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What is an Ice Slush and Is It Effective?

Ice slushes have see widespread use among top athletes during the Tokyo Olympics and the 2022 Tour de France. But what are they, how do they work and are they effective?

Ice slushes are a very low temperature drink made with blended ice and liquids. Popular at your local convenience store or movie theatre, ice slushes are also used for their sports nutrition benefits in hot weather. Ice slushes were used abundantly by athletes, coaches and teams at the very hot Tokyo Olympic Games. We also developed and employed them very effectively with Team Jumbo-Visma and other World Tour teams at the 2022 Tour de France. They can be used for pre-cooling before a race or competition, during exercise or breaks and post-race to reduce and maintain core temperature. A high body temperature contributes to fatigue and preventing very high temperatures has been shown to improve performance. This can be done by various ways of pre-cooling and perhaps also by cooling during exercise, although this is much more difficult from a practical point of view. In conditions such as those at the Tokyo Olympics with high temperature and little opportunity for evaporative cooling (cooling through sweating) because of the high humidity, cooling body temperature using other methods like cooling vests and ice slushes is especially effective.


Effects on physiology

Ice slushes can cool an athlete much more and more quickly than drinking a cold drink. The energy (heat) required to turn ice into fluid is over 80 times as high as the energy required to simply heat up cool water (based on the theory of enthalpy). Therefore, ice slush is a very effective way of reducing body temperature and is a process that occurs inside the body (and does not rely on environmental factors like sweating). One potential problem with both ingestion of ice slush and cold fluids is that they activate special thermoreceptors (receptors in the body designed to detect changes in temperature) in the abdomen. Activating these thermoreceptors with cold exposure leads to a reduction in sweating, no matter the temperature of the skin or the environment. This is problematic because sweating is the most effective method of removing heat when core temperature rises. Therefore, there are concerns that although consuming ice slush might make the athlete feel cooler, there may be no overall benefit to consuming it if it reduces sweating.


Pre-cooling with slush

Although drinking ice slush/cold fluids during exercise does reduce sweating, consuming these before exercise starts (when there is no sweating taking place), appears to be effective at lowering body temperature. Although different studies have used different amounts and under different conditions, ice slush can reduce core temperature by approximately 0.5°C. In a variety of studies, this reduction in core temperature leads to the ability to exercise for longer (increased exercise capacity). Many of the studies using ice slush have used endurance exercise, such as running or cycling, but some have also used intermittent, interval exercise, and ice slush appears to be effective at improving capacity and performance for both. This is thought to be due to the delaying in reaching a ‘critical’ core temperature, at which exhaustion occurs. Although sweating may be reduced in the early stages of exercise if ice slush is consumed beforehand, this is quickly overridden when no more slush is consumed, and the reduced core temperature remains. However, it may be possible to consume too much slush. Not only has this the potential to cause gastro-intestinal distress, if core temperature drops too much this can have negative effects as well. Therefore, care should be taken not to use too much ice slush or use it when the environmental conditions are cool.


Benefits of slush during exercise

Because consuming ice slush during exercise reduces sweating, slush does not lead to much of a reduction in core temperature when consumed during exercise. However, many of the studies that have used ice slush have used moderate temperatures (~25°C) which would be considered warm, but do not represent competitions or races in hot countries, or in the summer of temperate countries, where >30°C conditions are common. Many of the studies that have looked at ice slush in high temperatures (32-35°C) have shown a reduction in core temperature, and even improved performance. Above these temperatures, control of body temperature becomes more and more difficult, as the effectiveness of sweating reduces. Sweating is more effective at removing body heat when it can evaporate quickly. When temperatures are very high, sweat may be produced faster than it can evaporate, and it drips from the skin. This also occurs in humid conditions, where sweat does not evaporate quickly. In these conditions, ice slush appears to be most effective. Although consuming the slush may reduce sweating, in these conditions (high temperature and/or high humidity), more sweating is not necessarily effective at controlling core temperature. Therefore, the effects of slush on core temperature can be large.


Practical implications

Ice slush may be less effective for cyclists than runners or other athletes because it is most useful when sweating efficiency is low. When cycling, air movement past the body is very high, which can increase sweating efficiency even when the temperature and humidity are high. By the same token, it may be particularly useful for sports where the athletes are covered in clothing or protective gear (e.g. hockey, American football) and sweating efficiency is poor. Ice slush is probably beneficial when temperature and/or humidity is high, and sweat drips or pools, rather than evaporates. Although slush can be very useful, it is quite impractical in many cases. A blender or ice slush machine is usually needed to make it, which can make consuming it before or during a race difficult.


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