The most likely result is that you urinate more frequently to remove excess water.
But it is possible to go overboard. Normal kidneys can release up to a liter of fluid every hour. If you drink more than that, you will retain excess water in your body, which causes a condition known as hyponatremia and can be dangerous to your health. Mild hyponatremia causes few symptoms, but more severe cases (when blood sodium levels fall below 130 mEq/L) can cause brain swelling and progressive neurological symptoms, including confusion, disorientation, seizures, coma, and sometimes , death.
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Hyponatremia was the cause of death of a participant in a 2007 radio contest in California, in which the prize went to whoever could drink the most before having to urinate. The contestant died after drinking almost two gallons of water for two hours, clearly exceeding what her kidneys could handle, which would be about two quarts, or half a gallon, in two hours. Excessive water intake has also led to student deaths during the fraternity’s hazing rituals.
But it is important to remember that these are artificial situations that most people would never experience.
Your body carefully regulates your water intake.
To understand why this is unlikely to happen in your normal life, you need to know how we regulate the amount of water in our bodies. There are two controls of body water balance: thirst and the secretion of a hormone called arginine vasopressin (AVP, also known as antidiuretic hormone) from the pituitary gland.
When you’re outside on a hot summer day, sweat losses cause gradual dehydration. The brain detects changes in blood concentration, mainly increased sodium, and releases AVP, which travels to the kidneys and tells them to save water. As a result, very little urine is excreted; it also becomes very concentrated and has a darker yellow color.
But at a certain point, urine concentration alone is not enough to prevent dehydration. This is when the higher centers of the brain are activated to stimulate thirst.
When you’re not dehydrated and you drink too much water, your brain also detects this through opposite changes in blood concentration, primarily a decrease in sodium. AVP secretion is inhibited, signaling the kidneys to release more water. A large volume of dilute, very pale urine is then excreted to remove excess water.
You won’t feel thirsty either. But that doesn’t help much: we usually don’t drink liquids due to thirst. Instead, our fluid intake tends to be flavor-driven (like my daughter’s love of Diet Coke); our need to accompany solid food with a drink; and desired side effects, as is the case with coffee and alcohol.
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That said, there are certain conditions and activities that can limit the amount of urine the kidneys release and the likelihood of experiencing hyponatremia. These include health conditions such as syndrome of inappropriate antidiuretic hormone secretion; medications, including diuretics and antidepressants; and even nausea and exercise.
Perhaps the best studied of these is exercise-associated hyponatremia, particularly in marathon, triathlon, or ultramarathon races. These cases of hyponatremia were initially thought to be caused by sweat sodium losses, but later studies showed that they were primarily due to excessive fluid intake during the events.
Fluid intake during exercise rarely exceeds one liter per hour. But exercise stimulates AVP release, so even “normal” amounts of water intake can cause water retention and hyponatremia under these conditions. Nausea has a similar effect on AVP secretion.
So if you’re exercising or nauseated, you can’t excrete water as normally as when you’re sitting comfortably watching a movie—unless, of course, the movie makes you nauseous.
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So how much water should I drink?
I participated in consensus conferences on exercise-associated hyponatremia, which came to a very simple conclusion: You should drink according to your thirst, during exercise or at rest, but not an arbitrarily set amount. (I have consulted Otsuka Pharmaceuticals, which makes a drug to treat hyponatremia.)
The ideal fluid intake varies from person to person, depending on a variety of factors. There are some medical conditions where increased fluid intake is recommended, such as a tendency to form kidney stones.
Particular attention should be paid to the elderly, who generally have a decreased sensation of thirst. This usually does not cause dehydration, but in conditions of increased fluid loss (increased sweating during heat waves, diarrhoea), they should be encouraged to drink more fluids even if they are not thirsty.
But for most people, drinking more than is necessary to maintain fluid balance is not medically necessary or helpful.
In fact, the recommendation to drink eight eight-ounce glasses of water a day is not supported by evidence-based data or known physiological principles.
The bottom line: If you don’t feel thirsty, you’re probably well hydrated. But if you feel thirsty, drink as much as you need until the thirst is gone.
That is not only the best medical advice to maintain the water balance of the body, but also the common sense that has allowed the survival of the human species for millennia. We must not abandon that successful strategy now.
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