PETA provides us with a nice example of how having an ideological agenda can motivate an individual or a group to embrace dubious science. In an article currently on their website, and making the rounds in social media (this is repeating a claim from at least 2008, but the current article is undated), the group warns: Got Autism? Learn About the Link Between Dairy Products and the Disease.There are some anecdotes about the gluten-free, casein-free diet.
Such uncontrolled observations need to be confirmed by blinded observations. These studies have been done for both gluten-free and casein-free diets. A 2008 Cochrane review of these studies concluded:Current evidence for efficacy of these diets is poor. Large scale, good quality randomised controlled trials are needed.A more recent review from April 2014 came to a similar conclusion:We observed that the evidence on this topic is currently limited and weak.The pattern of evidence reveals that the methodologically poor studies, ones that are liable to confirmation bias, show some effect, but the properly blinded studies tend to show no effect. For example, a 2010 study (although small) observed children with autism on a gluten-free and casein-free diet, and then challenged them with either gluten, casein, or placebo in a blinded manner. There was no difference in behavior observed. A recent 2014 study also showed no association between dairy and behavior in autism.