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Water is often called the most forgotten nutrient, yet it is involved in almost every process that keeps the human body alive. Around 60 percent of an adult's body weight is water, and the proportion is even higher in babies and young children. Blood, which is mostly water, carries oxygen and nutrients to cells, removes waste products, and helps control body temperature. Because the body cannot store large amounts of water, supplies must be replaced every day.
The body loses water constantly, even when a person is resting. Every breath releases a small amount of moisture into the air, and the skin loses water through sweating, which increases sharply in hot weather or during exercise. The kidneys also remove water when they filter waste from the blood to produce urine. Even digestion uses a surprising amount of fluid. Altogether, a typical adult loses between two and three litres of water a day, and this loss must be matched by intake from drinks and food combined.
Water has many other jobs that people rarely notice. It cushions the brain and the spinal cord, lubricates the joints so they move smoothly, and helps digest food by forming part of saliva and gastric juices. It also allows the kidneys to flush out waste products, and people who drink too little over many years run a higher risk of painful kidney stones. Water also plays a quiet role in keeping body temperature steady, since sweat evaporating from the skin carries heat away. In addition, the fluid in and around cells allows chemical reactions to take place and helps transport hormones and other signals from one part of the body to another.
Many people have heard the advice to drink eight glasses of water a day. In fact, this figure is a rough guide rather than a scientific rule, because individual needs vary widely. Body size, activity level, climate, and diet all influence how much fluid a person requires. Someone who eats plenty of fruit and vegetables, for example, obtains a useful amount of water from food, which normally provides about 20 percent of daily fluid intake.
Drinks other than plain water also contribute. Milk, fruit juice, tea, and coffee all count toward daily totals, although health experts advise limiting sugary drinks such as cola because of their links with weight gain and tooth decay. Contrary to a common belief, moderate amounts of tea and coffee do not cause dehydration; their mild caffeine effect is easily outweighed by the water they contain.
Dehydration occurs when fluid loss exceeds intake. The earliest signs are often mild: thirst, a dry mouth, and darker urine. Health professionals often suggest checking urine colour, since pale yellow generally indicates adequate hydration while dark yellow suggests the need to drink more. If dehydration continues, headaches, tiredness, and dizziness can follow, and severe cases may require medical treatment.
Even mild dehydration can affect how well people think and feel. In one widely reported study, young women who lost just over one percent of their body weight in fluids after exercise reported poorer concentration, worse moods, and more frequent headaches than when they were properly hydrated. Similar research with office workers found that those who drank water regularly during the day performed better on memory and attention tests in the afternoon.
Some groups need to pay special attention to hydration. Athletes training hard in hot conditions can lose more than a litre of sweat per hour, so coaches encourage them to drink at regular intervals before, during, and after exercise. Older people are also at risk, because the sense of thirst becomes weaker with age, and many elderly people simply forget to drink. Young children, meanwhile, depend on adults to remind them.
For most healthy people, the practical message is simple. Keep a bottle of water nearby, drink a glass with each meal, and increase intake in hot weather or when exercising. There is no need to force down huge volumes, since drinking far more than the body needs offers no extra benefit. Paying attention to thirst and to urine colour is usually enough to keep this essential system in balance.