The question How much water do I need to drink? is often followed with the related question Can I drink too much of it? We're here to tell you that, while difficult to do accidentally, it is in fact possible to drink too much water – and the repercussions can be deadly.
Photo Credit: Lora Rajah | CC BY 2.0
Too much of anything will kill you. But then, "too much" is a relative concept. "The dose makes the poison," Paracelsus, the founder of modern toxicology, famously said. Today, we call that dose a substance's LD50. Also known as the "median lethal dose," LD50 is the amount of something needed to kill 50% of those exposed to it.
As you might imagine, median lethal doses can vary dramatically. The human tolerance for botulinum toxin, for example, is as close to zero as you'll find among substances known to science; with an estimated LD50 of 1.3–2.1 ng/kg, a few hundred nanograms of the potent neurotoxin (that's a few hundred billionths of one gram), delivered via injection, is enough to put just about anyone down for good.
For most substances, of course, the bar of lethality sits a lot higher than that of a notoriously deadly toxin. Among these substances is water, a compound so essential to life as we know it, scientists hunting for Earth-like worlds beyond our solar system have defined their search criteria as "planets capable of supporting liquid H2O." Water, we're taught from a young age, is everywhere. I don't even need to tell you what percent of the Earth's surface is covered in the stuff, or what proportion of your body is made up of it, or how many glasses of it you should drink daily to keep hydrated. (Fun fact: Nowhere is it written in stone that you must consume eight glasses a day to keep your body adequately watered, and the origins of this ubiquitous health tip remain elusive – much more on this below.) You may even know that, chemically and physically speaking, water is pretty strange, and that its strangeness actually probably has a lot to do with why life needs it in the first place.
But every substance, even a life-giving one like H2O, has its limit. Water's estimated LD50 is 90 ml/kg, a figure arrived at by studying rats. Assuming rat biology and human biology are interchangeable when it comes to the quick consumption of large volumes of water, if 100 people each weighing 150 pounds each drank about six liters of water all in one go, around fifty of those people would die, and the cause of death would probably be water intoxication.
Water intoxication is the extreme result of a bodily imbalance between electrolytes (the minerals in your blood and body fluid that carry an electric charge) and water, a condition called "hyponatremia." Chemically speaking, an electrolyte is any substance that ionizes, i.e. gains or loses an electron or electrons, when dissolved in a solvent like water. A classic example is table salt, chemical formula NaCl, which dissociates into a positively charge sodium ions and negatively charged chloride ions when dissolved in water.
Water just so happens to be the most versatile solvent there is, a property it owes to the arrangement of its hydrogen and oxygen atoms. Without getting too into the details, every H2O molecule possesses a partial negative charge near its oxygen atom and partial positive charge near its hydrogen atoms. The polar nature of water's structure allows it to dissolve more substances than any liquid on Earth (which explains why it's known in chemistry circles as "the universal solvent"), including electrolytes like sodium, potassium, calcium, chloride, magnesium, and hydrogen phosphate. There are many other electrolytes, but these, in particular, are important for normal physiological function. Water doesn't contain any electrolytes naturally, which is why you'll find these physiologically important ones listed on the label of your favorite sports drink and enhanced water products – though too much of these can actually lead to hyponatremia, too. As Deadspin reported last year, "neither sports drinks nor water can keep your sodium levels in line with where they are at a rest during prolonged exercise, and while Gatorade will have a slight improvement from water, drinking it to excess will do more harm than good." More on this below.
In severe cases of hyponatremia, the kidneys – which help, among other things, to regulate water and electrolyte levels in the body – become overwhelmed. At rest, under normal conditions, your kidneys are a supremely efficient filtration system. In an interview with Scientific American, Joseph Verbalis, chairman of medicine at Georgetown University Medical Center explains that a healthy kidney can process water at a rate of 800–1000 mL per hour without affecting the body's net H2O levels. Consume water at a faster rate than the kidneys can process it, however, and the excess is driven by osmosis to regions of the body with high electrolyte concentration, beginning with the blood and eventually seeping into the salt-rich interiors of cells throughout the body. The cellular influx of water is manifest as swelling known as edema; the cells of your body become turgid, ballooning and distending with their newly acquired water weight.
Edema can affect any cell and can therefore occur anywhere in the body, though it is most immediately noticeable in the hands, feet, ankles, and legs, which appear to plump. But it's when the cells in the brain begin to swell that edema becomes most problematic. Confined to the skull, ballooning neurons quickly run out of room to expand – and when the brain swells, things can get ugly. Early symptoms can, paradoxically, resemble heat stroke, and include headache, diarrhea, nausea, cramping, and vomiting. In cases where water intake greatly exceeds the kidney's processing abilities, M. Amin Arnaout, chief of nephrology at Massachusetts General Hospital and Harvard Medical School tells Scientific American that symptoms can escalate to include seizure, coma, respiratory arrest, brain stem herniation, and death.
It is, thankfully, quite difficult to flummox a healthy pair of kidneys accidentally. It is a testament to the water-processing abilities of this organ that most cases of water poisoning are restricted to extreme instances of hazing and drinking contests. An instance of the latter occurred in 2007, when a 28-year-old woman consumed six liters of water over a three-hour period for a radio station's on-air drinking contest. "She said to one of our supervisors that she was on her way home and her head was hurting her real bad," one of the woman's co-workers would tell reporters. "She was crying and that was the last that anyone had heard from her."
Athletes – endurance athletes, especially – are also recognized as being at risk of water poisoning. Results from a study published in 2005 in the New England Journal of Medicine suggest nearly one in six participants in 2002's Boston Marathon experienced some degree of hyponatremia. A 2006 study published in the British Journal of Sports Medicine states unequivocally that "exercise associated hyponatremia is due to overdrinking." In the article, sports physiologists Timothy David Noakes and Benjamin Speedy lament the biomedical community's slowness to acknowledge the risks over over-hydration in endurance athletes.
The paper recounts the upsetting story of a marathon runner in 1981, who, reporting to a hospital with symptoms resembling dehydration, received two liters of fluid delivered intravenously and was sent home. But the treatment didn't work. On the way home from the hospital, the woman suffered a grand mal epileptic seizure and descended into a coma. When she was readmitted to the hospital, blood tests revealed her serum sodium concentrations to be dangerously low. She was later diagnosed with "exercise associated hyponatraemic encephalopathy (EAHE) with neurogenic (non‐cardiac) pulmonary oedema, the first such known case." She was later treated intravenously with 0.9% saline solution, and made a slow recovery.
A handful of other exercise-related cases of hyponatremia would make headlines in the early eighties, but the biomedical community was slow to learn from them:
...by 1986 a body of evidence showed that: (a) EAHE is caused by excessive fluid consumption during exercise; (b) athletes who drink to excess during exercise usually do so on the well meaning advice of others, including race organisers; (c) the condition responds hardly at all to treatment with 0.9% normal saline, whereas recovery is rapid when hypertonic (3–5%) saline solutions are used. It would take almost another 20 years before this wisdom would be universally applied.
In their paper, Noakes and Speedy write that "the reluctance to accept the evidence may be because it conflicted with the prevalent message of the sports drink industry." Deadspin provided an excellent summary of the researchers' criticisms of the sports beverage industry last year, but their stance on the matter, and their opinion of how much people should actually be drinking, is well summarized by the title of another paper published in the British Journal of Sports Medicine in 2007, which reads: "Time for the American College of Sports Medicine to acknowledge that humans, like all other earthly creatures, do not need to be told how much to drink during exercise."