People can live several weeks without food, but only a few days without water. It is essential for all living things.
Human beings are made up of 65 to 70 percent water, and we need the life-giving liquid to maintain enough blood and other fluids so body systems function properly. Different types of tissue contain different amounts of water but the blood, bones, muscles and skin have the highest concentrations.
Water's function in the body includes regulating body temperature, transporting nutrients within and between cells plus aiding circulation and excretion. Drinking plenty of water also helps keep urine diluted and prevents kidney stones.
Each person loses four pints of water every day through breathing out, urination, bowel movements and sweating. To stay in good heath, experts recommend that adults drink about two quarts of fluid a day. Water comes both from the liquids we drink and from such foods as fruits and vegetables, even meat.
Drinking six to eight glasses of water a day is recommended rather than drinking soda, tea and coffee, because these can make your body lose liquid -- the diuretic effect. Soft drinks and juices also contain added calories.
Dehydration occurs if the body loses more fluid than it takes in. Dehydration can be a subtle problem to diagnose, said Dr. Eva Leonard, associate clinical professor at Stanford University.
"It is a multifaceted clinical problem and can mask other things like anemia or even pneumonia," said Dr. Leonard. "The underlying problem may not show up on blood tests or X-rays until we get the patient's fluid level back up."
Symptoms of mild dehydration include weariness, loss of appetite, flushed skin, heat intolerance, impatience, indistinct speech, stumbling and dizziness. More severe symptoms may include muscle spasms, delirium, shriveled skin, inability to swallow, sunken eyes, dimmed vision, painful urination, deafness and numb skin. With severe dehydration a person may have seizures, slip into coma and die.
"Older patients don't come in and say, 'I think I'm dehydrated,' " Dr. Leonard said. "We have to look at their history and symptoms. Sometimes they tell me they have bladder problems and can't drink water. So I need to look at their bladder functioning because they must increase their fluid intake."
She added that our senses age, including how often we get thirsty. "Some older people are habitually dry and are used to it," said Dr. Leonard. Women, in particular, may not want to bother going to the bathroom as much and after years have developed great control of their bladder. "They will find reasons not to drink fluids every day."
Diabetics need to watch their fluid intake carefully. Dr. Leonard said those with diabetes may crave fluids but need to choose the right type of liquid to quench their thirst. If they choose juice, for example, it may increase an already elevated blood sugar level.