What Body Water Percentage Means

It's commonly said that "The human body is mostly water." Given that this is the case, it's no wonder that body water is a key body composition measurement! What does body water refer to, and what is the normal body water range? What does it mean to be dehydrated, and what are its signs?
Body Water
In survival scenarios, the "rule of three" refers to priorities needed by the body to survive. You can survive 3 weeks without food, 3 days without water, 3 hours without shelter in a harsh environment, and 3 minutes without air. Suffice to say, body water is a crucial aspect of body function, and proper hydration is needed for ideal organ function, regulation of temperature, blood flow, and overall health. 
Normal Range
Normal range for body water varies with age and gender. Body water tends to be higher for babies (~75%), decreases as we grow into adulthood, and stays relatively stable throughout adult life. In general, men (50-65%) tend to have more body water than women (45-60%).
(Note that body water can change significantly during pregnancy)
What's particularly interesting is that body water correlates very strongly with muscle mass! As confirmed in a study published in the Journal of Nutrition, Health & Aging in 2019, researchers found there was a correlation coefficient of 0.89 between body water and muscle mass, and that Body Water was also positively correlated with the Barthel index score (functional performance) and gait speed in both men and women.
This is because muscle is high in water content, whereas fat contains relatively little water in comparison. (Note that this is a key principle that is utilized by Bioelectrical Impedance Analysis technology!) As such, people who have more body fat tend to have a lower body water percentage compared to those with less body fat. For example, someone with 20 percent body fat will have a higher body water percentage, compared to someone with 40 percent body fat.
Advice regarding how much water a person should drink each day is varied. The traditional guidelines of "X number of cups per day" is sometimes challenged by a seemingly intuitive "drink if you feel thirsty". Other common advice includes "your urine should be clear, not yellow. A yellow color is a sign of dehydration".
What does it mean to be dehydrated?
As a general guideline, dehydration - meaning not enough body water - is caused by not drinking enough fluid, or by losing more fluid than you take in. Physical exercise (particularly in hot weather), diet, or illness such as fever, persistent vomiting or diarrhea can all contribute to dehydration if fluids are not replenished sufficiently. Various physical symptoms can arise when the body is dehydrated and unable to carry out its normal functions smoothly, such as dark yellow urine, thirst, headaches, fatigue, muscle cramps, and more. 
In most cases, the easiest way to treat dehydration is simply to rehydrate by drinking plenty of fluids. In severe cases, intraveneous drips might be administered to replenish essential nutrients faster than drinking.
Is it possible to be "overhydrated"?
Generally speaking, the body effectively self-regulates body water automatically, and it is very hard to "overhydrate" by drinking too much water, as you will naturally have an urge to get rid of it and maintain a normal balance. 
However, it is possible for the body to hold on to too much water, or be unable to get rid of it as it normally should! This excess water is referred to as "water weight". Various illnesses such as those caused by kidney or cardiac problems correlate with excessive water weight, as the body has difficulty maintaining a proper balance.
Among otherwise healthy individuals, temporary retention of body water isn't necessarily indicative of a health problem. For example, increased salt intake can cause water retention.
Why is it important to estimate your body water?
You probably don't need a body composition analyzer to tell you that you're thirsty.However, knowing your body water level is very useful for tracking the source of weight change!
For example, when people begin a "diet" - which typically refers to a calorie-restricted diet - water weight is typically the first to go, before body fat starts to decrease significantly. So if you've noticed your weight dropping on the scale, it's important to first confirm if it's actually a decrease in body fat, or just losing water.
We hope that has helped clarify how body water can be a very useful measure of body composition!