Temporal divergence of percent body fat and body mass index in pre-teenage children: the LOOK longitudinal study.
BACKGROUND
The index of body mass related to stature, (body mass index, BMI, kgm(-2) ), is widely used as a proxy for percent body fat (%BF) in cross-sectional and longitudinal investigations. BMI does not distinguish between lean and fat mass and in children, the cross-sectional relationship between %BF and BMI changes with age and sex.


CONCLUSIONS
While BMI increases linearly with age from age 8 to 12 years in both boys and girls, %BF plateaus off between 10 and 12 years. Repeated measures in children show a systematic decrease in %BF for any given BMI from age 8 to 10 to 12 years. Because changes in BMI misrepresent changes in %BF, its use as a proxy of %BF should be avoided in longitudinal studies in this age group.


BACKGROUND
Body mass index (BMI, kgm(-2) ) is commonly used as an indicator of pediatric adiposity, but with its inability to distinguish changes in lean and fat mass, its use in longitudinal studies of children requires careful consideration.


OBJECTIVE
To investigate the suitability of BMI as a surrogate of percent body fat (%BF) in pediatric longitudinal investigations.


METHODS
In this longitudinal study, healthy Australian children (256 girls and 278 boys) were measured at ages 8.0 (standard deviation 0.3), 10.0 and 12.0 years for height, weight and percent body fat (%BF) by dual-energy X-ray absorptiometry.


RESULTS
The patterns of change in the means of %BF and BMI were different (P < 0.001). While mean BMI increased linearly from 8 to 12 years of age, %BF did not change between 10 and 12 years. Relationships between %BF and BMI in boys and girls were curvilinear and varied with age (P < 0.001) and gender (P < 0.001); any given BMI corresponding with a lower %BF as a child became older.


CONCLUSIONS
Considering the divergence of temporal patterns of %BF and BMI between 10 and 12 years of age, employment of BMI as a proxy for %BF in absolute or age and sex standardized forms in pediatric longitudinal investigations is problematical.
UC National Institute of Sports Studies, University of Canberra, Canberra, ACT, Australia; Clinical Trials Unit, Academic Unit of Internal Medicine, Canberra Hospital, Garran, ACT, Australia.
R D
RD
Telford
R B
RB
Cunningham
W P
WP
Abhayaratna
D015502:Absorptiometry, Photon; D000273:Adipose Tissue / Q000473:pathology*; D000375:Aging; D001315:Australia / Q000453:epidemiology; D001823:Body Composition / Q000502:physiology*; D050218:Body Fat Distribution; D015992:Body Mass Index; D002648:Child; D003430:Cross-Sectional Studies; D005260:Female; D006801:Humans; D008137:Longitudinal Studies; D008297:Male; D012737:Sex Factors
BMI; DEXA; childhood obesity; public health