LAUSR

Background: Children with autism spectrum disorder (ASD) are more likely to have obesity compared to children without ASD, but studies may report biased estimates because of limitations accounting for potentially important factors that may differ between these two groups of children. This study of siblings in "simplex" families (i.e., families that include only one offspring with ASD) avoids these potential pitfalls. Methods: The Simons Simplex Collection was used to create sibling dyads comprising a child with a clinically confirmed diagnosis of ASD and a full sibling without ASD within the same biological family. Child obesity, determined from measured heights and weights, was compared across three child age strata (4-7.9, 8-10.9, 11-18.0 years) and by parents' weight status using generalized estimating equations log-binomial regression models. Results: Among 1378 sibling dyads, 4-18 years of age, the prevalence of obesity significantly increased with age, with larger sibling differences at older ages. For ages 4-7.9 years, the obesity prevalence for children with ASD was 15.0% compared to 16.2% for siblings (p = 0.57). For ages 11-18.0 years, prevalence for children with ASD was 30.7% compared to 21.4% for siblings (p = 0.003). Parental obesity significantly affected sibling obesity. Conclusion: From this unique data resource that accounted for shared family environments, the prevalence of obesity diverged significantly at older ages between children with ASD and their full siblings without ASD and was associated with parental obesity status similarly for children with and without a diagnosis of ASD. Recognizing these age-related differences has important implications for targeting preventive interventions.