A diet popular as an alternative treatment for autism doesn’t appear to improve behaviors or symptoms of the condition, according to a small but rigorously conducted study published this month in the Journal of Autism and Developmental Disorders.

The new work, conducted by researchers at the University of Rochester Medical Center, examines the gluten-free, casein-free diet and offers what many experts in the field say perhaps the highest-quality evidence to date that eliminating proteins found in wheat and dairy doesn’t improve...