What percentage of the human body is water?
About 60 percent in adult men and 55 percent in adult women. Newborns are about 80 percent water, and older people 50 to 55 percent.
60%
In an adult man weighing 60 kilograms (130 pounds), about 36 kilograms is water. That is enough to fill eighteen 2-liter bottles. Our bodies are made of water to a greater extent than we tend to think.
The proportion isn’t the same for everyone, though. Babies and older people, men and women, all carry quite different amounts of water.
Babies 80 percent, older people 50 to 55
According to the heat illness manual published by Japan’s Ministry of the Environment, water makes up about 80 percent of a newborn baby’s body weight. The share then falls gradually, to about 70 percent in infants and 65 percent in young children, reaching about 60 percent in adult men and 55 percent in adult women. In older people it drops to around 50 to 55 percent.
In other words, the human body gradually “dries out” as we age.
Fat makes the difference
The gap between men and women comes mainly from differences in body fat. Muscle is about 75 percent water, while fatty tissue holds only about 10 percent. Women generally have a higher proportion of body fat than men, so their bodies as a whole contain a lower share of water. For the same reason, some people with obesity are only about 45 percent water. Figures like 60 or 55 percent are just averages; the real number depends on each person’s build.
Different parts of the body hold different amounts too. An analysis carried out in the United States in 1945 found the brain and heart to be about 73 percent water, the lungs about 83 percent, muscles and kidneys about 79 percent, and skin about 64 percent. Even hard bone was about 31 percent water.
Where is all that water?
Water in the body sits in three main places. In an adult man, an amount equal to about 40 percent of body weight is inside the cells, about 15 percent is in the spaces between cells, and about 5 percent is in the blood vessels as plasma, the liquid part of the blood. So two-thirds of the body’s water is inside cells.
This water is constantly at work: carrying nutrients and waste, and releasing heat as sweat. Keeping the concentration of body fluids steady is another of its vital jobs.
2.5 liters swapped every day
The water in your body doesn’t just stay put. For a 70-kilogram (155-pound) person living an ordinary life, about 2.5 liters (2.6 quarts) of water come in each day from drinks, food and “metabolic water,” which forms inside the body when nutrients are used. A similar amount leaves as urine, feces, sweat and breath.
Japan’s Ministry of the Environment recommends drinking about 1.2 liters a day as beverages, on top of the water in food. When you sweat heavily during exercise, the advice is to drink enough that your weight drops by no more than 2 percent between the start and end of the workout.
When the body loses water, there is less blood available to carry heat away, so it becomes harder to cool down. Muscles, the brain, the liver and the kidneys may not get enough blood either, which can lead to cramps or a hazy, confused state. And beer or other alcoholic drinks are no way to replace water lost through sweat, because alcohol makes you pass more urine.
Older people start out with less water in their bodies, and they find it harder to sense thirst even as dehydration sets in. That is one reason they are more vulnerable to heat illness in hot weather. In fact, people of any age are said not to feel thirsty when only mildly dehydrated. Whatever your age, the advice is to drink a little and often, before you feel thirsty.