45: BMI Changes During Teen Years in Girls are Influenced by BMI at Age 9

Abstract

Previous studies report that increases in BMI (body mass index) among children and teens are due to increases in fat-free mass. We examined longitudinal changes in BMI and components of BMI, fat mass index (FMI) and fat-free mass index (FFMI). Girls were recruited at age 9 from the NHLBI Growth and Health Study (NGHS), and followed at annual examinations (n = 1156) over ten years. Girls were grouped into low (<20th %tile), middle, and high (> 80th %tile) BMI on the basis of race-specific BMI %tile rankings at age 9. BMI was partitioned into fat and fat-free mass (FMI and FFMI) (wt/ht2 = fat mass/ht2 + fat free mass/ht2). Race-specific timing of maturation was based on early (<20th %tile), typical, and late (> 80th %tile) age of menarche. Fat mass index was based on percent body fat calculated from sum of skin folds. Independent of initial BMI, girls accrued fat at a greater rate than lean body mass, and the ratio of fat-free to fat mass decreased steadily from ages 9 through 18. Girls with greater BMI at age 9 had greater FMI and FFMI than girls with lower BMI at age 9, and had greater BMI, FMI, and FFMI at age 18 than girls with middle or lower levels of BMI at age 9. The ratio of fat free to fat mass at 18 was greater in later maturing girls and girls at lower BMI at 9. There was a significant decline in this ratio after age at peak height velocity in all girls, but with a significant interaction with timing of puberty. There were strong correlations between BMI at age 9 with several outcomes at age 18: BMI (0.78) and FMI (0.73), weaker but significant with FFMI (0.39), and inverse with ratio of fat-free to fat mass (-0.62). Changes in BMI during the teen years result from increases in fat and fat-free mass, are dependent upon childhood BMI levels, and lead to different outcomes in body composition at age 18. Higher BMI levels during childhood lead to greater than expected changes in BMI, and those changes in BMI are driven by disproportionate increases in fat mass, especially after the pubertal growth spurt. These finding suggest that interventions to decrease overall prevalence and level of obesity within an individual should focus on preteens and at the time of the pubertal growth spurt. None.