Change in body mass index in Australian primary school children, 1985–1997

Abstract

BACKGROUND: Childhood obesity is an important, potentially modifiable risk factor for a range of concurrent and later morbidities. Despite concerns about recent increases in children's body mass index (BMI), supporting data in Australia (as elsewhere) are scant. OBJECTIVE: To seek anthropometric evidence of a recent secular increase in BMI in primary school children in Victoria, Australia. DESIGN: Data from two cross-sectional population-based surveys of primary school children (the Victorian subsample of the 1985 Australian Health and Fitness Survey and the 1997 Health of Young Victorians Study) were compared. Similar stratified random sampling and standardized measurement methods were employed in the two studies. Subjects were all children aged 7–12 y with complete height and weight data. Body mass index (BMI (weight/height²)) was used as the index of relative adiposity. Non-parametric and parametric methods were used to examine the pattern and magnitude of change in BMI over the 12 y interval. RESULTS: Data for 1421 children (50% male, 68% response) from the 1985 survey and 2277 children (51% male, 75% response) from the 1997 survey were analysed. At all ages, mean height and median weight were greater in 1997 than 1985 for both boys and girls. Median BMI was significantly higher in the 1997 sample for all but 12 y-old girls and for boys aged 7, 8 and 10 y (Mann–Whitney U test). The magnitude of the overall increase in BMI was estimated using analysis of covariance for log-transformed BMI adjusted for exact age, which indicated an increase of 1.03 kg/m² for boys and 1.04 kg/m² for girls (both P<0.001). Plots of BMI against BMI percentile clearly showed a pattern of higher BMI at any given percentile, especially at the upper percentiles, for all ages and both genders. CONCLUSIONS: Primary school children in Victoria have become more obese over the last decade. Increases in BMI are most marked at the heavier end of the distribution. Lesser increases in median and mean BMI (confirmed by both parametric and non-parametric statistical models) may also have major public health implications.