The "8 glasses per day" rule

Eight glasses of water a day is a common mantra of health gurus, personal trainers, and those promoting weight-loss diets. Is that much water really necessary?

Dehydration means loss of water

The condition of having lost too much water from the body is called dehydration. Symptoms of dehydration include thirst, headache, fatigue, and dizziness. Muscles may cramp or ache, especially if exercise is attempted or continued. Because blood pressure may fall, you may feel lightheaded when sitting or standing, and, in severe cases, there may be loss of consciousness, kidney failure, and death.

In general, there are two major mechanisms that contribute to dehydration: too little water intake or too much loss. In fact, most cases of dehydration are due to a combination of the two; for the amount of water lost from the body, there is not enough intake. It is quite true that dehydration is common. But are you at risk?

Who gets dehydrated?

Other than those who have no access to water, the person who is most likely to become dehydrated is a person exercising without drinking enough, a sick child, or a sick elderly person. For a child who is too young to drink on his or her own or express thirst, adequate water intake may become a problem, especially if the child is losing excessive fluids from an illness.

Since fever leads to loss of body fluids (through the skin and breathing, called insensible losses), an infection that causes fever, diarrhea, nausea, or vomiting are important risk factors for dehydration, especially in infants. A reduced sense of thirst in the elderly also is thought to play a role; the reason for this dampened thirst mechanism is not entirely clear. Finally, medications that many people take that promote water loss (such as diuretics, or "water pills" for high blood pressure) also increase the risk of dehydration.

Who does not get dehydrated?

The vast majority of people who have access to water get plenty by what they ordinarily drink and in the foods they eat. Though we don't often think of foods as sources of water, they are - in fact, food contributes in a significant way to the average person's fluid intake. For example, a carrot is 87% water, and broccoli is over 90% water. Dehydration is unusual in healthy children and adults who eat a balanced diet, drink when thirsty, and take some common-sense precautions when spending time in hot or humid environments, especially if exercising.

Why the recommendation of eight glasses a day?

The origin of the recommendation to drink eight glasses of water per day is not clear. One explanation I've heard is that it came from World War II estimates of the absolute minimum a soldier needs each day while in combat. Since water is heavy to carry around, knowing this minimum average amount is important to those responsible for providing soldiers with their nutritional and hydration needs. Of note, they often carried heavy gear through their grueling activities all day, and their food contained very little water. As a result, their water requirements were likely to be much higher than the average nonmilitary person.

Another explanation comes from estimates of how much fluid would need to accompany the "average" person's 2,000-calories-per-day food intake to maintain normal sodium and electrolyte balance. But that doesn't take into account the fluids in the foods themselves.

The suggestion for adults to drink eight glasses a day probably persists because it's not harmful and may have other benefits besides preventing the unlikely event of dehydration. For example, it may reduce hunger and promote weight loss by briefly filling the stomach. And to be fair, the suggestion to drink eight glasses of water per day is not wrong; it could be helpful to the person at risk (such as a person who doesn't experience thirst normally or cannot communicate their needs). It simply may be unnecessary for most people and gives the impression there is a problem when none exists.

There is limited evidence that increasing water intake comes with health benefits. In a review of 18 prior trials, about half reported positive results and half reported negative results. Most studies were too small and/or short-term to be considered definitive or widely applicable.

Can you drink too much water?

Drinking eight glasses of water each day is generally safe; the body has a sophisticated system that regulates how much of what is ingested is kept and how much is eliminated, so if more than enough water is taken in, the body simply gets rid of it.

But there is such a thing as too much water, and for those who think that "if some is good, more is better," it is possible to cause harm. The body's sodium concentration will fall (a condition called hyponatremia) if there is enough extra water in the body, and this can lead to swelling in the brain (cerebral edema), nausea, fatigue, confusion, even seizures or coma - it's appropriately called water intoxication. Fortunately, it is difficult to drink enough extra water to cause any of these problems.

The bottom line

If you have risk factors for dehydration (such as an illness with fever and diarrhea, or prolonged exercise in hot weather), it makes sense to pay attention to your fluid intake and try to take in extra water (and salt). But for most of us, the advice to drink eight glasses of water each day can be safely ignored; if you drink when you are thirsty, eat a balanced diet, and feel generally well, you are likely getting all the water you need.