"Vitamin-D deficiency does not seem to affect adult cognition, at least not according to a recent review article (Annweiler et al., 2009). But there may be early developmental effects. When rat fetuses are deprived of vitamin D, the newborn pups have lar- ger brain volumes and show more cell proliferation throughout their brains. This is consistent with the antiproliferative effect of this vitamin on body tissues. Prenatal vitamin-D deficiency seems to increase the rate of neuronal proliferation while de- creasing the rate of neuronal cell death (Eyles et al., 2003). If prenatal vitamin-D deficiency affects humans similarly, the result may be improved cognitive performance, albeit at a high cost for homozygous individuals. As with Tay-Sachs, the more numerous heterozygous individuals should enjoy a lower cost/ benefit ratio."

source: http://www.scirp.org/journal/PaperInformation.aspx?paperID=21701