Let’s start with some facts. Water is the single most important substance we consume. You can survive for about two months without food, but you would die in about seven days without water. Water makes up about 75 percent of an infant’s weight and 55 percent of an older person’s weight.
--"Dehydration: Risks and Myths," Jane Brody, NY Times, 5/9/16
Being mildly dehydrated is often the root of headaches, fatigue, constipation, dry eyes, among many other ailments. If you're feeling under the weather and otherwise healthy, ask yourself, when's the last time I drank a glass of water?David Shute, MD