Potential selection effects when estimating associations between the infancy peak or adiposity rebound and later body mass index in children

Aim Lots of research has been conducted to identify early life factors or growth characteristics in infancy and childhood related to an unfavorable weight development. For instance, a late age at infancy peak (IP), an early age at adiposity rebound (AR) as well as body mass index (BMI) at IP and AR were shown to be positively associated with later adiposity status. The present study aims to investigate the usefulness of the IP and AR in comparison to other measures of BMI growth as indicators of later weight status. For the first time, the selection effect possibly occurring when excluding children with non-identifiable IP or AR will be explored. Methods Identification and Prevention of Dietary- and Lifestyle-Induced Health Effects in Children and Infants (IDEFICS)/I. Family is an ongoing cohort study conducted in eight European countries with 16,229 children participating in the baseline survey in 2006/2007 [1] . In a subset of 4744 children with at least four repeated measurements of height and weight in the age interval from 0 to 8 years (37,998 measurements in total; survey data supplemented with records of routine child visits), fractional polynomial mixed-effects models were used to derive individual BMI trajectories. Based on these trajectories, age and BMI at IP and AR, BMI values and growth velocities at selected ages as well as the area under the BMI curve were estimated. The BMI growth measures were standardized and related to later BMI z-scores (mean age at outcome assessment: 9.2 years). Results Age and BMI at IP and AR were not identifiable in 5.4% and 7.8% of the children, respectively. These groups of children showed a significantly higher BMI growth during infancy and childhood. In the remaining sample, BMI at IP correlated almost perfectly (r ≥ 0.99) with BMI at ages 0.5, 1 and 1.5 years, whereas BMI at AR correlated perfectly with BMI at ages 4–6 years (r ≥ 0.98). In the total study group, BMI values in infancy and childhood were positively associated with later BMI z-scores where associations increased with age. Associations between BMI velocities and later BMI z-scores were largest at ages 5 and 6 years. Results markedly differed for children with non-identifiable IP and AR, demonstrating a selection effect. Conclusions IP and AR may not be estimable in children with higher-than-average BMI growth. Excluding these children from analyses may result in a selection bias that distorts effect estimates. BMI values at ages 1 and 5 years might be more appropriate to use as predictors for later weight status instead.