Infant feeding is associated with disease comorbidities including autism in the neurodevelopmental disorder fragile X syndrome Cara Westmark, Department of Neurology, University of Wisconsin, Madison, WI Breastfeeding is associated with numerous health benefits in infants, but early life nutrition has not been specifically studied in the neurodevelopmental disorder fragile X syndrome (FXS). Herein, we evaluate associations between the consumption of breast milk versus formula during infancy and the prevalence of autism, allergies, diabetes, gastrointestinal (GI) problems and seizures in participants with FXS. The study design was a retrospective survey of families enrolled in the Fragile X Online Registry and Accessible Research Database (FORWARD). There was a 73% rate of breastfeeding in the enrolled participants with an 81% success rate for at least 3 months, 73% for 6 months and 41% for 12 months of age or longer. There was a 1.7-fold reduction in the prevalence of autism in participants with FXS who were fed breast milk for 12 months or longer and a 1.9-fold decrease in autism in participants reporting no use of any infant formula. The most common reasons for cessation of breastfeeding were reduced milk production and age of the child. There were strong negative correlations between increased time the infant with FXS was fed breast milk and the prevalence of autism and seizures and moderate negative correlations with the prevalence of GI problems and allergies. However, participants reporting GI problems or allergies commenced these comorbidities significantly earlier than participants that were not fed breast milk. Parsing the data by sex indicated that males exclusively fed breast milk exhibited decreased prevalence of GI problems and allergies. These data suggest that long-term and exclusive use of breast milk are associated with reduced prevalence of autism, GI problems and allergies in FXS, although breast milk is associated with the earlier development of GI problems and allergies.