A mouse study helps explain why vitamin D is so important for cognition. After 20 weeks of no vitamin D, the healthy adult mice showed a significant decline in their ability to remember and learn. They also showed a pronounced reduction in perineuronal nets in the hippocampus. These nets provide a supportive scaffold around certain neurons, stabilising their connections with other neurons. There was also a substantial reduction in both the number and strength of connections between neurons.
It’s suggested that vitamin D helps keep perineuronal nets stable, and that when vitamin D levels drop, they’re more easily degraded by enzymes. The hippocampus may be most vulnerable, and thus affected first. It also seems that the right hippocampus is more affected than the left.