To study the change in body mass index (BMI) from childhood and adolescence and development of obesity into adulthood.

We performed a longitudinal study of 480 individuals (49% male; 67% white) with height and weight measures in childhood (mean age 7 years), repeated in adolescence (mean age 16 years) and adulthood (mean age 39 years). Weight status in childhood was defined as low normal weight (0-<50 BMI percentile); high normal weight (50-<85 BMI percentile); overweight (85-<95 BMI percentile); obese (≥95 BMI percentile). Adult weight status was defined as normal weight (18.5-<25 kg/m²); overweight (25-<30 kg/m²); obese (>30 kg/m²).

Adult obesity (%) increased with weight status in childhood (low normal weight 17%; high normal weight 40%; overweight 59%; obesity 85%) and similarly with adolescence. Children in a lower category in adolescence than in childhood had lower risk of having adult obesity than did those who maintained their childhood category. Among adults with obesity, 59% (111 out of 187) were normal weight as children, with 75% (83 out of 111) from the high normal weight children; and 50% of adults with obesity were normal weight (n = 94/187) as adolescents, with 84% (81 out of 94) from the high normal weight adolescents. Only 6% of 143 normal weight adults had either overweight (n = 9) or obesity (n = 0) during childhood.

This study shows the high risk for adult obesity in children and adolescents who have overweight or obesity. A majority of adults with obesity had a 50-85 BMI percentile as children. Those who did not move to higher weight status between childhood and adolescence had lower probability of adult obesity.