This study found that adults with vitamin D deficiency have impaired spatial memory and reduced brain structures called dendritic spines in the hippocampus, the brain region responsible for learning and memory. The researchers identified that low vitamin D decreases nitric oxide production in the brain, which is important for forming and maintaining the synaptic connections needed for memory formation. Importantly, when vitamin D was supplemented back to deficient mice, the brain’s ability to produce nitric oxide was restored, suggesting vitamin D supplementation could potentially improve cognitive function in vitamin D-deficient individuals.