There are a lot of bogus claims about autism out there on the Internet. Countless quacks and angry Internet commenters will tell you that vaccines cause autism, despite a lack of scientific evidence to back them up.
Unhelpfully, PETA have also made claims about autism. In a campaign that has recently resurfaced to widespread criticism, PETA linked drinking milk to autism. The campaign has received criticism from a prominent vegan chef, as well as other commentators, both for the unscientific nature of their claims and the negative way it portrays people who have autism.
In a post that's still on their site, despite numerous people debunking it, PETA say that drinking milk "worsens" autism.
"It isn’t surprising that dairy products may worsen this condition," they write on their site. "Considering that milk has already been strongly linked to cancer, Crohn’s disease, and other serious health problems. Anyone who wants to alleviate the effects of autism should try giving cow’s milk the boot and switch to healthy vegan alternatives instead."
PETA cite a study as well as anecdotal evidence in their post. "More research is needed, but scientific studies have shown that many autistic kids improve dramatically when put on a diet free of dairy 'products'. One study of 20 children found a major reduction in autistic behavior in kids who were put on a casein-free diet (casein is a component of cow’s milk)."
They then say that the reason why dairy "worsens autism" is up for debate, but that "some suggest that the gastrointestinal problems so often caused by dairy products cause distress and thus worsen behavior in children with autism."
The problem is that this isn't the case. PETA cites a study that has just 20 participants, which isn't enough to be recommending dietary changes for anyone. More than that, it's a study that has been discredited by two independent overviews.