The TRENDS Project: development of a methodology to reliably monitor the obesity epidemic in childhood.

AIMS: The government has set a target to halt the rise in childhood obesity in those aged under 11 by 2010, but no system is in place to ascertain if this has been achieved. We aimed to develop a simple and reproducible methodology to monitor trends in childhood obesity.
METHODS: A purposive sample of 10 primary schools and three secondary schools was selected. Children were measured with parental "opt out" consent in reception class, year 4, and year 8 (ages 5, 9, and 13 years, respectively). Measurements were compared with those obtained locally in 1996-2001. Calculations were then performed to ascertain the sample size required to confidently identify a halt in the rise in obesity using three growth measures.
RESULTS: A total of 999 children were measured with ascertainment of 95% in primary and 85% in secondary schools. The proportion of overweight and obese children aged 9 and 13 years had increased since 1996-2001, although only 9 year olds showed a significant rise. A general trend of an increase in obesity was observed with increasing age. Calculations showed that 1900-2400 children per age group are needed to detect a halt in the rise in obesity based on mean body mass index (BMI) standard deviation scores (SDS) by 2010 with 90% power, whereas 4200-10 500 children are needed for other measures.
CONCLUSION: We have developed a simple, cost effective methodology for accurately measuring the epidemic and recommend the use of mean BMI SDS for demonstrating if a halt has been achieved.