OBJECTIVE:Although the body mass index (BMI, kg/m 2 ) is widely used as a surrogate measure of adiposity, it is a measure of excess weight, rather than excess body fat, relative to height. We examined the relation of BMI to levels of fat mass and fat-free mass among healthy 5-to 18-y-olds. METHODS AND PROCEDURES: Dual-energy X-ray absorptiometry was used to measure fat and fat-free mass among 1196 subjects. These measures were standardized for height by calculating the fat mass index (FMI, fat mass/ht 2 ) and the fat-free mass index (FFMI, fat-free mass/ht 2 ). RESULTS: The variability in FFMI was about 50% of that in FMI, and the accuracy of BMI as a measure of adiposity varied greatly according to the degree of fatness. Among children with a BMI-for-age Z85th P, BMI levels were strongly associated with FMI (r ¼ 0.85-0.96 across sex-age categories). In contrast, among children with a BMI-for-age o50th P, levels of BMI were more strongly associated with FFMI (r ¼ 0.56-0.83) than with FMI (r ¼ 0.22-0.65). The relation of BMI to fat mass was markedly nonlinear, and substantial differences in fat mass were seen only at BMI levels Z85th P. DISCUSSION: BMI levels among children should be interpreted with caution. Although a high BMI-for-age is a good indicator of excess fat mass, BMI differences among thinner children can be largely due to fat-free mass.

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.

BACKGROUND: Childhood overweight develops during`critical periods', but the relationship of body mass index (BMI) patterns during`critical periods' from childhood into adulthood with subsequent overweight and adiposity has not been previously investigated. BMI patterns during early childhood, pubescence and post-pubescence and their independent effects on overweight and body fatness at 35 ± 45 y of age were examined along with birth weight and the effects of adult lifestyle factors. METHODS: BMI parameters describing the timing, velocity minimum (min) and maximum (max) values from 2 to 25 y of age were related to adulthood BMI values and total and percentage body fat (TBF, %BF) at 35 ± 45 y. These data were from 180 males and 158 females in the Fels Longitudinal Study. RESULTS: There was no sex difference in the timing of BMI rebound, but the age of BMI maximum velocity and maximum BMI were both earlier in girls. Children with an earlier BMI rebound had larger BMI values at rebound and at maximum velocity. Children who reached maximum BMI at later age had larger maximum BMI values. Maximum BMI was a strong predictor for adult BMI and in females, a strong predictor of adulthood TBF and %BF. Maximum BMI was closely related to maximum BMI velocity in females and in males, BMI at maximum velocity is a strong predictor of TBF and %BF. CONCLUSIONS: Changes in childhood BMI were related to adult overweight and adiposity more so in females than males. BMI rebound is a signi®cant important period related to overweight at 35 ± 45 y in females but not in males. However BMI patterns during and post-adolescence were more important than the BMI rebound for adulthood TBF and %BF status. There is marked tracking in BMI from approximately 20 y into 35 ± 45 y. The pattern of BMI changes from 2 to 25 y had stronger effects on subsequent adult overweight than birth weight and adult lifestyle variables.

Among girls, higher amounts of physical education may be associated with an academic benefit. Physical education did not appear to negatively affect academic achievement in elementary school students. Concerns about adverse effects on achievement may not be legitimate reasons to limit physical education programs.

OBJECTIVE. Our aim was to examine the degree to which changes in BMI percentile reflect changes in body fat and lean body mass during childhood and how age and gender affect these relationships.METHODS. This analysis used serial data on 494 white boys and girls who were aged 8 to 18 years and participating in the Fels Longitudinal Study (total 2319 observations). Total body fat (TBF), total body fat-free mass (FFM), and percentage of body fat (%BF) were determined by hydrodensitometry, and then BMI was partitioned into its fat and fat-free components: fat mass index (FMI) and FFM index (FFMI). We calculated predicted changes (⌬) in FMI, FFMI, and %BF for each 10-unit increase in BMI percentile using mixed-effects models.RESULTS. FFMI had a linear relationship with BMI percentile, whereas FMI and %BF tended to increase dramatically only at higher BMI percentiles. Gender and age had significant effects on the relationship between BMI percentile and FFMI, FMI, and %BF. Predicted ⌬%BF for boys 13 to 18 years of age was negative, suggesting loss of relative fatness for each 10-unit increase in BMI percentile.CONCLUSIONS. In this longitudinal study of white children, FFMI consistently increased with BMI percentile, whereas FMI and %BF had more complicated relationships with BMI percentile depending on gender, age, and whether BMI percentile was high or low. Our results suggest that BMI percentile changes may not accurately reflect changes in adiposity in children over time, particularly among male adolescents and children of lower BMI. URRENT RECOMMENDATIONS SPECIFY the 85th and 95th percentiles of the Centers for Disease Control and Prevention reference values of BMI to identify children who are aged 2 to 20 years and "at risk for overweight" and "overweight" and who may require additional testing and/or monitoring. [1][2][3][4][5][6][7][8][9][10] These percentiles were chosen as clinical action points for obesity prevention in children because they correspond to adult BMI values (eg, 25 kg/m 2 ) at which the risk for morbidity and mortality are significantly elevated. 3,6,11,12 Evidence also indicates that at or above these BMI percentiles, childhood cardiovascular health status is negatively affected, 13-16 future risk for overweight and obesity is elevated, 17 and future health is compromised. [18][19][20] Given that BMI percentiles and their recommended cutoff points are more easily interpretable than other BMI metrics and can be compared with nationally representative reference data, they provide a useful clinical tool for screening and prevention of obesity for children. 10 Accordingly, experts recommend that BMI percentile changes be monitored during annual well-child visits for the purpose of obesity prevention. 1 Although high correlations have been reported between BMI and both total body fat (TBF) and percentage of body fat (%BF) during childhood 21-26 and although BMI above the 85th percentile is predictive of elevated fat mass and %BF in children, BMI is not a precise indicator of the underlying ...