cheap and practical. But now, a New Zealand endurance athlete and exercise researcher says he has found a method that is.

Ice, ice baby
All you have to do is drink an ice slurry, also known as a slushie, before exercising. In a new study, he reports that young male recreational athletes who drank a syrup-flavoured ice slurry just before running on a treadmill in a hot room could keep going for an average of 50 minutes before they had to stop. When they drank only syrup-flavoured cold water, they could run for an average of 40 minutes.

There are limitations — the running test was indoors, so there was no cooling effect from breeze on the skin. In those artificial circumstances, precooling might’ve had more dramatic effects than it would outdoors. And what athletes really want is to go faster in a race, not run until they drop. But the study tested endurance, not performance, which is typical of such research.

Short-term benefits, only?
The effect was short-lived, according to the senior investigator, Paul Laursen, at the New Zealand Academy of Sport in Auckland and a competitor who has raced in 13 Ironman triathlons (a 2.4-mile swim followed by a 112-mile bike race followed by a marathon-distance 26.2-mile run). It would not even begin to last long enough to run a marathon or do a century (100-mile) bike ride, for example. But it would be perfect for a sport like tennis or for a 5- or 10-km race or for team sports like soccer or football, and it might give endurance athletes in longer events a boost by letting them beat the heat, to a certain extent, for the first 50 minutes or so.

It’s not so clear why ice slurries or any other method work, though. As Dr Laursen notes in a review article, “The mechanisms underlying the performance effects associated with precooling are not yet completely understood.”

One possibility, which Dr Laursen said might have explained the ice slurry results, was that slurries lowered body temperature before the young men ran, letting them run for a longer time before their bodies became critically hot.Dr Montain is not sure. The men did not get that hot, he said. At exhaustion, the mean body temperature of those who drank the slurry and those who drank cold water was about 101 degrees.

Ice-cream headaches
He said the limiting factor might have been the men’s heart rates. In both groups, exhaustion occurred when heart rates reached about 185 beats per minute. Those who drank the slurry hit that heart rate 10 minutes later than those who drank cold water. Dr Montain says the heart beats faster and faster, trying to get blood to the skin, for cooling, and to the muscles for exercising, until it just can’t keep up.

“The heart can’t send blood everywhere without the blood pressure falling,” Dr Montain said. “At some point, you can’t maintain your blood pressure.” But, Dr Montain said, what mattered in the end was that the simple solution of drinking an ice slurry worked. He might even try drinking a one himself if he’s stuck racing in the heat, something he tries to avoid.

“I try to race when the weather is good,” Dr Montain said.
Dr Laursen, though, is drinking slurries. He gets ice cream headaches when he drinks them — everyone does, he said — but he puts up with the pain. It’s worth it.
When he has a slurry before a gruelling bout of exercise in the heat, he said, “I feel so much better.”