Making up about 66 percent of the human body, water runs through the blood, inhabits the cells, and lurks in the spaces between. At every moment water escapes the body through sweat, urination, defecation or exhaled breath, among other routes. Replacing these lost stores is essential but re-hydration can be overdone. There is such a thing as a fatal water overdose.
"Ingesting more water than the body needs can increase the total blood volume. Since blood volume exists within a closed system - the circulatory system - needlessly increasing body blood volume on a regular basis puts unnecessary burden on your heart and blood vessels. Our kidneys must work overtime to filter excess water out of the circulatory system. The kidneys are not the equivalent of a pair of plumbing pipes whereby the more water flushed through the kidneys, the cleaner they become; rather, the filtration system that exists in the kidneys is composed in part by a series of specialized capillary beds called glomeruli. Glomeruli can get damaged by unnecessary wear and tear over time, and drowning the body system with large amounts of water is one of many potential causes of said damage."
In the long run, says Dr. Ben Kim, a holistic and alternative doctor, it can be doing more harm than good.
You may or may not have heard the story (2007) where a 28-year old Californian wife and mother of three children accidentally died from drinking too much water. After entering a radio show contest called, "Hold Your Wee for a Wii, (where contestants were promised a new Wii gaming system if they held out from the loo after drinking the most amount of water. She was in her home, dead before the game had even ended.
Perhaps you have never even thought of the fact that there have been other unfortunate deaths by water. In the last decade, college fraternities have been getting more creative in their hazing rituals. "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 that close to one sixth of marathon runners develop some degree of hyponatremia, or dilution of the blood caused by drinking too much water."
These stories demonstrate the body's need for balance, and what we may think is a good and healthy liquid consumption (in this case water) may not fair so well. Take wine or alcohol as another example. There are many studies that say a glass a day, a few times a week may be beneficial for the heart (etc). We all know what too much of that can do to our bodies and mental health.
In humans the kidneys control the amount of water, salts and other solutes leaving the body by sieving blood through their millions of twisted tubules. When a person drinks too much water in a short period of time, the kidneys cannot flush it out fast enough and the blood becomes waterlogged. Drawn to regions where the concentration of salt and other dissolved substances is higher, excess water leaves the blood and ultimately enters the cells, which swell like balloons to accommodate it.
Wolfgang Liedtke, a clinical neuroscientist at Duke University Medical Center explains this dangerous reaction in the body. “Most cells have room to stretch because they are embedded in flexible tissues such as fat and muscle, but this is not the case for neurons. Brain cells are tightly packaged inside a rigid boney cage, the skull, and they have to share this space with blood and cerebrospinal fluid. Inside the skull there is almost zero room to expand and swell.” This is why brain edema, or swelling, can be disastrous. "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."
n a 2002 review for the American Journal of Physiology—Regulatory, Integrative and Comparative Physiology, Heinz Valtin, a kidney specialist from Dartmouth Medical School, decided to determine if the common advice to drink eight, eight-ounce glasses of water per day could hold up to scientific scrutiny and published his findings. After scouring the peer-reviewed literature, Valtin concluded that no scientific studies support the eight glasses of eight dictum (for healthy adults living in temperate climates and doing mild exercise). In fact, drinking this much or more could be harmful, both in precipitating potentially dangerous hyponatremia and exposure to pollutants, and also in making many people feel guilty for not drinking enough. Since Valtin’s findings were published not a single scientific report published in a peer-reviewed publication has proven the contrary.
Most cases of water poisoning do not result from simply drinking too much water. Georgetown University Medical Center states that 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.
Every hour, a healthy kidney at rest can excrete 800 to 1,000 milliliters, or 0.21 to 0.26 gallon, of water and therefore a person can drink water at a rate of 800 to 1,000 milliliters per hour without experiencing a net gain in water. If that same person is running a marathon, however, the stress of the situation will increase vasopressin levels, reducing the kidney's excretion capacity to as low as 100 milliliters per hour. Drinking 800 to 1,000 milliliters of water per hour under these conditions can potentially lead a net gain in water, even with considerable sweating.
While exercising, people should balance what they are drinking with the amount of sweating, including sports drinks, which can also cause hyponatremia when consumed in excess. If you are sweating 500 milliliters per hour, that is what you should be drinking for replacement. 
But measuring sweat output is not easy. How can a marathon runner, athlete, or any person, determine how much water to consume? As long as you are healthy and equipped with a thirst barometer unimpaired by old age or mind-altering drugs, drink to your thirst. It is said to be the best indicator, but do not disregard your sense of thirst and strive to ingest several glasses of water a day just because you have been told that doing so is good for your health (and do not wait until you are thirsty to drink, for you are may already be dehydrated).
By observing the color of your urine as a dehydration symptom, it may be an easy way to check if you are in need of water. The idea is that clear urine indicates that you are well hydrated, while yellow urine indicates that you need more water in your system. While this advice is somewhat useful, it is important to remember that some food additives (including some synthetic nutrients) and heavily pigmented foods (like red beets) can add substantial color to your urine. Taking daily vitamins and drinking juices will also add more color to your urine. Aim for clear liquids.
The answer to this question depends on each person’s unique circumstance, including diet, exercise habits, and environment.
If you eat plenty of foods that are naturally rich in water, such as vegetables, fruits, and cooked legumes and whole grains, you may not need to drink much water at all. If you do not use much or any salt and other seasonings, your need for drinking water goes down even further.
Conversely, if you do not eat a lot of plant foods and/or you add substantial salt and spices to your meals, you may need to drink several glasses of water every day.
Regardless of what your diet looks like, if you sweat on a regular basis because of exercise or a warm climate, you will need to supply your body with more water (through food and/or liquids) than someone who does not sweat regularly. Remember to listen to your body signals and hydrate with good clean water and wholesome foods.
Resources: Dr. Ben Kim: http://drbenkim.com/drink-too-much-water-dangerous.html
Chemistry.com, ScientificAmerican.com, MensHealth.com, FitSugar.com, LiveStrong.com
