According to NCBI, “In conclusion, vitamin D deficiency is common in patients after TBI and is associated with worse cognitive function and more severe depressive symptoms. ” Vitamin D helps stimulate the production of neurotransmitters and improves synaptic density . Additionally, insufficient vitamin D intake has been linked to poor immune response, muscle weakness, and fatigue. It is believed that vitamin D may protect the brain against dementia and cognitive decline. Vitamin D is the vitamin produced from exposure to the sun, and it is important to maintain normal blood levels of calcium and phosphorus. With that said, if you were in a hospital for a long time, or do no get adequate sunlight, Vitamin D might benefit you. This is where a supplement may be required, but should also be approved by a treating physician first.