How much do children's body mass indices change over intervals of 6-12 months? Statistics from before and during the obesity epidemic.

Background Researchers need to evaluate changes in children's body mass index (BMI) over periods of 6 or 12 months, yet reference statistics are limited for change in BMI. Objectives We aim to estimate the distribution of changes in children's BMIs over periods of 6 and 12 months. Methods We analyze data on two cohorts of children in the Fels Longitudinal Study: an older cohort born 1946–1970 and a recent cohort born 1971–1995. Between ages 3 and 18 years, we calculate changes in BMI over intervals of 6 and 12 months. For each age, sex and cohort, we estimate the mean, standard deviation, skewness, kurtosis and percentiles of change in BMI. Results Median BMI growth peaks around age 12–13 years for girls and 13–15 years for boys. Large BMI gains are common in adolescence, and BMI losses are not uncommon at any age. Percentiles of BMI change are quite dispersed, especially for girls and especially in adolescence. In the recent cohort, the adiposity rebound is earlier and BMI gains are larger, especially at the high percentiles. Conclusions Researchers can use these estimates to evaluate data quality, evaluate effect sizes and calculate the sample size needed to detect an effect.