The prevalence of autism spectrum disorders (ASDs) is causally dependent on genetic and environmental influences. We investigated whether autism spectrum disorders are associated with month of birth compared to the general population using a retrospective study, comparing ASD cases (n = 3478) with the general population (n = 2,716,876) born between 1995 and 2008. Associations were examined using χ2 tests and Walter and Elwood’s seasonality χ2 tests for the total ASD group, and separately for autistic disorder and Asperger syndrome. For the total ASD group, the distribution of month of birth was different compared to the general population (p < 0.0001), with July as the highest contributor, and a season-of-birth effect was found for this group (p = 0.02). For the autistic disorder group, the months of birth distribution were different (p = 0.01), with July as the highest contributor. No season-of-birth effect over the year was found (p = 0.09). No association was found for the months of birth of individuals with Asperger syndrome (p = 0.06), with no seasonal trend over the year (p = 0.60). In conclusion, a drop in sun exposure during the first trimester of pregnancy might explain the peak in July births and the associated risk for ASD development.
Part of the book: Autism
1,25(OH)2D is the hormonally active form of vitamin D known for its pleiotropic immunomodulatory effects. Via altering gene transcription, 1,25(OH)D exerts immunosuppressive effects and stimulates immune regulation. Recently, the interest in vitamin D in association with autism spectrum disorder (ASD) has been triggered. The prevalence of ASD has increased excessively over the last few decades, emphasizing the need for a better understanding of the etiology of the disorder as well as to find better treatments. Vitamin D levels in ASD patients are observed to be lower compared to healthy individuals and maternal vitamin D deficiency has been associated with an increased risk of ASD. Moreover, vitamin D supplementation improves ASD symptoms. These recent clinical findings strongly suggest that vitamin D is a factor in ASD onset and progression. Yet, possible mechanisms behind this association remain unknown. This review summarizes immunomodulatory properties of vitamin D and peripheral immune dysregulation in ASD, after which possible mechanisms via which vitamin D could rebalance the immune system in ASD are discussed. Although promising clinical results have been found, further research is necessary to draw conclusions about the effect and mechanisms behind the effect of vitamin D on ASD development.
Part of the book: Vitamin D