To drink or to pour: How should athletes use water to cool themselves?

It’s almost that time again. With the 2016 summer Olympics in Rio just around the corner, the season of dreams is upon us. It’s the time when we watch seemingly real life super heroes push themselves to the limit, while bringing back early memories of when we wanted to be those athletes on TV that children look up to. At the Rio games, not only will these super humans compete against each other, they must also contend with the hot and balmy conditions. Some of us, during sporting activities, may be familiar with battling against our own minds and dealing with the oppressive force of the heat, when we would happily accept any measure that alleviates the discomfort of our exertions. With respect to endurance sports in the heat, this relief can come with sipping cool water, spraying our face with a cool mist, or wrapping an ice towel around our necks. The cover photo of this edition of Temperature illustrates a scenario of two elite triathletes, Andrea Hewitt (on the left) and Rachel Klamer (on the right), dousing themselves with water from their bottles in order to cope with both the internal heat they are producing through muscular contractions, and the external heat from the surrounding environment. We sometimes see athletes self-dousing with water, or have done it ourselves, in order to attain an immediate relief from the heat rather than taking the time to drink. But is this a smart move? Shouldn’t we just drink the water instead, or even better – drink iced water? To answer this question, we must consider how much heat we can lose to water in its various forms. The most straightforward way is to do so via conduction following the ingestion of cold water. The amount of heat lost is determined by the temperature difference between the ingested water and the body core, the volume of water drunk, and the specific heat capacity of water, i.e. the amount of heat energy needed to warm up 1 g of water by 1 C, which is 4.184 J/g/ C. We can dramatically increase the amount of heat lost to water by adding ice into the mix, as the amount of heat required to melt ice, known as the latent heat of fusion, is much greater than the specific heat capacity of water at 334 J/g. It is this much greater potential for heat loss that has led to the recent trend of athletes consuming ice slurry drinks, a mixture of shredded ice and water, before or during their athletic activities. Despite this improved potential for heat dissipation, melting ice is still a far cry from the amount of heat we can lose through the evaporation of water, as just one gram of evaporated water results in the liberation of a massive 2430 J of latent heat energy. To put these different cooling strategies into context, we can directly compare heat loss potential with a fixed volume of water (Fig. 1). Assuming a core body temperature of 38 C, drinking one glass (250 ml) of 1 C water would result in a net heat loss of 39 kJ. Whereas if the contents of that glass were changed to half-water and halfice, the potential for heat loss would more than double to 81 kJ. However, if we could somehow spread that 250 ml of water across our skin surface so that it all evaporated, the resultant heat loss would be a whopping 607 kJ. One caveat is that, while it is relatively easy to ingest water without spilling it, ensuring 250 ml of water is distributed across the skin in a way that it all evaporates is much more difficult. However, it is important to keep in mind that if just 15% of that water evaporates from the skin, the heat loss would still be greater than ingesting the entire 250 ml ice slurry. Another consideration is that the exercise modality and environment in which we perform exercise may alter the effectiveness of dousing ourselves with water, based on how likely the water is to evaporate. For example, dry air and high wind speeds greatly favor evaporation, so cycling in the desert may be an ideal situation for selfdousing with water, as most of it is likely to evaporate. Conversely, high levels of ambient humidity and low air