Why is your thirst quenched so quickly after drinking? And does dehydration negatively impact performance even if you don't know you're thirsty?

I recently wrote a blog on whether you should just drink water to thirst. In my background reading for that I came across a couple of recent research papers I'd not seen before, but I didn't have the time to fully dissect them, so didn't include my thoughts on them in the piece.

Having found some time to get my head around them, I've written a follow-up post to bring this extra research into the discussion. The papers essentially ask two different questions, both of which are linked to the 'drink to thirst v hydration plan / supplementation' debate...


Paper #1 - "Hierarchical neural architecture underlying thirst regulation" (‘Nature’, March 2018)

Basic question: Why is your thirst quenched so quickly after drinking?

This paper is pretty heavy going for non-scientists but this NPR summary does a great job of distilling the main points into a more digestible format.

Essentially, what the researchers from the California Institute of Technology managed to demonstrate was the critical role that receptors (cells that respond to light, heat or other external stimuli and transmit a signal to a nerve) in the throat might play in thirst and the quenching of it when you drink.

They messed around in the heads of some mice, inhibiting a previously identified ‘thirst centre’ in the brain's connection to some receptor cells in the throat. They discovered that they could induce significant ‘over-drinking’ behaviour in the rodents, even when they were in fact fully hydrated.

Normally when we get dehydrated (as measured by an increasing concentration of solutes in our blood) thirst kicks in to make us drink water to dilute our blood back down to an acceptable level. But, once we start drinking, this sensation generally subsides within a minute or less, even if we were really thirsty.


Thirsty dog


This is puzzling because we know that it takes 15 minutes or so for whatever you drink to be absorbed into your blood. If it were only your blood concentration that controlled thirst (as is hinted at by Professor Tim Noakes in his simplified model of thirst-only hydration management in his book 'Waterlogged'), then we would continue to drink for ~15 minutes before our thirst went away and we'd become ‘Waterlogged’ in the process.

What the authors of this paper suggest is that receptors in the throat - and the thirst centre in the brain that's connected to them - are able to reliably identify the action of swallowing fluid (as opposed to swallowing food) and have the ability to ‘switch thirst off’ long before the fluids have been absorbed properly. It’s like the receptors send an advance signal that says "don’t worry brain, water is on the way, you can stop being thirsty now".

This seems to make perfect sense and, although the paper only experimented on mice, it doesn’t seem like a ridiculous stretch to apply the conclusions to humans too.

The question is, when you're an endurance athlete being challenged by very high levels of fluid loss over many hours (potentially in the heat), is it possible that there could be a disconnect between this ‘thirst quench prediction’ system and your actual fluid needs in the face of ongoing heavy sweat losses over several hours? Especially when the amount of fluid you're able to take in and absorb is very likely to be less than the amount you're losing through sweating...

This seems like something that needs further investigation in order to add another dimension to the whole ‘drink to thirst’ debate…


Paper #2 - "Dehydration Impairs Cycling Performance, Independently of Thirst: A Blinded Study." ('Med Sci Sports Exercise', March 2018)

Basic question: Does dehydration negatively impact performance even if you don't know you're thirsty?

The 2nd paper I read saw researchers trying to ‘blind’ athletes to their actual hydration status whilst also preventing them from becoming thirsty. This was to try to separate ‘dehydration’ from ‘thirst’ and see what effect all of this had on performance.

The participants (two groups of cyclists in a lab) were given tiny sips of water every so often during a steady 2 hour ride to make sure that they wouldn't get especially thirsty.


Horse wearing blinkers


Both groups were then asked to do an all-out 5km time trial. One group had their hydration status fully maintained by water pumped directly into their stomach via a nasal tube (sounds fun!). This would get them close to replacing 100% of their sweat losses. The other group had less fluid pumped into their stomachs in order to make sure that they became dehydrated by ~1-2% of their bodyweight.

The results showed that the athletes who were better hydrated tended to put out more power (at a lower core body temperature) than those who were allowed to become somewhat dehydrated, even though none of the athletes had a decent understanding of their hydration status at the time. 

The authors concluded that their results clearly demonstrate that dehydration can negatively impact performance independently of the sensation of thirst. Taken at face value, this certainly seems to be the case.

But, it's worth pointing out that the study only had 7 participants. That's not unusual in this kind of study, but it does somewhat limit the power of the conclusions as a result. Also, having read their prior work, the researchers involved do have a pretty well established ‘position’, believing that drinking to thirst alone is a questionable strategy for athletes. So I tend to put these results in the categories of ‘interesting’ and ‘illustrative’ but a not quite definitive.


Anyway, I thought that it was adding these studies into the mix in the ‘drink to thirst’ debate and hope that this post has made it clear that there's more to this topic than meets the eye…

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