Applying the authors' method of estimating the body density which was reported previously, the human body density of young men from 7 to 22 years old was estimated, and the fat content in the body was calculated by Keys' method.The human body density is low in childhood and increases with growth and attaining the maximum at the age of about 16 or 17 years old, the value remains almost constant or tends to decrease. The body fat content (in percentage of body weight) decreases with age as the body density increases fill about 16 years old, while tends to increase thereafter.It follows that the total fat content in the body increases little by little until 16 years and after 17 years old, it increases remarkablly.From these results, it is concluded that the increase of body weight of young boys who are younger than about 16 or 17 years old, is mainly due to the increase of lean body mass, while the increase of body weight after 17 years old is not only due to both lean body mass but also to the content of fat in the body.
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