Retrospective individual tracking of body mass index in obese and thin adolescents back to childhood.

The objective of this study is to track body mass index (BMI) in obese or thin adolescents from adolescence to childhood on an individual basis. This was performed at a single school with a 12-year combination education system in an urban city in Japan. A total of 617 students in the 3rd grade of senior high school (17 years old) during 2005-2009 were enrolled. Based on the Japanese BMI reference in childhood adjusted for age and gender, obesity and thinness were defined as >=90th percentile and <=5th percentile, respectively. Sixty-three (10.2%) and 84 (13.6%) students were found to be obese and thin, respectively. Complete annual tracking of BMI back to the 1st grade of elementary school (6 years old) (1994-1998) was possible in 47 obese (74.6%) and 67 thin students (80.0%). The most common ages when obesity was first detected were 6-8 years for males, and 12-14 years for females, and the most common ages when thinness was first detected were 12-14 years for males, and 15-17 years for females. Once obesity or thinness started, these conditions remained until 17 years old in most students. Obese students whose obesity started earlier tended to have higher BMIs at 17 years old in both genders. This will be the first tracking study of BMI in obese and thin adolescents on an individual basis. A longitudinal study of BMI during childhood is useful for establishing intervention programs to prevent obesity or thinness in adolescence.