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Randomized controlled trial finds that high dose vitamin D supplementation (4,000 IU/day) for 18 weeks improves visual memory in healthy adults but low-dose vitamin D (400 IU/day) does not.
Serum 25 hydroxy vitamin D levels increased significantly more in the high dose group (from 27 ng/ml to 52 ng/ml) than the low dose group (24 ng/ml to 34 ng/ml). What is interesting is that even though 34 ng/ml is considered sufficient, it still was not enough to improve visual memory.
It has been suggested that visual memory tasks place greater demands on cognitive processing, organization, planning, and overall executive functioning than do verbal memory tasks, and require more fluid cognitive processes.