This analysis provides trajectories for FMI, FFMI, and WC for black and nonblack adolescents. The trajectories varied remarkably by age and gender and to a lesser extent by race. There were gender and age differences in the measurements after age 10 years, with a strong divergence between boys and girls in the growth pattern of FMI; boys decreased in FMI with age, and girls continued to increase in FMI. This finding was similar to those from other studies,8,15,16
except in one,8
FMI values in boys did not decrease until after age 12 years.
Although BMI growth pattern and rate were similar for boys and girls, the BMI components differed greatly between genders, particularly during adolescence. In both genders, BMI was composed predominantly of lean mass. For boys, the age-related increase in BMI was concurrent with a large increase in FFMI and a decrease in FMI, which may explain the changes in boys’ physiques as they become more lean and muscular during adolescence. These results were similar to those of other studies.3,5,15,17
Demerath et al.3
reported that white boys aged 13–18 years experience an increase in BMI percentile that may be accompanied by a uniform increase in the FFM component across the range of BMI percentiles; further, older boys increase faster in FFM than younger boys and experience a decrease in percent body fat, even in the overweight range.
Waist circumference trajectories differed by gender and race. Black girls had a larger WC compared with other gender–race groups at all ages, similar to the findings in another report.18
Waist circumference did not differ between black and nonblack boys, which supports findings19
of no differences in the rate of visceral and subcutaneous abdominal adiposity between gender groups. Of the four adiposity trajectories presented in the current study, WC showed the fewest differences among the four gender–race groups and generally was most similar to BMI and FFMI, except that black boys had a steeper increase in FFMI. An explanation for the increase in WC is that it represents the measurement for truncal fat, including muscles and bones without differentiation of visceral and subcutaneous fat, and the increase for boys may represent a greater increase in FFM than in fat mass.
The trajectories for all measurements varied remarkably by age and gender and to a lesser extent by race. In the study cohort, black girls, at all ages, had higher values of FMI, BMI, and WC than other race–gender groups, likely making them the highest risk group for CVD and its risk factors. In a study of healthy multi-ethnic children aged 5–18 years,7
distribution of lean and fat mass was similar to the current findings. The current findings are also in agreement with national data indicating that BMI values for black girls are higher than those for nonblack girls during this age period.20
The analyses did not seek to determine the etiology of excess fat mass development in black girls, but they clearly demonstrate the excess fat mass that occurs in adolescents, which likely increases their risk for development of CVD risk factors and CVD as adults. Some possible mechanisms for the differences between black girls and other youth in this study are physiologic, genetic, cultural, and socioeconomic.
Our findings support the recommendation17
to use FFMI and FMI in determining the deeper meaning of BMI. These two measures of adiposity allow independent evaluation and assessment of the contribution of FFM and fat mass relative to body size.4
Calculation of FFMI and FMI in the context of BMI enable the clinician to identify children with normal BMI and excess adiposity (large FMI) to initiate intervention. Similarly, they enable identification of children who are overweight as a result of a large FFMI for primary prevention counseling.
Standards for FFMI, FMI, and WC need to be established. The current findings contribute to the understanding of FFMI, FMI, and WC. A possible weakness of the findings is that FFMI and FMI calculations require measurement of percent body fat. This measurement can be done using bioelectric impedance analysis or dual x-ray absorptiometry. Neither of these measures is readily available in the majority of outpatient pediatric clinics. Another limitation of the current study is that the population was predominantly white with few black children, and there were no other race/ethnic groups for comparison. Finally, the participants were at a variety of pubertal stages, which may account for significant differences in trajectories between boys and girls and between races.