Figure shows the median curves for body mass index in the six datasets by sex from birth to 20 years. A wide range of values spans several units of body mass index in both sexes. These show the different extents of overweight across datasets, reflecting national differences in fatness. The median curves are all about the same shape, although the curve for Singaporean males is more curved, being lowest at ages 6 and 19 and highest at age 11.
Median body mass index by age and sex in six nationally representative datasets
Averaging the median curves would be a simple way to summarise the age trend in body mass index through childhood. But the resulting position of the curve at each age would depend on the overweight prevalence of the countries in the reference set, and so would be comparatively arbitrary. In any case the median is not an extreme centile and is ineffective as a cut off point. So averaging the median curves is not the answer.
Instead the centile curves are linked to adult cut off points of 25 and 30 kg/m2, positioned at age 18 to maximise the available data. These values are expressed as centiles for each dataset, and the corresponding centile curves are drawn. Figure shows the centile curves for overweight and obesity for the British reference.
Figure presents the centile curves for overweight for the six datasets by sex, passing through the adult cut off point of 25 kg/m2 at age 18. They are much closer together than the median curves (fig ), particularly above age 10, because the national differences in overweight prevalence have been largely adjusted out. The divergence of the Singaporean curve is more pronounced than in figure .
Centiles for overweight by sex for each dataset, passing through body mass index of 25 kg/m2 at age 18
Figure gives the corresponding centile curves for obesity in each dataset, all passing through a body mass index of 30 kg/m2 at age 18. There is less agreement than for the centiles for overweight, and again Singapore stands out.
Centiles for obesity by sex for each dataset, passing through body mass index of 30 kg/m2 at age 18
Table gives the centiles for overweight corresponding to a body mass index of 25 kg/m2 at age 18 for each dataset by sex. For example, they approximate the 95th centile for Dutch males and the 90th centile for British males—that is, prevalences of overweight of 5-10%. The centiles for obesity corresponding to a body mass index of 30 kg/m2 in table are mainly above the 97th centile, less than 3% prevalence, and they show more variability.
Table 2 Centiles and z scores for overweight corresponding to body mass index of 25kg/m2 at age 18 years in six datasets, derived from fitted LMS curves
Table 3 Centiles and z scores for obesity corresponding to body mass index of 30kg/m2 at age 18 years in six datasets, derived from fitted LMS curves
The curves in figures and are reasonably consistent across countries between ages 8 and 18, although those for Singapore are higher between ages 10 and 15. This is due partly to the increased median (fig ) and partly to greater variability. The LMS method estimates the coefficient of variation (or S curve) of body mass index during the centile fitting process, and figure compares the S curves for the six datasets. Between ages 6 and 15 the coefficient of variation in Singapore is greater than for the other countries. The range of values for the coefficient of variation in puberty is greater for males than females, and for Brazil, Singapore, and the United States the curves for both sexes show a peak in puberty.
Plots of coefficient of variation of body mass index by age and sex for each dataset
The amount of skewness, as measured by the sample L curves, is similar across countries. The Box-Cox powers are consistently between –1 and –2 indicating extreme skewness (not shown).
Table shows international cut off points for body mass index for overweight and obesity from 2-18 years, obtained by averaging the centile curves in figures and . From 2-6 years the cut off points do not include Singapore because its data start at age 6 years. Figure shows the cut off points, with the values at 5.5 and 6 years adjusted slightly to ensure a smooth join between the two sets of curves.
Table 4 International cut off points for body mass index for overweight and obesity by sex between 2 and 18 years, defined to pass through body mass index of 25 and 30kg/m2 at age 18, obtained by averaging data from Brazil, Great Britain, Hong Kong, (more ...)
International cut off points for body mass index by sex for overweight and obesity, passing through body mass index 25 and 30 kg/m2 at age 18 (data from Brazil, Britain, Hong Kong, Netherlands, Singapore, and United States)