Aim: To construct reference curves for body mass index (BMI) and estimate the prevalence of obesity or being overweight at ages eighteen and 21 years.
Methods: Smoothed BMI curves were derived from data from a birth cohort born in Dunedin in 1972-73, followed up at two yearly intervals from age three to fifteen years, and then at ages eighteen, 21 and 26 years.
Results: Nine curves 0.67 SD apart, corresponding to the 0.4, 2, 9, 25, 50, 75, 91, 98, 99.6 centiles are provided. The prevalence of obesity, a BMI of 30 kg/m2 or more, was 2.7% for men and 2.6% for women at age eighteen years, and 5.8% for men and 5% for women at age 21 years. At age eighteen years, 14.9% men and 16.8% of women were overweight, with a BMI between 25 kg/m2 and 30 kg/m2. The prevalence increased to 24.7% for men and 24.2% for women at age 21 years.
Conclusion: At age eighteen years, the value of the 98th centile was close to WHO criteria defining obesity in adults. This suggests that it could be used to describe obese children and adolescents. The 75th centile could be used in a similar way to delineate those regarded as overweight.