When the water content of our blood drops, neurons in the brain tell us that we are thirsty. But how do we know when enough is enough?
The water content in our body is tightly regulated. Dehydration can lead to dizziness, delirium, and unconsciousness. Drinking fluids restores this balance or homeostasis.
But it takes time for water to travel from our mouths through the body. We stop drinking a long time before this happens.
If we kept drinking during this delay, we would be at serious risk of water intoxication, or water poisoning, which is potentially deadly.
Scientists are beginning to unravel the sophisticated mechanisms that stop us from drinking too much water, and the answer lies in the brain.
What controls thirst?
The brain's thirst control circuit is a small region in the forebrain called the lamina terminalis (LT).
Once the LT network is activated, we become thirsty. A study published last week in the journal Science demonstrated that thirst creates an uncomfortable feeling in mice, which is alleviated by drinking.
There is one other thing that triggers thirst: eating. As soon as we start to eat, our thirst is stimulated. This is known as prandial thirst.
Water is necessary for us to digest the food that we eat. It also stops electrolytes in food from disturbing homeostasis by balancing out the fluid levels.
Why do we stop drinking?
Zachary A. Knight, Ph.D. - from the Department of Physiology at the University of California, San Francisco - and his team reported in the journal Nature that neurons in the subfornical organ (SFO), which forms part of the LT, might be at the heart of things.
The authors explain that "much normal drinking behavior is anticipatory in nature, meaning that the brain predicts impending changes in fluid balance and adjusts behavior pre-emptively."
For their study, the researchers used mice and restricted their access to water overnight. "When water was made available," the authors write, "mice drank avidly and, surprisingly, [SFO] neurons were inhibited within 1 min."
This drop in neuronal signaling happened much faster than the water was able to reach the blood.
"Drinking resets thirst-promoting SFO neurons in a way that anticipates the future restoration of homeostasis," they add. This means that our brain anticipates how much water we need to drink to restore homeostasis.
Signals from the mouth to the brain
What is not yet clear is how the brain knows when we are drinking fluids. A recent study published in the journal Nature Neuroscience pointed the finger at receptors in our mouth.
The team - led by Yuki Oka, Ph.D., who is from the Division of Biology and Biological Engineering at the California Institute of Technology in Pasadena - showed that water changes the acid balance in the saliva, which activates acid-taste receptors.
So, what is the best way of quenching thirst? A study by Sanne Boesveldt, Ph.D. - from the Division of Human Nutrition at Wageningen University & Research in the Netherlands - and her team, which will be published in the October edition of the journal Physiology & Behavior, set out to answer this question.
The authors explain that cold drinks are already known to be more thirst quenching, as are sour, flavored, and carbonated drinks.
In their study, the team found that cold, flavored popsicles were significantly more thirst quenching than cold liquids. The most effective flavor was lemon.
So, while the days may be getting colder as fall gets underway in the Northern hemisphere, a lemon popsicle might still be a good option the next time thirst calls.