2. Changes in body composition during growth
While girls are only slightly fatter than boys at birth and
throughout infancy and childhood, the sex difference in body composition becomes
significant during adolescence. The adolescent spurt in weight and stature is
accompanied by increases in lean body mass (LBM) and body fat; the boy acquires
considerably more LBM than the girl, while she accumulates a larger percent body
fat. Between the age of 10 and 20 years, the average boy puts on 33 kg LBM, the
average girl only 16 kg. This sex difference remains when LBM is referred to
stature: adolescent boys have a higher LBM:height ratio than girls. Not only is
the adolescent spurt in LBM more intense in the male, but it also lasts longer;
maximum LBM is achieved by the girl at about 18 years, but in the boy not until
about age 20 (Figure 2).
Upper section: Mean
body weight and lean body mass (LBM) during late childhood and adolescence.
Lower section: Estimated LBM velocity; note the more intense and prolonged LBM
spurt in boys. Author's data by potassium-40 counting.
This sex difference in LBM growth has obvious nutritional
implications. Based on available data, the average boy will accumulate during
the second decade of life 770 g Ca, 2.08 g Fe, 0.98 g Zn, and 7300 g protein.
For the average girl these values are much less, namely 400 g Ca, 0.84 g Fe,
0.66 g Zn, and 3650 g protein. The sex difference is similarly striking for the
estimated daily increments in body content at the peak of the adolescent
growth spurt: 400 mg Ca for the boy versus 240 mg for the girl, 3.8 g protein
for the boy, 2.25 g for the