A reply to Wang T, Shan L, Du L, Feng J, Xu Z, Staal WG, Jia F. Serum concentration of 25-hydroxyvitamin D in autism spectrum disorder: a systematic review and meta-analysis. Eur Child Adolesc Psychiatry. 2015; doi: 10.1007/s00787-015-0786-1.

The author of this correspondence has recently published findings of an ecological investigation wherein herbicide use was proposed as an instrumental variable that predicts healthcare utilization among subjects with severe ADHD impairment via individual exposure to nitrous oxide (N2O) emissions. Vitamin D deficiency, a possible risk factor for autism spectrum disorders (ASD) as discussed by Wang et al., may be a homeostatic response to increasing chronic environmental N2O exposures. The author explains how deficient and insufficient states of vitamin D may promote vagal withdrawal and tolerance to increasing opioid exposures in the environment and how these characteristics are particularly relevant in neurodevelopmental disorders, like ASD and ADHD.