Get access to everything we publish when you sign up for Outside+.
If you’ve ever run a marathon, half-marathon, or any other long race, you’ve undoubtedly had a hydration plan: a detailed assessment of what you intend to drink, when you intend to drink it, and how you intend to obtain it. But a paper in tomorrow’s issue of Science may cause you to rethink exactly how you do this, suggesting that to a larger degree than people have thought, you may be able to trust your sense of thirst to tell you when you are and aren’t getting enough fluids.
Not all that long ago, runners were told that thirst was a “slow” reflex that lagged well behind the body’s actual needs. But it actually appears to be exquisitely sensitive, integrating an amazing amount of information in real time, says Christopher Zimmerman, a neuroscientist at Princeton University, Princeton, NJ, who won the Science magazine’s 2020 Eppendorf prize for his work in the field.
A Sophisticated Thirst Sensor
Decades ago, Zimmerman says, researchers realized that the sense of thirst was regulated by a tiny part of the brain called the subfornical organ (SFO), about the size of a grain of rice, buried deep within the brain.
What made it interesting, he says, is that it was located outside of the blood-brain barrier, the membrane that separates the brain from the bloodstream, insulating the brain from toxins and pathogens that might be in the bloodstream. Being outside that barrier allowed it to monitor the blood for important changes related to hydration, such as the “osmolarity,” or salt and protein content, of the blood.
If osmolarity goes up, you are losing fluid, and need to replace it. If it’s low, you’ve had enough, and don’t need to drink any more.
It was a simple concept, and a major contributor to the idea that thirst is a slow reflex, because by the time your blood osmolality has changed, you are already beginning to dehydrate. Just as surgical patients are often told to not “get behind” on their pain meds, runners were told not to “get behind” on hydration, under the theory that by the time they realized they were thirsty, it was too late.
Zimmerman’s research throws a cold bucket on that notion.
Working with mice, whose drinking behavior, he says, is very similar to humans, he inserted tiny fiber optic cables into their brains in an effort to spy on their SFOs. The cables were small enough, he adds, that while the mice were probably aware of them, they didn’t impede their normal activities.
Using these, he and his colleagues then examined how the mice’s SFOs reacted when the mice were dehydrated…and how they reacted to various thirst-related stimuli.
What they found was paradigm breaking. The SFO didn’t just react to osmolality in the blood. It anticipated it. When thirsty mice were allowed to drink (which laboratory mice do by licking water from a tube), their SFOs immediately turned down their thirst signals, long before the water could possibly have entered their blood.
“It’s like these neurons are counting how much water the animal has consumed and are using that to find when to start and when to stop,” Zimmerman told PodiumRunner.
They also appear to get information from the gut on how much salt the mouse has recently consumed — a possible explanation for why salty food makes you thirsty long before the salt has had time to enter your bloodstream.
Even more interestingly, their SFOs reacted to the temperature of the water, reducing thirst signals more rapidly if the water is colder.
That’s interesting Zimmerman says, because like most people, he himself prefers cool water. “[It] feels better when I am thirsty,” he says. “It quenches the thirst faster.”
He himself is a runner, though not a racer, logging 40-60 minutes a day. He too knows that cold water tastes “way better” after a run. “It’s the first thing I do when I get back,” he says.
What all of this means, he says, is that the sense of thirst is regulated very quickly, partly by signals from the mouth and throat. “We don’t know exactly which cells [do this],” he says. “That’s future work.” Though, he adds, “we think swallowing plays a role.”
As for why cold water turns the sense of thirst down faster? That’s an open question. There is research that says cold water absorbs more rapidly from the gut, so it’s possible that the SFO knows that and accounts for it. Or, perhaps, it knows that cold water will, to some extent, cool your core temperature, reducing your need for more water in the future. Nobody really knows.
Nor, Zimmerman says, do we yet know what other sensors might be involved in determining thirst during a long race. Could the SFO also have sensors tied into your sweating rate? “That’s totally possible,” he says. “When you sweat, our understanding is that these neurons [the SFO] should know. It should be one of the signals these cells get.”
Other scientists are impressed. Zimmerman, says Zachary Knight, a neuroscientist at the University of California, San Francisco, has done a remarkable job of showing that the sensation of thirst is vastly more complex than we once realized…and far more rapid.
“It is remarkably elegant,” Knight says.
If You Thirst, Drink
How best to apply this to your own hydration plans remains uncertain.
Zimmerman notes that if you are going out for a long run on a hot day, you might still want to plan ahead. But his research also supports findings by South African exercise physiologist Tim Noakes (author of the encyclopedic tome Lore of Running and the revolutionary Waterlogged) that dehydration isn’t the bugaboo many people have long claimed it to be.
Noakes argues that our distant ancestors evolved to run long distances during the day, slowly dehydrating as they chased game over the African savannas. Then, they’d rehydrate at night, after they’d run their quarry to ground. The same genes, Noakes has argued, mean that you do not need to remain fully hydrated during a long race. Rather, he says, you can, to a large extent, trust your sense of thirst.
Zimmerman isn’t a marathoner and doesn’t want to weigh into this debate. Though, he says his research makes one thing clear. “If you feel thirsty, you definitely need to drink.”
Knight is a little bolder. “I’m not sure I can say how much this impacts runners,” he says. “It’s very basic research into the neurobiology of thirst.” But, he says, “my understanding of Tim Noakes’ work is that his advice for runners is that they should drink when they are thirsty. I strongly endorse this view. If Chris [Zimmerman’s] work has shown anything, it is that the thirst system is very robust.”