Controversial. The effects of sugar and other foods or additives in chidlren with autism or adhd are highly controversial. Best reserach suggests that, especially with chidlren under age 5 or 6 years, if may have an effect. One problem is that sugar rarely is the only variable--context (party, after school, etc.) may have a big impact. Check for yourself; if it helps, restricting it may be worth the effort.
Not alone. For sugar to alter behavior with or without autism other factors shoild be present.
Many studies have. Been done since 1973 when Dr. Feingold suggested a link between food asditives & behavior in children with ADHD. None has shown a causal link between sugar intake & behavior in any subset of children. Still, some parents report a temporal link between refined sugar intake & increased hyperactivity in their children with ADHD, with or without Autism, & they assume a causal link, though unproven.