Water accounts for about one-half to two-thirds of an average adults body weight, depending on their age, sex, and physical conditioning. For example, the body of a fit, 180-pound young man can contain nearly 120 pounds (~54 kg) of water. (A gallon of water weighs approximately 8.3 pounds.)

Because muscle tissue contains five times more water than fat tissue (79% vs 15%), muscle mass accounts for most of the variance in body water content between individuals.

*Source: IRCP Publication 23- Report of the Task Group on Reference Man

There is no scientific evidence to support the long-held belief that adults should aim to drink 8 glasses of water a day. According to the 2004 Dietary Reference Intake (DRI) guidelines from The Institute of Medicine (IOM), American women need approximately 2.7 L (11.5 cups) of water a day, including approximately 9 cups as total beverages including drinking water. Men need approximately 3.7L (15.5 cups) of water a day, including approximately 13 cups as total beverages including drinking water.

The body obtains water from both beverages and food, not just drinking water. On average, the IOM estimates that about 80 percent of an adult American's total water intake comes from beverages, including drinking water, and about 20 percent comes from food. The IOM report also states that all beverages, including those with caffeine can contribute towards this recommendation.

To learn more about this topic, see: Hydration More than Just Water.

Although caffeine may have a mild, short-term diuretic effect in individuals who do not normally consume caffeine, recent studies have shown that this is not the case for people who regularly consume caffeine-containing beverages. As a result, the Institute of Medicine concluded in its 2004 report on water needs that all beverages, including those with caffeine, can contribute to hydration.

To learn more about this topic, see Laying the Caffeine-Dehydration Myth to Rest.

The adult human body can rarely survive without water for more than a week. Inevitably, how long a healthy person can live without any source of water depends on the rate at which fluid is lost. Although urine accounts for most fluid loss in the hydrated individual, this rapidly reduces in the presence of dehydration, and, unless there is other sources of water loss (for example vomiting and/or diarrhea), sweat becomes the most important source. Sweat losses depend on air/room temperature and level of physical activity.

Coma and death can be expected once dehydration amounts to ~20% of body weight. In a 175 pound (80 kg) man, this represents 35 pounds (16 kg) or ~4 gallons (16 L) of water. Observations made on healthy soldiers during the Second World War found that to maintain adequate homeostasis, a man resting at low ambient temperature (61°F/16°C), required about 8 cups (2L) of water a day. Thus, time for death from dehydration with zero water intake in relatively healthy individuals would therefore be expected to fall in the range 1-8 days, depending on environmental circumstances.

Reference: British Medical Journal website (BMJ.com), which also references Adolph EF. Physiology of Man in the Desert. Haffner Publishing. NY 1947.

Paying attention to urine output is actually an easy way for most people to determine if they are drinking enough.

Urine with a "light straw" or pale yellow color suggests adequate hydration. Urine that is dark in color suggests one is not drinking enough, although some medications and vitamin supplements can affect urine color. Another indicator is urine frequency. Normally, health people should urinate at least four times a day. And, of course, thirst should not be ignored. However, although thirst is a strong signal that the body needs fluids, lack of thirst may not always be an accurate indicator of adequate fluid intake.

Learn more about Monitoring Hydration Status and How to Calculate Your Sweat Rate, which is key for developing a personal hydration plan for regular vigorous physical activity.

Adequate hydration is important for proper function of your brain. Mild dehydration, as little as 1% to 2% loss in body weight due to fluid deficiency, can impair your ability to concentrate. And loss of more than 2% body weight due to dehydration can affect your brain's processing abilities and impair short-term memory.

Adequate hydration is also important to maintaining healthy and elastic skin. Water is also an important part of the fluids that help cushion joints and digest food. Your body's water also helps dissipate heat, regulating your overall body temperature.

Learn more about the role of hydration in the body with our Hydration & You tool.

Only about 7- 8 percent of the total amount of water in the body is found in the bloodstream. The majority (about 57 percent) is actually present inside cells, while about 20 percent is found in the space around the cells*.

The water in fluids between cells (called interstitial fluid) contains nutrients and helps carry metabolic wastes away from the cells. The water inside cells (intracellular fluid) facilitates chemical reactions and helps maintain cell structure. In fact, water comprises ~ 80% of most body cells' weight. Adipose (fat) tissue, which is about 15% water, is the exception. Fat cells contain almost no water, but there is interstitial fluid between fat cells.

*Source: IRCP Publication 23- Report of the Task Group on Reference Man

Most beverages, including milk, juice and soft drinks, are at least 85 percent water. Solid substances like the calcium and protein in milk, the pulp and sugars in juices and the flavoring ingredients and sugars in soft drinks account for the rest. Diet and Light soft drinks and beverages containing low- and no-calorie sweeteners can contain more than 99 percent water.

Some people are also surprised to learn that foods also contribute water to the body. Cooked rice, pasta and cereals are also between 65 - 90 percent water by weight. And even "dry" foods like nuts and seeds contain 1 to 5 percent water.

To learn more about this topic, see: Hydration: More Than Just Water