Dehydration occurs when the amount of water in the body is too low. Causes of dehydration include not drinking enough fluids, or losing a lot of fluids through sweating, particularly during hot weather, exercise or a fever, or a combination of these factors.
How much fluid you need depends upon several things, including:
Age: Kids need plenty of fluids; they can get dehydrated much more easily than adults. Older people may need more fluids because of health conditions or because they tend to lose their sense of thirst.
Gender: Men need more fluids than women. (And pregnant women need more fluids than other women.)
Weight: Heavier people need more water.
Health: Conditions such as diabetes, cystic fibrosis, and kidney disease can boost your need for fluids.
Environment: You need more fluids in extreme weather conditions (especially hot, humid, or cold) and at high altitudes.