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Dehydration itself may be less potent than fearing it

 •  • by Paul Ingraham
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How thirsty do you have to get before it starts to reduce your ability to perform feats of endurance? When you lose about 3% of your body mass in fluids, you will start to falter. But what if you’re only a little dehydrated … and you don’t even know it?

Slightly dehydrated cyclists had impaired athletic performance when they believed that they were dehydrated … but not when they didn’t, a new study reports. 🤯

Funnell et al studied nine cyclists, all active young men in their mid-20s, working hard in high heat (34˚C, 92˚F). Uncomfortable! More exotically, and even more uncomfortable, the cyclists also endured gastric feeding tubes, so that they couldn’t tell how much water they were actually getting. In fact, they all got the same amount: they all were slightly dehydrated by 2% when it was time to perform in three tests. The only thing that varied was what they were told! For one test, they were told that they were dehydrated, and for the other two tests they were told that they had been rehydrated by two different sport drinks … and they thought the point of the test was to compare the effect of the drinks.

Sneaky researchers!

Seven of the nine cyclists did less work when they thought they were dehydrated, with quite a bit of variability and an average of 5.6%, give or take 6. While not the strongest signal imaginable, it was comfortably over the required threshold for calling it “statistically significant.” The effect size isn’t huge, but 10% less work is certainly a real difference, and even 5% can easily be the difference between winning and losing a race.

This result hits a sweet spot of being surprising enough to be interesting, but not too surprising to be believable — because there is already other evidence like this. Alex Hutchinson has written about how “a little thirst isn’t the end of the world” for athletes. Endurance is clearly a function of a cocktail of physical and psychological factors, as explored in detail by Alex in his book Endure: Mind, body, and the curiously elastic limits of human performance. “Curiously elastic” indeed. (Highly recommended book.)

Beyond the obvious neato-factor of this result, I like it because it’s probably a good reason not to go through life being excessively fearful of dehydration. And there is a plague of such fear, which I’ve written a lot about. This post has now been integrated into one of the oldest articles in the library: Water Fever and the Fear of Chronic Dehydration: Do we really need eight glasses of water per day?

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