The magnitude of the association between childhood levels of BMI and body fatness (determined by dual-energy x-ray absorptiometry, densitometry, and other methods) has varied substantially across studies, and relatively modest (r
, approximately 0.5) associations have been reported.26–28
In the current study, of the 200 children who had a BMI for age between the 85th and 94th percentiles, about 20% had a body fatness that was comparable with levels among children with higher BMIs, while about 30% had a body fatness within the range of children with normal weight (<85th percentile). Although the misclassification of body fatness among children who had a BMI for age between the 85th and 94th percentiles did not vary significantly by sex or age, black children with BMIs in this range were more likely to have a normal body fatness than were other children. In addition, children with BMI for age between the 85th and 89th percentiles were more likely to have normal body fatness than those with levels between the 90th and 94th percentiles.
Although a high BMI (for a child’s sex and age) is a good indicator of elevated body fatness,29
BMI has been considered to be “almost useless as an estimator of percentage of body fat in normal-weight children.”15
We have previously reported14
that BMI differences among children with normal weight can largely reflect variability in fat-free mass, and our current results indicate that there is substantial misclassification of body fatness by BMI for age among thin children. For example, only 42% of the 144 children who had a BMI for age lower than the 25th percentile had a correspondingly low body fatness, but 93% of the 120 children with very high levels of BMI for age (≥97th percentile) had an elevated body fatness. We have previously shown30,31
that skinfold thickness measurements can substantially improve the prediction of body fatness in children with a BMI for age lower than the 95th percentile, and it is likely that these measurements could identify children with moderately elevated BMIs who truly have elevated body fatness.
In addition to the variability in levels of fat-free mass, the misclassification of body fatness by BMI for age may also reflect the differing scales for BMI and BMI for age. Among boys at age 12 years, for example, the difference between the 50th (BMI, 17.8) and 97th (BMI, 26.0) percentiles of BMI for age is 8.2, whereas the difference in BMI between the third (BMI, 14.7) and 50th percentiles (also a 47-point percentile difference) is only 3.1. The asymmetry arises from the skewed distribution of BMI levels. Because BMI-for-age categories among heavy children span greater BMI ranges than do categories among thinner children, one might expect BMI for age to be a better indicator of the body fatness of relatively heavy children. We have previously reported30
that the combination of age, race/ethnicity, and BMI for age is a good predictor of percent body fat, with multiple R2
values of 0.81 (boys) and 0.82 (girls).
About 70% of children in the current study with a BMI for age between the 85th and 94th percentiles had a high (moderate or elevated) level of body fatness; this positive predictive value did not significantly vary by sex or age. Black children with BMIs in this range (both boys and girls), however, were less likely to have high body fatness than other children (white, Asian, and Hispanic). These results are in agreement with our previous report16
that at similar levels of BMI for age, the mean body fatness of black children is about 3% lower than that of white children. Other investigators have also reported racial differences in body composition, with black individuals having a higher fat-free body density32
and less body fatness33
than white individuals.
All classification systems of body fatness are somewhat arbitrary, and there is little agreement on the classification of excess body fatness among adults or children.23
We used cutoffs for percent body fat that were consistent (in terms of identifying similar numbers of children within each sex-age group) with various categories of BMI for age, and other studies34–36
have used similar approaches. Although some investigators have attempted to base fatness cutoffs on cross-sectional associations with cardiovascular disease risk factors,37–39
this approach may not be optimal. The underlying associations between risk factors and body fatness are continuous without obvious inflection points, and some of the proposed cutoffs37
do not account for the large age and sex differences in body fatness. Furthermore, it is possible40,41
that various risk factors are as strongly correlated with levels of BMI as with body fatness.
There are several limitations of the current study that should be considered. Children in the current study were recruited from 1995 to 2000 and cannot be considered to represent the general population. However, we have reported14
that mean levels of BMI for age in this sample differ only slightly (z
score, about 0.1) from those of children examined in the Bogalusa Heart Study and in the National Health and Nutrition Examination Survey. Furthermore, because the proportion of children in the current study who had a BMI for age at or above the 95th percentile (16%) and between the 85th and 94th percentiles (17%) were almost identical to those among 6- to 19-year-old adolescents examined in the National Health and Nutrition Examination Survey from 2003 to 2006,13
our results may be generalizable. It should also be realized that we are not proposing that our cutoffs for body fatness () be used to classify excess body fatness in other studies. We feel, however, that the approach of matching prevalences of body fatness and BMI-for-age categories is useful in examining the correspondence of categories of BMI and body fatness.
In summary, our results indicate that BMI for age is an appropriate screening test to identify individual children and adolescents who should have further evaluation and follow-up. However, BMI for age cannot be considered to be diagnostic of body fatness; this is particularly true for children who have moderately elevated levels of BMI for age (between the 85th and 94th percentiles). Although 70% of children with a BMI for age between the 85th and 94th percentiles had a body fatness that corresponded to (moderate) or was higher than (elevated) the level indicated by their BMI for age, 30% had a body fatness that was comparable with levels among children with normal weights. This misclassification was highest among black children and those with lower levels of BMI for age.