Tracking of BMI in Japanese children from 6 to 18 years of age: Reference values for annual BMI incremental change and proposal for size of increment indicative of risk for obesity

Background: A large incremental increase in BMI indicates excess fat deposition in most children, but the reference values for identifying those at risk for developing obesity have not been defined.Aim: To determine the mean and SD of annual incremental change (ΔSDS) in BMI for Japanese school children. Subjects and methods: A cohort of 669 Japanese children in one private school in Tokyo in whom height and weight were measured annually between 6-17 years of age. Each child's BMI was converted to SDS as based on the 1978-1981 Japanese references for the 12 annual measurements to derive the correlation coefficient, r, between two successive measurements. Using the formula, SD of ΔSDS = √2(1-r), the mean and SD of ΔSDS were obtained. Results: Excess BMI gain was defined in terms of ΔSDS in Japanese children. Annual incremental increase greater than 2 SD of ΔSDS, equivalent to 1-2 BMI units/year for younger children and 2-3 BMI units/year for older children, respectively, indicates rapid increase in body fat in Japanese children. Conclusion: Based on analysis of incremental change in BMI in this cohort, a cut-off has been identified that can be used to identify children at risk for developing obesity.