Weight gain in early years and subsequent body mass index trajectories across birth weight groups: a prospective longitudinal study

Background: Rapid weight gain (RWG) in early-life is associated with increased risk of childhood obesity and is common among low-birth weight infants. Few studies have compared body mass index (BMI) trajectories of children experienced RWG to those who did not, across birth weight groups. We investigated the association between RWG in early-life and subsequent BMI trajectory and whether the association differs by birth weight.Methods: We included term singletons from the UK Millennium Cohort Study (n = 10 637). RWG was defined as an increase in weight z-scores (derived using UK-WHO growth reference) between birth and 3 years >0.67. Mixed-effect fractional polynomial models were applied to examine the association between RWG and BMI trajectories (5-14 years). Models were further adjusted for confounders and stratified by birth weight-for-gestational-age group. Results: Mean BMI trajectories were higher in children who experienced RWG in early-life, compared with their non-RWG counterparts. RWG was associated with higher BMI at five years [by 0.76 kg/m(2) (95% CI: 0.67-0.85) in boys and 0.87 kg/m(2) (0.76-0.97) in girls]; the difference persisted into adolescence [1.37 kg/m(2) (1.17-1.58) and 1.75 kg/m(2) (1.52-1.99) at 14 years, respectively]. Differences remained after adjustment and were particularly greater for children born large-for-gestational-age than those born small- and appropriate-for-gestational-age. Mean BMI trajectories for large-for-gestational-age children with RWG exceeded international reference curves for overweight (for obesity at some ages in girls). Conclusions: RWG was associated with higher BMI trajectories throughout childhood and adolescence, especially in large-for-gestational-age children. Strategies for obesity prevention need to address factors during and before infancy and preventing excessive weight gain among infants who have already had adequate growth in utero.