Associations of birthweight and infant growth with body composition at age 15--the COMPASS study.

Size at birth and postnatal growth have been positively associated with obesity in adulthood. However, associations between postnatal growth and body composition later in life have rarely been studied. The overall purpose was to explore the associations between birthweight, weight gain during first year of life and height, weight, body mass index, fat free mass index (FFMI), fat mass index, % fat mass (FM) and waist circumference in adolescence. The COMPASS study is a population-based study of adolescents from a well-defined area in Stockholm County, Sweden. Birth characteristics and weight during childhood were collected from registers and child health centre records, and body composition at age 15 years was measured by bioelectric impedance by trained nurses. Complete data were available for 2453 adolescents. Associations between predictor and outcome variables were assessed with linear regression modelling. Birthweight was positively associated with all outcome variables, except for %FM among girls. FFMI increased by 0.49 kg/m(2)[95% CI 0.34, 0.63] (boys) and 0.25 kg/m(2)[0.12, 0.38] (girls) per 1 SD increase in birthweight. Increased weight gain in infancy showed strong, positive associations with all measures of body composition. FFMI increased by 0.73 kg/m(2)[0.60, 0.87] (boys) and 0.63 kg/m(2)[0.50, 0.76] (girls) per unit increase in weight z-score during first year of life. The effect of increased weight gain in infancy was not modified by birthweight. Birthweight and postnatal growth were both positively related to body composition in adolescence. Increased weight gain during the first year of life had stronger effect than prenatal growth, suggesting infancy to be a more critical period.