Of all the silly urban legends …
Lord knows where these silly ideas come from. I often hear medical personnel advise patients to “drink plenty of fluids” when sick. Medical personnel are uncomfortable telling a patient with an ordinary cold that there is nothing really to be done, and critically ill folks do often appear dehydrated, so maybe that was the start of it. But a healthy person should not drink water if not thirsty. I suspect that the makers of all the various bottled beverages pay dieticians “consulting fees” to write health articles in magazines and newspapers to promote the unnecessary and excessive drinking of liquids.
In any event, the whole thing is a 
scam. There is not a shred of evidence from any well conducted clinical trial that shows drinking extra water helps the skin, promotes weight loss, or treats colds or flu. The idea that drinking plenty of water “flushes” out the system, as if the human body is some sort of big pipe in a mechanical sewage treatment plant, is also completely false. The notion that regular folks going about their regular lives, working, eating, sleeping, and playing tennis or jogging are in constant danger of “dehydration” is absurd. This whole extra water drinking thing is utter and complete nonsense.
In fact, excessive water drinking is potentially hazardous. To understand why, we must review the principle of osmosis from high school biology while staying awake. Imagine you’re the owner of a fancy Belltown nightclub. You show up one Saturday night when there is a convention of flashy, out-of-towners in Seattle, and 10% of your club is now populated by big guys in fancy sport coats. You demand to know why your bouncers let them in. The bouncers say the out-of-towners are rich and buy lots of liquor. As long as 90% of the patrons are your local customers, having 10% big spenders makes a good balance.
Maybe you’re happy with that, so you instruct the bouncers not to let any of the big spenders out, but for cryin’ out loud, don’t let any more in! You want your loyal customers to come and go as they please. You’ve now created a semi-permeable membrane, a barrier that lets some smaller molecules pass but holds up the bigger molecules. In the human body, water can freely pass in and out of cells, but sodium and other larger molecules are restricted in their movements.
If a crowd shows up outside the club with the same mix of 10% big spending out-of-towners and 90% loyal customers, as the loyal folks come and go, the percentages inside the club will not change. But if 100% of the people showing up are loyal customers, then by their random coming and going, more people will end up inside the club. The out-of-towners are stuck in there, held in place by the bouncers who want them to spend, and when additional loyal customers arrive, the club will swell up. If there are 50% out-of-towners and 50% loyal customers outside the club, then there will be a net migration of loyal customers out and the club will shrink down. This is osmosis.
The same principle can be observed when you stay in the bath too long. Some plain water from the bath is absorbed by the fingertips and the skin swells up. In order for the now swollen skin to fit in the same space, it has to wrinkle.
So if you drink too much water, the saltiness of your blood becomes lower than the saltiness inside your cells. That abnormally low saltiness of the blood is called hyponatremia (hypo = below, natr = sodium, emia = in the blood). Over time, the kidneys compensate by creating very un-salty urine and peeing out the extra water, thereby restoring balance. But in the short haul, osmosis will result in more water moving into cells than moving out and this will cause cells to swell up. Most parts of the body can handle this pretty easily, but your brain can’t. When the brain swells, it has nowhere to expand because it is inside your skull, which is a hard protective case. The pressure in the brain increases. A headache ensues. Next, the blood flow to the brain may be inadequate and further brain swelling can result. In the end, the brain is forced down through the hole in the base of your skull since it has nowhere else to go, and this is fatal. Last year, a radio station ran a water drinking contest. One woman drank two gallons of water in a short period of time and died as a result. The cause of her death was hyponatremia, which resulted in rapid brain swelling. There was too much water drinking without giving the kidneys a chance to compensate.
Before the mid-1980s, athletes only drank water when they were thirsty, and medical problems from excessive water drinking during athletics were extremely rare. Then folks started on this psychotic water bandwagon. During marathons, there were water stations every ten feet and everyone was warned not to get “dehydrated.” As a result of this unwarranted fear, athletes suffered from hyponatremia because they drank too much water during exercise. One runner died in the Boston Marathon as a result of drinking water excessively.
You might think that all you have to do is drink salty water to prevent this problem. Unfortunately, drinking electrolyte-rich “sport drinks” will not work because they have a far lower salt concentration than the normal human body. The saltiness would have to be equal to that of tears, and not only is that too much salt for us to tolerate in big gulps, but our bodies are not designed to drink down huge quantities of salt water.
When you are exercising, your normal instinct, without reading silly urban legends in health magazines, is to drink some water, but not enough to fully replace what you lose, and that is exactly what the body wants to do. This would be on page five of the human owner’s manual if we had one. People naturally eat and drink in the ensuing hours following a long period of exercise, and this helps slow any shifts in sodium concentration. It also gives the kidneys a chance to make whatever adjustments are necessary. It is perfectly acceptable to become mildly “dehydrated” during a marathon. In fact, winners of marathons tend to be somewhat dehydrated at the end of the race, just as nature intended. It is not reasonable, healthy, or possible to fully replace everything you lose with exercise. You are supposed to replace most of it later. This is what people have been doing since the dawn of time. Dehydration, with a normal or near normal sodium level is much, much safer than developing hyponatremia.
If you are a vigorous exerciser and can’t break yourself of the habit of drinking beyond your body’s sense of thirst, then check your weight at the beginning of a race and make sure it never goes up during exercise. Gaining weight is proof you are becoming overhydrated and developing hyponatremia, unless you are eating hamburgers during your workout.
If it is way too hot or humid outside and you exercise vigorously,
no amount of fluid consumption will
really prevent heat stroke.
We should also distinguish heat stroke from dehydration. Heat stroke means the body is too hot and could not cool itself adequately. Sure, if you’re so dehydrated that you no longer sweat, your body temperature will rise, but this is unlikely to occur under virtually any reasonable exercise condition. Sweating is just one of many mechanisms we have to cool the body. If it is way too hot or humid outside and you exercise vigorously, no amount of fluid consumption will really prevent heat stroke. Don’t think you can drink your way out of that problem. The only way to drink and prevent heat stroke if you’re planning on running a marathon on an extremely hot, humid day is to skip the race and hang out at the bar.
In any event, if you are running that marathon, your own body has thirst sensors which work quite well and will encourage you to replace some, but not all, of your body water losses. If you develop heat stroke, it is mostly because you could not get rid of the excess heat, not because you became seriously dehydrated.
If you are in a developing country with no access to medical care, and ill with severe diarrhea, then drinking beyond normal thirst may be beneficial in the short term. This is called oral rehydration and should not apply to most of us.
In conclusion, there are essentially no health benefits to drinking liquids beyond what normal thirst dictates. In certain conditions, it can be hazardous to do so. There is no hard and fast rule about eight glasses of water per day. That is just made-up nonsense. Mild dehydration during exercise is normal and safe. Folks should simply rely on their thirst instincts and drink water only when thirsty.