At http://www.vitamindcouncil.org/ there is an extensive section explaining the theory that low levels or inadequate metabolism of vitamin D may cause or make a child susceptible to Autism. It is the 5th topic down on the left side of the screen. Once you click on "autism" you will see the summary or contents of the entire section. You can click on any heading and read that portion.
If you don't have time to read it all here is a very brief summary of what's going on:
Vitamin D is produced by the body in response to sunlight exposure. The article on the Vitamin D Council website proposes that the rapid rise in autism coincides with an increased tendency toward sun avoidance in our culture.
Vitamin D is important to the developing brain early in life and has several functions.
The different effects that estrogen and testosterone have on vitamin D metabolism could explain why more boys have autism than girls.
Vitamin D has been shown to lessen oxidative brain injury due to heavy metals exposure (in rats). This could show a link between lack of adequate vitamin D, mercury in vaccines, and autism.
The currently recommended amount of vitamin D in most supplements is not adequate to replace the vitamin D lost due to lessened sun exposure.
After reading the information about the theory linking less Vitamin D to Autism, I went to the healthfood and herb shop in town and bought 1000 IU gel caps that I now give my children daily - and take myself.
This isn't saying Vitamin D is a cure for autism; but its lack could be a triggering factor for autism.