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Med J Aust 2000; 173 Suppl 7 August: S8-S9

Body mass index

The most convenient way of measuring overweight and obesity in clinical practice is to determine body mass index (BMI; weight/height²). Note that precise and accurate measurements of height and weight are required, and that self-report or parent's report of the heights and weights of children and adolescents is not reliable.

BMI is significantly correlated with fatness in childhood and adolescence,²⁸ and is specific for those with the greatest amount of fatness.^29,30 In childhood, BMI changes substantially with age, falling during the preschool years and then rising again into adulthood. For this reason, age-related reference charts are necessary for assessing BMI in children and adolescents.

Which BMI-for-age reference chart should be used? Members of the Childhood Obesity Working Party of the International Obesity Task Force (IOTF) have amalgamated BMI-for-age data from several nationally representative studies in order to develop a globally representative BMI-for-age chart.³¹ This work has only just been published³² and is yet to be used clinically. The current clinical practice in Australian paediatric practice is to use readily available BMI-for-age reference charts derived from measurements of United States children taken in the early 1970s (see Box 5).³³

To date, only arbitrary cut-off points for BMI-for-age in children (eg, the 85th and 95th centiles) have been used to mark the transition from "normal weight" to "overweight" and thence to "obesity".³⁴ Interestingly, the recently published IOTF BMI-for-age reference values³² incorporate a novel approach to setting the childhood percentile for overweight and obesity based on adult morbidity cut-off points.³¹

Assessment of childhood obesity in general practice
General practitioners are well placed to assess obesity in childhood. Careful measurements of height and weight are essential, as is the routine charting of height-for-age, weight-for-age and, if possible, BMI-for-age on appropriate growth charts.³⁵ Serial growth measurements are particularly useful for monitoring growth and nutritional status or the development of overweight or obesity. Serial measurements of waist circumference or waist-to-hip ratio may be helpful for identifying children with a more central fat distribution, and hence increased cardiovascular risk factors.³⁶

More detailed information on the clinical assessment of obese children is provided in a recent review.³⁷

The prevalence of childhood obesity in Australia
"Simple" figures on the prevalence of obesity in childhood and adolescence are not readily available. Nevertheless, there is clear evidence that the prevalence of obesity and overweight in Australian children is increasing. A comparison of anthropometric data on children from the 1985 Australian Health and Fitness Survey with those from the 1997 Health of Young Victorians Study showed substantial increases in BMI for children aged 7-12 years between 1985 and 1997.³⁸ This is consistent with worldwide trends. An as yet unpublished comparison of several recent Australian studies with the new IOTF BMI-for-age reference values suggests that approximately a quarter of Australian children are overweight or obese (Michael Booth, Coordinator, Centre for the Advancement of Adolescent Health, Royal Alexandra Hospital for Children, Westmead, personal data).

Important factors contributing to the prevalence of overweight in Australian children include ethnicity and socioeconomic status. One study showed that children from Mediterranean and Middle Eastern backgrounds had a higher relative weight than children from other backgrounds, while those of Asian ethnic origin were lighter in weight.³⁹ In addition, children from families of lower socioeconomic status were found to be more overweight. More recent studies of NSW school children support these general findings.^40,41

Louise A Baur

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