Drink to it: why ice is your hot-weather friend

Sports Performance Bulletin investigates recent research on the merits of consuming ice-cold drinks and ice slurries during hot-weather training and competition

When the mercury rises, endurance performance can suffer – particularly in longer events. One reason is that a combination of heavy sweating and inadequate fluid replacement can lead to dehydration. While the body can tolerate modest amounts of dehydration without impacting performance, higher levels (above 2-3%) can significantly impair performance. There’s also the effect of heat itself. When the body diverts blood flow to the skin in order to lose heat, the blood and oxygen supply to exercising muscles is reduced, which then affects maximum work capacity.

It’s no surprise therefore that in hot conditions, strategies that can help prevent the body’s core temperature from rising excessively can help maintain performance. Wearing cool garments and using water sprays are one way to help cool the body on the move. Another is to cool the body from the inside by using very cold or iced drinks. Ice is particularly useful for cooling because it takes a lot of heat energy to turn ice at 0C to water at 0C (more technically known as the latent heat of fusion). For this reason, ice slurry drinks have been investigated by researchers for their cooling properties, and some research suggests that consuming an ice slurry drink can be as effective at protecting performance in the heat as undergoing heat acclimatisation!

The research

In one study, scientists investigated the performance effects of a pre-cooling ice drink consumed before a cycling time trial performed in the heat [Int J Sports Physiol Perform. 2018 Feb 1;13(2):228-234]. In particular, they wanted to see how these benefits compared with the more traditional way of coping with heat - a period of heat acclimatisation training. To do this, fifteen male cyclists/ triathletes completed two cycle time trials in the heat, with a 12-day training programme in between. In the first trial, all the participants consumed 7g 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 acclimatisation 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 acclimatisation group drank water during the trial (as they had done in the first trial). However, the pre-cooling group used a different strategy and consumed a large ice drink before commencing the trial. The heat acclimatisation group were obviously expected to improve (having become acclimatised). The key question was whether the unacclimatised pre-cooling subjects would also derive a benefit from their pre-event ice drink, and if so, how it compared to proper heat acclimatisation?.

The results

When the results of the second time trial were compared, both groups improved compared to the first trial. Moreover, it became clear that the pre-time trial ice drink was just as effective at improving performance (a 166-second improvement) as heat acclimatisation training (a 105-second improvement). Indeed the ingestion of ice produced a greater improvement in times, but because of the small number of subjects in the trials, the scientists were unable to conclude whether this extra gain was statistically significant. What was clear was that when ice was ingested, the cyclists’ core temperatures stayed lower for longer and they also started sweating later into the time trial. Also, they reported lower levels of ‘thermal distress’, all of which led the researchers to conclude that ‘pre-cooling with ice ingestion offers an alternate method of improving endurance cycling performance in hot conditions if heat acclimation cannot be attained’.

Implications for sportsmen and women

These results are in line with previous recent research, which has found consuming ice slurry before and during endurance events in the heat can improve performance^{Scand J Med Sci Sports. 2015 Jun;25 Suppl 1:39-51. doi: 10.1111/sms.12366}. However, what’s interesting about this research is that it tells us these benefits are significant, and at least equal to twelve days of heat acclimatisation 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 acclimatisation. It would have been good to have a third group of athletes who underwent heat acclimatisation AND consumed ice drinks, to see whether these benefits are additive. But that’s another study for another day!

Practical advice

• In hot conditions, always start any exercise well hydrated.
• Prior to starting (eg on the race start line), try to keep in the shade and consume very cold or (even better) iced drinks.
• 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 for further cooling.
• Use a compact, portable water spray for additional cooling.
• Don’t forget 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 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.