Science File / An exploration of issues and trends affecting science, medicine and the environment

Q.: How much of the human body is composed of water?

A.: Surprisingly, the answer varies with age, according to the book “Total Nutrition: The Only Guide You’ll Ever Need,” by Dr. Victor Herbert. The body of a newborn infant is 75% to 80% water. But a gradual process of water loss continues throughout life, until, in old age, water accounts for only about 50% of body weight.

The difference lies in the water composition of individual tissues. Muscle tissue is about 75% water, while body fat is only about 25% water and bone is about 20%. As muscles decay with age, the percentage of water in the body declines.