In my previous letter, I highlighted our study showing that when athletes were unaware of their hydration levels and exercised in conditions simulating the outdoors, including real-like wind speeds, they did not suffer any negative impact on their exercise performance even when dehydrated up to -3% of their body weight. The practical lesson learned from this was that we should drink fluids when we feel thirsty. We also found that despite the hype, there were no documented cases of deaths from dehydration, but there were cases of deaths due to overdrinking.

Now, you might wonder how and why overdrinking can lead to such fatal outcomes.

As fate would have it, during my time as a young professor in Australia, I embarked on a research fishing experiment. Excitingly, a plethora of new equipment had become available, including a thermal pill system to measure core temperature and an iStat blood electrolyte monitor to measure sodium levels, in addition to the standard heart rate, power, and GPS monitors we commonly use today. This presented a unique opportunity to conduct a case study during the Ironman Bussleton event in Western Australia. With an entire research team studying my performance, I willingly became the guinea pig for this investigation, which was subsequently published as a case report.

The race was a hot one that peaked at 33 degrees Celsius, and to cut a long story short, it turned out to be a dreadful race for me. I had a decent swim and a good bike ride, but after 20 kilometres of running, I experienced a complete breakdown, suffering from intense full-body cramps like I had never encountered before. The unpredictability of these cramps was the most novel and distressing aspect. I couldn't anticipate when or where they would strike, adding immense stress during the final hours of the race. It became a very long and painful walk resulting in my slowest time in all 18 Ironman races I had participated in (11:38).

A bit more context. Prior to this race, I held a biased belief that I needed to pre-empt my thirst by consuming the maximum amount of fluids tolerable. This notion was prevalent at the time and translated into an estimated intake of 1.7 litres per hour during the bike phase for me; likely more than double the emptying ability of my stomach. It was hot out, I was feeling hot (my core temperature got to 39.3 degrees Celsius in the run), and I thought that more fluids would improve the situation allowing me to enhance my performance.

All in all, this challenging experience served as a crucial opportunity for reflection and data analysis (see figure).

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But the learning didn't end there. Upon returning my accommodation to recover, I experienced vertigo and hallucinations. Something was seriously happening to my mind, and it wasn't nice. Because we were taking blood samples, we later discovered that my sodium levels had plummeted to 128 mEq/L (clinical hyponatremia; normal is ~140), just two hours after the race. With all the fluid I had consumed, I had inadvertently put myself at risk of brain swelling, which can be fatal. Fortunately, my body righted itself, but the lesson was clear. Drinking ahead of thirst is unnecessary and dangerous.

In summary, my experience as a case study highlighted the dangers of overdrinking and the potential consequences of disregarding our body's natural signals during intense physical activities. Proper hydration is essential, and drinking when thirsty remains the best approach.

Having followed these accounts, the next questions that may arise are how much body weight you should expect to lose in racing, and whether the issue of hyponatremia could be prevented by supplementing with sodium. In the forthcoming discussions, I will share my perspective on these matters to offer valuable insight.

To your health,

Paul Laursen, PhD

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