Thirst and Dehydration

Thirsty? Then your body is on its way to becoming dehydrated. For the average adult, water accounts for 60 percent of body weight. That water plays a critical role in nearly every bodily process. And being a quart or two low can affect how you feel. Water is key in body temperature regulation. 

Nearly everything you do during the day—typing, talking, walking, even eating—requires energy. That energy, in turn, generates heat, which escapes the body through perspiration. Perspiration is the mechanism that keeps the body cool.

Most adults lose about 1.5 liters of fluid a day in urine. An additional liter is lost through breathing, sweating, and bowel movements. An average woman needs about 11 cups of water (2.7 liters)—from all beverages and foods—each day, and the average man needs about 15.5 cups (3.7 liters), according to 2004 recommendations from the Institute of Medicine. Because about 80 percent of a person's total water intake comes from drinking water and beverages—including caffeinated beverages—and the remaining 20 percent comes from food, the average woman should drink about nine cups of liquid a day; the average man should drink about 12 cups. But water needs are very individual, depending on many factors in your daily life, so many doctors simply advise their patients to drink until they are no longer thirsty.

These are situations that require you drink more liquid:

  • Exercise

  • Being in hot or humid weather

  • Indoor heating during wintertime that causes dry air

  • Being at a high altitude

  • Illnesses that cause high fever, diarrhea, vomiting, or frequent urination

  • Pregnancy or breastfeeding


Dehydration occurs when the body doesn't have as much water and fluids as it should. Dehydration can be caused by not drinking enough water or by losing too much fluid. Infants, children, older adults, and people with certain illnesses are especially vulnerable to dehydration.

The loss of just 2 to 3 percent of body-water weight can affect some of the functions in the body. Dehydration is classified as mild, moderate, or severe, depending on how much fluid is lost or not replaced. Mild or moderate dehydration can be treated by drinking small to moderate amounts of fluid repeatedly until the dehydration improves; drinking too much too quickly can cause vomiting. Severe dehydration is a life-threatening emergency and requires medical treatment.

These are symptoms of dehydration that mean you should call your health care provider:

  • Low or no urine output; concentrated urine appears dark yellow

  • Lack of tears

  • Sunken eyes

  • In an infant, markedly sunken fontanels (the soft spot on the top of the head)

  • Lethargic or comatose


To guard against dehydration, drink up. Don't depend on feeling thirsty, though. Thirst may not be a reliable guide to tell you when you're dehydrated, especially for older adults. Drink more when the weather is hot, you're exercising, or ill. Urine that is light yellow is a good indication that you're getting enough fluid.