When the thermometer climbs past thirty-five degrees, the instinct is to reach for an ice-cold drink. A poor choice? Not quite so simple. Here is a quick look, with the science to back it up.
1. Iced drinks: a false friend
A very cold drink is refreshing in the moment, there is no denying it. Yet there is a catch: on receiving that signal of cold, the body cuts down its perspiration. And it is precisely sweat, as it evaporates, that truly cools you down. The result is immediate relief, but body heat that is released less efficiently over time. Not to mention the thermal shock, which can be hard on the digestion.
2. Hot drinks: a paradox that holds up
It seems counterintuitive, yet the work of the thermoregulation researcher Ollie Jay, at the University of Sydney*, has shown it: a hot drink can actually help you cool down. By raising your temperature slightly, it makes you sweat a little more, and it is that evaporation which brings the body's temperature down. A word on the conditions, though: it only works if the sweat can evaporate, which means dry air and uncovered skin. In full humidity, or buttoned into a suit, all you gain is the heat. The mint tea of hot countries, then, is no accident.
3. The right compromise: cool, not iced
Between the two extremes, practical sense wins out. Health authorities recommend neither: their message is to hydrate regularly, with cool rather than iced water, and to steer clear of alcohol and excess sugar. Temperate water has one concrete advantage, namely that it is better tolerated, so you drink more of it and hydrate more effectively. And what of the skin in all this? Good internal hydration means a less dull complexion and a barrier that holds firm. In a heatwave, your face is every bit as thirsty as you are.
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* Bain AR, Lesperance NC, Jay O., "Body heat storage during physical activity is lower with hot fluid ingestion under conditions that permit full sweat evaporation", Acta Physiologica, 2012.