The difference between under-hydration and dehydration is not often discussed. Dehydration is a familiar concept. We know the feeling when we lose too much fluid: sluggishness, headaches, muscle cramps, and other symptoms. We know when we are dehydrated.

Underhydration, however, is much more sneaky. Many of us—likely most—do not drink enough water daily. We experience mild dehydration so often that it becomes normal to us. Yet this underhydration, with serum sodium levels around 143–145, is often associated with obesity and the worsening of several eye conditions, such as retinopathy and glaucoma, as well as heart disease. Underhydration is a serious issue. A reasonable daily intake recommendation is about two liters (or thirty-six ounces) for most men and slightly less for most women. With exercise and in hot weather, this amount should increase by at least another half liter (18 ounces). Adequate electrolytes are necessary for good intracellular hydration and to prevent depletion. If you drink water and urinate it out right away, then your cells didn’t absorb it.