Cool it: beat the heat with ice

Ice and heat acclimatization

As the interest in ice slurry drinks for aiding performance in hot weather steadily grew, scientists began looking at how ice slurry ingestion compared to heat acclimatization. In a 2018 study, 15 male cyclists and triathletes completed two cycle time trials in the heat, with a 12-day training program in between⁽⁹⁾. In the first trial, all the participants consumed 7 grams of water per kilo of bodyweight prior to completing a time trial in hot and humid conditions (35°C and 50% relative humidity). The participants were then split into two groups:

• Pre-cooling group – the subjects trained for twelve days as usual in normal weather conditions (ie not hot and humid).
• Heat acclimatization group – the subjects trained for twelve days in hot conditions (35°C and 50% humidity).

After the 12-day intervention, the time trial was repeated in hot conditions. The cyclists in the heat acclimatization group drank water during the second trial (7g per kilo, as they had done in the first trial). However, the pre-cooling group used a different strategy and consumed a 7g per kilo of an ice drink before commencing the trial. By following this protocol, the scientists were able to compare the effectiveness of ice drinks vs. 12 days of heat acclimatization training.

The results showed that pre-time trial ice drink was just as effective at improving performance (a 166-second improvement in trial #2) as heat acclimatization training (a 105-second improvement in trial #2). Also, cyclists’ core temperatures stayed lower for longer, they started sweating later into the time trial, and reported lower levels of thermal discomfort. In short, the performance benefits of consuming 7g per kilo of bodyweight of ice slurry prior to exercise was at least equal (and likely superior) to twelve days of heat acclimatization training! This could be a real boon for athletes who are competing in hot events but don’t have the time or money to travel early and undergo heat acclimatization.

The latest research

If anything, the research on ice slurry drinks prior to and during exercise has accelerated over the past five years with a number of further published studies, so what does the most up-to-date research have to say? In November of last year, a team of scientists carried out a comprehensive review of all the previous research on ice slurry drinks and performance⁽¹⁰⁾. The researchers scoured the literature for studies on ice slurry for exercise performance, and which met the following criteria:

• Studies that include the details related to pre-exercise ice-slurry ingestion, thermoregulation and exercise performance of athletes.
• Studies that were conducted in concern of age group 20–35 yrs.
• Studies that were published in peer reviewed journals.

In total, 11 articles (see a list here) met the inclusion criteria and were analyzed. The conclusions was that the majority (six studies) reported a significant decrease in core or rectal or gastrointestinal or skin or body surface temperature. Meanwhile, six studies reported a significant increase, or positive influence on exercise performance, and of these, three studies also reported decreased temperatures. When the researchers looked at the data from the studies showing performance benefits, it revealed that ingesting ice slurry drinks delivering 7-14 grams of slurry per kilo of bodyweight was preferable; consuming lower or higher amounts was less effective. Also, it didn’t seem to matter whether slurry drinks were prepared from plain crushed ice, or sports drinks, or carbohydrate and electrolyte drinks – they all had a similar effect on thermoregulation and exercise performance.

In a study carried out in June last year, scientists compared the effects of ingesting ice slurry or very cold fluid during a cycling task in the heat (35^oC [95^oF)⁽¹¹⁾. Unlike other studies where the fluid/ice intake was fixed, the cyclists in this trial were allowed to consume as much or as little as they liked. The purpose was to find out whether, given the choice, athletes would be likely to consume more fluid as ice or less compared to a standard cold drink. The results showed that when consuming ice slurry, the cyclists consumed half the volume of fluid compared to the cold drinks trial. The really interesting thing however was that when consuming ice slurry, the cyclists performed just as well, stayed just as hydrated, sweated the same amount and experienced no more thermal stress than when they drunk double the amount of fluid. The clear implication is that because it’s much more effective at cooling, smaller ice slurry amounts are required to maintain hydration in the heat.

Half-marathon performance and ice slurry ingestion 

In study earlier this year, scientists investigated half-marathon performance in hot and humid conditions – specifically whether ice slurry ingestion or water at ambient temperatures resulted in superior performance⁽¹²⁾. Ten well-trained runners were initially required to run two half marathons: one while consuming either ice slurry at -1°C and another where they consumed water at 37°C. In both of these initial half marathon trials, the runners were free to consume as little or as much ice/water as they wished.

