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(The Conversation)The warmer weather and longer days have inspired reminders to "stay hydrated" and drink eight glasses of water -- or about two liters -- a day.
Not to burst anyone's water bottle, but healthy people can actually die from drinking too much water. I am an exercise physiologist, and my research focuses on overhydration and how drinking too much water affects the body.
Since water -- and sodium -- balance is essential to life, it is extremely rare for people to die from drinking too much -- or too little -- fluid. In most cases, your body's finely tuned molecular processes are unconsciously taking care of you.
Water out, water in
As spring unfolds, hydration challenges take root across schools, sports and workplaces. These heavily marketed hydration challenges serve to cultivate both camaraderie and friendly competition to ensure that we drink compulsory amounts of water throughout the day.
Hydration and "Gallon Challenges" support the widely held belief that water consumption beyond physiological need -- or thirst -- is healthy.
But this is not so. Individual body water needs -- intake -- are primarily based upon how much water people lose. How much water each person needs to drink mainly depends on three factors:
- Body weight. Bigger people need more water.
- Environmental temperature. When it's hotter, people sweat and lose water.
- Physical activity levels. Increased exercise intensity increases sweat water losses.
Therefore, a "one size fits all" fluid replacement strategy, such as drinking eight glasses of eight ounces of water per day, is inappropriate for everyone.
It remains unclear where the "8 x 8" water intake recommendation comes from. Perhaps, this two-liter intake threshold is derived from a misinterpretation of original recommendations offered by the U.S. Food and Nutrition Board in 1945 as well as the 2017 European Food Safety Authority, which states the daily recommended amount of water includes all beverages plus the moisture contained in foods.
This means that the moisture contained in foods, especially fresh fruits, sodas, juices, soups, milk, coffee and, yes, even beer, contributes to this daily recommended water requirement. These guidelines go on to suggest that most of the recommended water content can be accomplished without drinking additional cups of plain water.
And, it is important to note that while alcohol has diuretic properties -- ethanol acts directly on the kidneys to make us pee more -- caffeinated beverages, like tea and coffee, do not increase urinary water losses above the amount of water contained in these beverages.
Now, you may be wondering why this is so. After all, you've heard from a lot of people that you need to drink more, more, more.
Because total body water balance, or what we exercise scientists call homeostasis, is complicated, mammals survive by making real-time adjustments at the kidney. That's why when it comes to hydration, our kidneys are king.
Within each kidney -- we need only one (i.e., we are born with a spare, just in case) -- is an undercover network of aquaporin-2 (AQP-2) water channels that respond to a hormone called arginine vasopressin. This is the body's main anti-diuretic (water retention) hormone. It is secreted by the posterior pituitary gland in response to nerve signals sent from specialized brain sensors which detect subtle changes in water balance. These specialized sensors are called circumventricular organs.
The kidneys will make molecular adjustments to both underhydration and overhydration within 40 seconds in response to any upset in the water balance. These adjustments result from the mobilization armies of AQP-2 water channels, numbering about 12 million per collecting duct cell.
This is why when we drink more water than our body needs -- above thirst -- we immediately have to pee out any excess water. Or when we forget our water bottle during practice, we stop peeing to conserve body water. This quick coordinated action between the brain, cranial nerves and kidneys is far more efficient and precise than any phone app, gadget or personalized recommendation available.