After my own very positive experiences with vitamin D therapy for seasonal affective disorder (S.A.D) which I wrote about a couple of months ago, my interest was peaked this week when I came across a story linking autism to deficiency of the "sunshine vitamin".

Researchers from Cornell University carried out a "more refined" analysis of data collected during a 2006 study and confirmed that autism rates are highest in the rainiest counties of the three Pacific coast states of the US: Washington, Oregon and California.  The study is published in the Archives of Pediatric and Adolescent Medicine.

The original 2006 study, conducted by the National Bureau of Economic Research was highly controversial, as the authors were not medical researchers and blamed higher amounts of TV viewing by children in the rainiest areas for the higher incidence of autism.

I reported on this research when it was originally published: Autism Associated with Higher Levels of TV Viewing

Other studies have also linked autism to rainier weather. In 2003, a survey by the U.S. Department of Education found the highest autism rates in northern states such as Indiana, Maine, Massachusetts, Minnesota and Oregon, and the lowest rates in dry, sunny states such as Colorado, Mississippi, New Mexico, Oklahoma and Tennessee.

Researchers involved in the latest Cornell study and others who have noted the association between autism rates and precipitation have concluded that environmental factors, most likely vitamin D deficiency, are to blame rather than TV viewing habits. This would seem like a much more logical conclusion given that sun exposure is required for vitamin D production in the skin and the fact that immune dysfunction is increasingly implicated in autism and vitamin D's role in immune system regulation is also becoming ever more apparent.

Over the past few years researchers from the University of California Davis's MIND institute have found various pieces of evidence pointing to overactivity of the immune system in autistic children; these include 40% more natural killer cells, 20% more B lymphocytes (antibody producing cells), and strong expression of genes associated with natural killer cell activity.

Meanwhile vitamin D is now known to play a pivotal role in the regulation of the immune system, particularly the innate immune system (of which natural killer cells are a part).

Lead researcher on the Cornell study, Michael Waldman, said: "If you look at the autism literature now, they're much more open to an environmental trigger."

Lack of sunlight exposure and resulting vitamin D deficiency looks like a prime candidate. Waldman also suggests other environmental factors linked to high levels of precipitation that might play a part including greater exposure to household chemicals, or to pollutants carried by the rain/snow itself.