So here’s a little brain teaser to think about when you’re next exercising: what makes you thirstier – a serious swim, a long cycle or a full-on running session?

I got to thinking about this the other day when someone asked me whether you sweat when you swim. Good question.

First things first, though. What actually drives your thirst?

Thirst is ‘the physiological urge to drink’ or, in other words, it’s your body’s reaction to a change in fluid levels.

Like most of the physiological processes in the human body, specific areas in the brain regulate thirst. So when the body gets low on water, the ‘thirst centre’ in the brain is stimulated and prompts the need for fluid intake. For example, when you become dehydrated, which can happen because of heavy sweating after strenuous exercise, your body’s response is obvious – you ‘feel’ the sensation of thirst until you drink enough water to restore your body fluids to a normal level.

Now, about the sweating… So, whether you’re swimming, cycling or running, all athletes sweat, right? Yes, but the rate at which you sweat – and lose water by doing it – depends on two factors: the environment (i.e. temperature) and your metabolic rate (which is determined by how hard you are exercising).

According to Dr Phil Nathan from the School of Exercise and Sports Science at the Australian Catholic University, the tendency to sweat while swimming is reduced because swimming has a 25 times higher ‘thermal conductivity’, which is a measure of the ability of a substance to conduct heat, compared to air. In other words, the water temperature is often cooler than the human body temperature (21 degrees compared to 37 degrees, respectively), therefore the body will lose more heat in water compared to air, so the tendency to sweat is reduced.

As for the metabolic rate, research shows that running has a 10 per cent higher work output compared to cycling, and a 15 per cent higher work output compared to swimming – meaning that running generates the most body heat, and therefore a higher metabolic rate.

However, cyclists lose heat faster due to travelling through air a lot quicker, meaning cyclists will cool more efficiently compared to runners. On the other hand, runners rely heavily on sweat to cool the body down, meaning they lose water a lot quicker.

So to answer the question about which exercise is the thirstiest, in descending order it is running, cycling and then swimming.

Phew, even writing about it makes me thirsty…

Kathleen Alleaume

Kathleen Alleaume is an accredited exercise physiologist and nutritionist and founder of The Right Balance