"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."
Rats are not humans. Hello, neocortex!
Since D deficiency is so detrimental to babies (rickets, anyone?) I cannot imagine that it is helpful to a fetus.
The D status of the fetus is completely reliant on the D status of the mother and there is growing evidence of the benefit of D sufficiency for a healthy pregnancy:
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