Childhood Body Composition in Relation to Body Mass Index

OBJECTIVE: The aim is to describe body composition in relation to body mass index (BMI; body weight/stature(2)) to provide health care professionals insight into the meaning, significance, and limitations of BMI as an index of adiposity during childhood. METHODS: Data from 387 healthy, white children 8 to 18 years of age from the Fels Longitudinal Study were analyzed. Measurements were scheduled annually and each child was examined 1 to 11 times, totaling 1748 observations. Total body fat (TBF) and fat-free mass (FFM) were determined from hydrodensitometry. Stature and weight were measured using standard methods and BMI and the components of BMI, TBF/stature(2), and FFM/stature(2) were calculated. Analyses included correlations between BMI and body composition variables; age-related patterns of BMI, TBF/stature(2), and FFM/stature(2); and annual changes in BMI, TBF/stature(2), and FFM/stature(2). RESULTS: Generally, correlations between BMI and body composition variables were strong and significantly different from zero. Means for BMI throughout childhood were similar for boys and girls, although significantly larger values were observed for girls at ages 12 to 13 years. Age-related patterns of TBF/stature(2) and FFM/stature(2) differed between sexes. In each sex, annual increases in BMI were driven primarily by increases in FFM/stature(2) until late adolescence, with increases in TBF/stature(2) contributing to a larger proportion of the BMI increases in girls than in boys. CONCLUSIONS: Unlike adults, annual increases in BMI during childhood are generally attributed to the lean rather than to the fat component of BMI. Because the properties of BMI vary during childhood, health care professionals must consider factors such as age and sex when interpreting BMI.