In stage two of the study, the runners then performed two other half marathons where, during one, they were required to ingest an amount of water equivalent to the amount of ice they had consumed during the initial trial, and in the other, they had to ingest an amount of ice slurry equivalent to the amount of water consumed in the initial trial. During all these half marathons, the temperature was maintained at 33^oC and humidity at 60% (humid!). The times achieved in each condition were then compared.

The key finding was that when consuming water/ice freely in the initial stage, the runners tended to ingest much less ice slurry (around 350 grams) during the half marathon compared to water (635 grams). However, performance, heart rates, perceived exertion, body temperatures, and thermal perception were no different between the two trials. Remarkably however, then the runners swapped over (consuming more ice and less water), no differences in any of these parameters emerged – ie 635g of ice slurry ingestion conferred no advantage over 350 grams of water ingestion. But when the data was further analyzed, it emerged that consuming 350 grams of either ice or water resulted in better times than 635 grams of ice or water. This suggests that over relatively short distances with minimal levels of dehydration and core temperature rises, consuming smaller amounts of ice or water during exercise could actually be easier on the GI system, and lead to better performance.

Summary and practical recommendations

Let’s draw this research together and make some practical recommendations. The first thing to say is there’s now very solid evidence that in hot and humid conditions, the consumption of ice slurry drinks do offer significant performance and comfort benefits to endurance athletes. However, the studies to date have also revealed nuances in terms of what circumstances they can offer these benefits. Let’s summarize these:

• For shorter events (under 90 minutes) ice slurry drinks seem to be most effective when consumed immediately prior to the event. This is most likely because it takes a while for the ingested ice to melt and draw heat away from the rest of the body.
• For longer events in the heat such as marathons, ultra-marathons and longer triathlons, consuming ice slurry during the event is likely to provide performance benefits later on in the event. For triathletes, the ideal time to consume ice is during the cycling leg.
• The optimum amount of ice slurry needed to produce beneficial effects seems to be somewhere between 7 and 14 grams per kilo of bodyweight.
• When combating the deleterious effects of heat and humidity, it doesn’t seem to matter what is used to make the ice slurry drink (ie plain water or carbohydrate drink or electrolyte drink).
• Compared to water ingestion, smaller amounts of ice slurry can have the same cooling effect, giving the same performance benefits. This maybe especially advantageous for athletes with sensitive tummies.
• Athletes travelling abroad to hot climates can use ice ingestion before and during the event to help combat the heat, thus reducing the time needed for heat acclimatization.

In terms of the practicalities of using ice slurry drinks, all that is needed is a blender, some ice from the freezer and some water/juice carbohydrate drink – and of course an insulated flask in which to store it. When ice is removed from the freezer, it comes out at around -18^oC. To produce ice slush in a blender, you will need to mix the ice and room-temperature water in the ratio of around two parts of ice to one part of water then transfer it immediately to an insulated flask. Make up the ice slurry as near to your event as possible and try and keep your flask as cool as possible en route to your event/race (eg store it in a shaded part of the car and ensure your air con is running!). For shorter events, consume the ice slurry during the 15 minutes prior to the event start. For longer events, you will need a friend of family member to hand you your drink somewhere along the route!

Finally, while ice-slurry drinks can help performance, it’s important not to forget that there are other useful tips and strategies that are important for success in the heat, which can be used with or without ice slurry drinks:

• In hot conditions, always start any exercise well hydrated.
• Prior to starting (eg on the race start line), try to keep in the shade.
• If you have support from family friends, ask them to bring a cold box and supply iced drinks en route and/or pass you ice packs/vests for further cooling.
• Use a compact, portable water spray for additional cooling.
• Don’t forget your clothing; white reflective clothing will be cooler than dark colours, and materials that wick away moisture from the skin will help reduce unpleasant thermal sensations further.
• If you can undergo some pre-event heat acclimatisation, this will still help you in the later stages of a hot race, when the benefits of ice have worn off and when further ice drinks may not be available. 

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