Changes in fat mass and fat-free mass during the adiposity rebound: FLAME study.

OBJECTIVE: To determine the changes in body composition from 3 to 7 years of age in children undergoing adiposity rebound (AR) at different ages.
METHODS: Body composition was measured bi-annually by bioelectrical impedance in 229 children from a birth cohort. Age at AR was calculated from changes in weight and height velocity over time. Early AR was defined as < 6.1 years (boys) and < 5.6 years (girls). Differences in fat mass (FM) and fat-free mass (FFM) and the velocity of change in these measures were calculated between early and late rebounders. Physical activity (accelerometry), time in sedentary activity, birth factors, and parental weight were compared.
RESULTS: Children with early and late AR did not differ in body composition at 3 years of age, except for greater FFM in boys (by 0.8 kg, P = 0.022). In both sexes, change in body mass index (BMI) was significantly higher in early compared with late AR, and was entirely due to differences in the rate of weight gain, rather than any discrepancy in height velocity. This weight differential is predominantly due to increased deposition of FM in girls and FFM in boys. However, in both sexes, children with early rebound have significantly greater increases in FM velocity from 5 years of age. Few differences in any environmental influences were observed.
CONCLUSIONS: Variation in BMI associated with the timing of AR is due to differences in weight rather than height, and sex differences in the relative contribution of FM and FFM to this additional weight gain are apparent.