Do You Really Need To Drink 8 Glasses Of Water A Day? An Exercise Scientist Explains Why Your Kidneys Say ‘No’

The warmer weather and longer days have brought back memories to “stay hydrated” and drink eight glasses of water – or about two liters – a day.

Not for bursting someone’s water bottle, but healthy people can actually die from drinking too much water. I’m an exercise physiologist and my research focuses on overhydration and how drinking too much water affects the body. Since the water and sodium balance are vital, people very rarely die from drinking too much or too little fluids. In most cases, your body’s finely tuned molecular processes are subconsciously taking care of you.

A high school soccer player drinks water while exercising. Many coaches have emphasized hydration in recent years. Darrin Klimek / Getty Images

Water out, water in

As spring unfolds, hydration challenges are rooted in schools, sports, and the workplace. These heavily marketed hydration challenges encourage both camaraderie and friendly competition to ensure we drink mandatory amounts of water throughout the day.

Hydration and “gallon challenges” support the popular belief that water consumption beyond physiological needs – or thirst – is healthy.

But that is not the case. The individual body water requirement – the intake – depends primarily on how much water people lose. How much water everyone needs to drink depends mainly on three factors:

  • Body weight. Taller people need more water.
  • Ambient temperature. When it’s hotter, people sweat and lose water.
  • Physical activity. Increased training intensity increases the loss of sweat water.

Therefore, a fluid exchange strategy that is suitable for everyone, such as drinking eight glasses of eight ounces of water a day, is unsuitable for everyone.

It remains unclear where the recommendation for water intake “8 x 8” comes from. This two-liter intake threshold may result from a misinterpretation of the original recommendations of the US Food and Nutrition Board from 1945 and the European Food Safety Agency 2017, which stated that the recommended daily amount of water for all beverages as well as humidity contains contained in food.

This means that the moisture found in foods, especially fresh fruits, sodas, juices, soups, milk, coffee, and even beer, all contribute to these daily recommended water needs. These guidelines further suggest that most of the recommended water content can be achieved without drinking additional cups of plain water.

And it’s important to note that while alcohol has diuretic properties – ethanol works directly on the kidneys to make us pee more – caffeinated beverages like tea and coffee do not increase urinary water loss beyond the amount of water in those beverages.

King kidney

Now you may be wondering why that is. After all, you’ve heard from many people that you need to drink more, more, more.

Because the body’s overall water balance, or what we call homeostasis, is complicated, mammals survive by making real-time adjustments to the kidney. That’s why our kidneys are king when it comes to hydration.

In every kidney – we only need one (meaning we are born with a replacement just in case) – is a covert network of aquaporin-2 (AQP-2) water channels that respond to a hormone called arginine vasopressin. This is the body’s main antidiuretic (water retention hormone). It is secreted by the posterior pituitary gland in response to nerve signals sent by special brain sensors that detect subtle changes in the water balance. These special sensors are called circumventricular organs.

The kidneys make molecular adjustments to both underhydration and overhydration within 40 seconds when the water balance is disturbed. These adjustments result from the mobilization armies of the AQP-2 water channels, which amount to approximately 12 million per collecting channel cell.

Therefore, if we drink more water than our body needs – beyond our thirst – we have to pee out excess water immediately. Or if we forget our water bottle while exercising, we stop peeing to save body water. This rapidly coordinated action between the brain, cranial nerves and kidneys is far more efficient and precise than any available phone app, gadget or personalized recommendation.

Good Morning America hosts accepted a water challenge. These competitions validate the idea that it is good to drink eight glasses of water a day.

Is there anything good to be made of it?

The data suggest that drinking about two liters of water a day reduces kidney stone formation in people with a history of kidney stones and decreases the number of bladder infections in people with a history of cystitis.

An improvement in the complexion of the skin, kidney function and constipation with increased water consumption is not clearly supported by science. Drinking extra water alone will not help children lose weight unless the water intake replaces higher calorie beverages like soda or makes people feel “full” before meals.

Drinking water can affect the state of mind of some people. Some studies report better cognitive performance after increasing water intake; Women with anxiety report compulsive water intake, which makes them feel better, likely due to activation of reward circuits that increase dopamine. Many schizophrenic patients are compulsive water drinkers, claiming that “voices” tell them to drink and that drinking water suppresses those voices.

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Notably, brain imaging studies confirm that excess drinking is uncomfortable and requires greater muscle effort than drinking when thirsty. Our brain tries to prevent chronic overdrinking or polydipsia, because “social polydipsia” causes chronic peeing (polyuria), which can lead to changes in internal lines such as bladder distension, ureteral dilation, hydronephrosis and kidney failure.

So do you have to drink eight glasses of water a day? Unless you’re thirsty, drinking extra water likely doesn’t offer superior health benefits, but it’s also unlikely to be harmful. However, if the kidneys could speak, they would say that hydration problems are nothing more than highly marketed pissing competitions.

Tamara Hew-Butler, Associate Professor of Exercise and Exercise at Wayne State University

This article is republished by The Conversation under a Creative Commons license. Read the original article.