This isn't surprising given everything we're starting to discover about the connection between gut bacteria and mental health[1], and that sugar has been shown to promote the growth of unhealthy gut bacteria[2].

[1]http://www.theatlantic.com/health/archive/2015/06/gut-bacter...

[2]https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3448089/