A quote to begin: "Levels of serum 25(OH) D in participants with ASD [autism spectrum disorder] were significantly lower than controls, suggesting that lower vitamin D level might be a risk factor for ASD."
That was the bottom line finding reported by Tiantian Wang and colleagues  following their systematic review and meta-analysis of the existing peer-reviewed science literature looking at whether serum concentrations of 25-hydroxy vitamin D - the typically measured compound reflective of vitamin D status - might be something to look at when it comes to autism.
Of course these results are nothing new to this blog and the various occasions that I've covered research suggesting that lower vitamin D levels may be 'associated' with autism (see here and see here for example). That body of work also includes the idea that some of the genetics of the vitamin D receptor might also be implicated in some autism (see here).
There is still quite a bit more to do in this area notwithstanding the idea that screening for vitamin D issues in cases of autism should be rather more prevalent than it already is. The use of vitamin D supplements in cases of insufficiency / deficiency with autism is mind is a bit of a growth area as per other research from the Wang research group (see here). The idea that vitamin D issues in autism might also overlap with various other data from other conditions / labels is also of interest (see here) given that quite a few of those labels seem to be over-represented when it comes to a diagnosis of autism or ASD (see here and see here for example). With such multiple associations in mind, one wonders whether quite a bit more 'integrated' research taking a wider view of autism and labels such as depression might be in order...
Music: The Chemical Brothers - Block Rockin' Beats.
 Wang T. et al. Serum concentration of 25-hydroxyvitamin D in autism spectrum disorder: a systematic review and meta-analysis. Eur Child Adolesc Psychiatry. 2015 Oct 29.
Wang T, Shan L, Du L, Feng J, Xu Z, Staal WG, & Jia F (2015). Serum concentration of 25-hydroxyvitamin D in autism spectrum disorder: a systematic review and meta-analysis. European child & adolescent psychiatry PMID: 26514973