Water makes up 55% to 78% of the human body depending on the body size. As recommended by health experts, approximately 2 to 3 liters of water are required in minimum to maintain proper hydration in the body. But have you ever heard of fatal water overdose?

According to a video, “How Much Water Can Kill You?” released by the American Chemistry Society, it takes about 6 liters of water to kill a 165-pound (74.8 Kg) person. But believe it or not, it all has to do with the levels of sodium in the blood.

Healthy young woman drinking water

Sodium is one of the most essential minerals and electrolytes present in the body. Although it often gets a bad rap, it has many important functions such as – in regulating blood pressure and in helping conduct nerve impulses. But what’s probably sodium’s most important function of all is to maintain proper fluid balance in the body.

Now that you are familiar with the importance of sodium, do you know it can wreak havoc in our body, too, if its concentration falls below or rises above normal levels? Since this post deals with water intoxication, we are only going to discuss about what happens if its concentration falls below than what health experts consider normal.

Normal concentration of sodium in our body ranges from about 135 to 145 milliequivalents per liter (mEq/L) of blood.  But, if you drink too much water, the amount of water in your blood increases. And as a result, the electrolytes in your blood, especially sodium become diluted, causing the levels to drop below 135 milliequivalents per liter (mEq/L).

This condition where a person has low sodium concentrations in blood is called hyponatremia. Severe cases of hyponatremia can lead to water intoxication. Symptoms include headache, fatigue, nausea, vomiting, frequent urination, elevated blood pressure, double vision and mental disorientation.

According to Scientific American, there has been several cases of death by water, of which some of them are:

  • A 28-year-old California woman died after competing in a radio station’s on-air water-drinking contest. After downing some six liters of water in three hours in the “Hold Your Wee for a Wii” (Nintendo game console) contest, Jennifer Strange vomited, went home with a splitting headache, and died from so-called water intoxication.
  • In 2005 a fraternity hazing at California State University, Chico, left a 21-year-old man dead after he was forced to drink excessive amounts of water between rounds of push-ups in a cold basement.
  • Club-goers taking MDMA (“ecstasy”) have died after consuming copious amounts of water trying to rehydrate following long nights of dancing and sweating. Going overboard in attempts to rehydrate is also common among endurance athletes.
  • A 2005 study in the New England Journal of Medicine found that close to one sixth of marathon runners develop some degree of hyponatremia, or dilution of the blood caused by drinking too much water.

The kidneys also play a role in regulating the amount of salt and water in the blood. Millions of tubules inside them filter water and other solutes from blood supplied by renal artery. Most of the water and solutes filtered out by tubules are sent back into the blood further down the tubules, and the remaining fluid (a.ka urine) passes on to the bladder. However in the case of water intoxication – that is when you drink too much water way more than your body needs – the kidneys cannot function properly.

The kidneys can excrete about 20 to 28 liters of water a day, but it does not exceed 0.8 to 1.0 liter per hour. So when a person drinks too much water in a short period of time, let’s say 3 to 4 liters in an hour, the kidneys will not be able to flush it out fast enough and the blood will become waterlogged. As a result, the excess water dilutes blood sodium levels and enters the cells causing them to swell.

Most cells can resist the swell caused by excess water as they are embedded in flexible tissues. But when it happens to brain cells, it can be life-threatening-  as the brain is confined within the skull. And with no room to expand or swell, excess fluid accumulation can lead to brain edema (or, swelling) and even death.

“Rapid and severe hyponatremia causes entry of water into brain cells leading to brain swelling, which manifests as seizures, coma, respiratory arrest, brain stem herniation and death,” explains M. Amin Arnaout, chief of nephrology at Massachusetts General Hospital and Harvard Medical School.

Joseph Verbalis, chairman of medicine at Georgetown University Medical Center told Scientific American, “Most cases of water poisoning do not result from simply drinking too much water, it is usually a combination of excessive fluid intake and increased secretion of vasopression (also called antidiuretic hormone). Produced by the hypothalamus and secreted into the bloodstream by the posterior pituitary gland, vasopressin instructs the kidneys to conserve water. Its secretion increases in periods of physical stress—during a marathon, for example—and may cause the body to conserve water even if a person is drinking excessive quantities.”

So if you want to avoid hyponatremia symptoms, do not drink more than 0.8 to 1.0 liter of water per hour. Anything more than that is too much for you and can take a toll on your kidneys. Remember that drinking a lot of water in a short period of time can lead to water intoxication. But if you drink the same amount over a much longer period of time, or at different intervals of the day, you have less risk of developing water intoxication.

So answering your question: How much water does it take to kill you? Well, just about 6 liters, but also only if you drink all up in a short period of time, otherwise it is completely safe. Just note that kidneys cannot eliminate more than 1 liter of water an hour, so it’s not a good idea to drink more than that.

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