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As is often the case with your health, it’s best to start with the obvious: Your thirst tells you to drink. For healthy people under normal circumstances, thirst is a reliable mechanism to indicate the body’s need for more fluid. However, your thirst doesn’t tell you exactly what to drink. It just tells you that you’re thirsty. Of course, billions of dollars are made by persuading you to reflexively reach for a soft drink or something like that, when really the best choice usually is water.

How much to drink?
You may have read or heard that you need at least eight glasses of water per day. This quantity won’t hurt a healthy adult. But doctors say such one–size–fits–all answer fails to tell the whole story about the body’s necessary balance of fluid intake and loss. Humans normally lose about 10 cups (2.4 liters) of fluid a day in sweat, urine, exhaled air and bowel movements. What is lost must be replaced to maintain a fluid balance.

Dehydration poses a particular health risk for the very young and the very old. Not all fluid replacement must come from water. Other drinks consist mostly of water. Foods contain water as well. However, as mentioned earlier, your thirst is generally a good guide for when you need to replace fluids, and water is generally the best choice. If you are normal and have a normal set of kidneys and lungs, I think the maximum amount of water tolerated is huge and the minimum is less than eight glasses a day. Caffeinated and alcoholic beverages are actually dehydrating because they increase urine output, so don’t count on those to replenish fluid loss.

Are there exceptions?
Of course! For example, if you have kidney stones, drinking eight or more glasses of water a day is highly recommended. You also should drink extra amounts of water when you are experiencing any dehydrating conditions (e.g. hot, humid or cold weather, high altitudes, physical exertion, etc.).

What does the body do with all that water?
Among other things, water regulates the body’s temperature through perspiration, carries away wastes in the urine and moves nutrients and other substances throughout the body. There’s tremendous movement of water to and from organs and tissues in your body. Blood itself is more than 80 percent water. A healthy person’s urine output is a very small fraction of the total quantity of fluid filtered by the kidneys. Most is reabsorbed and used elsewhere in the body.

What can excessive thirst mean?
Increased thirst and increased urination (both in volume and frequency) can be symptoms of diabetes mellitus. Excessive thirst and urination also are symptoms of diabetes insipidus. Diabetes mellitus is due to a deficiency of or resistance to insulin, a hormone produced by the pancreas that enables your body to use glucose to provide cells with energy. This is not to be confused with diabetes insipid us, which results from a deficiency of ant diuretic hormone (ADH). A shortage of ADH, which is produced by the posterior lobe of the pituitary gland in the brain, causes your body to lose control of its water balance. If you notice unexplained increases in thirst and urination, consult your physician to determine the cause. There are other possible causes to be considered. Some people consume excessive amounts of water and experience increased urine output not associated with any underlying disease.

What about hydration during physical exertion?
Thirst is not always an adequate indicator of your body’s need for fluid replenishment during exercise. Studies show that during vigorous exercise, an important amount of your fluid reserves may be lost before you are aware of thirst. Make sure you are sufficiently hydrated before, during and after exercise. Again, water is your best bet. Sports drinks are generally not necessary unless you are exerting yourself for 90 minutes or more (60 minutes if the activity is particularly intense or temperatures are very hot). During exercise, it’s recommended to replenish fluids at least every 20 minutes. Remember, your body has limits to its ability to adjust to fluid loss.