The study examined the variation of autism prevalence by state in the US for those aged 6-17 years in 2010. It found that states with higher solar ultraviolet-B (UVB) doses in summer or autumn had half the rate of autism as states with the lowest.
Similar geographical variations have been noted for incidence and mortality rates for about 15 types of cancer in the US, 'vitamin D council' reported.
One of the mechanisms whereby vitamin D might reduce the risk of autism is through reducing the risk of sporadic DNA mutations from influencing fetal development.
Another is through reducing the risk of influenza and other infectious diseases during pregnancy, which have been linked to increased risk of schizophrenia. Also, vitamin D reduces inflammation by shifting cytokine production towards less inflammatory cytokines.
If vitamin D deficiency during pregnancy is a risk factor for autism, then risk could be reduced by having pregnant women take 4000 IU/d vitamin D3 and raising serum 25(OH)D concentrations to above 40 ng/ml (100 nmol/l).
This amount has been shown to be both safe and necessary to increase concentrations of 1,25-dihydroxyvitamin D (calcitriol), the active metabolite of vitamin D, to optimal levels in a randomised controlled trial by Bruce Hollis, Carol Wagner and colleagues at the Medical University of South Carolina.
Vitamin D deficiency in early life could be a risk factor for autism, although this remains to be proved.
Vitamin D also strengthens the body's innate immune system by inducing production of cathelicidin and defensins, which can combat bacterial and viral infections. It shifts cytokine production away from T-helper 1 (Th1) proinflammatory ones toward Th2 cytokines.
Recent studies also show vitamin D increases neurotrophins, upregulates glutathione, increases DNA repair enzymes, and protects against mitochondrial damage.
Once autism develops, symptoms may be reduced by treating vitamin D deficiency in autistic children, the study found.
The study was published in the peer-reviewed journal Dermato-Endocrinology.
