Vitamin D, which can come from food or our skin when stimulated by sunlight, is essential for our health and brain development. People with lower vitamin D levels appear to have a higher risk of age-related diseases, including cognitive decline and dementia. While a few small studies suggest that vitamin D supplementation may improve some aspects of cognitive functions, no studies have confirmed that it can protect against dementia. Vitamin D is usually safe when used as directed.
Multiple meta-analyses and systematic reviews of observational research have examined the relationship between vitamin D levels and cognitive function. A few studies have tested the effects of vitamin D treatment on cognition, but randomized controlled trials on dementia risk or cognitive decline are lacking. Our search identified:
• 7 meta-analyses of observational studies examining the link between blood levels of vitamin D and cognitive function
• 1 randomized controlled trial testing the effects of vitamin D and calcium in healthy older people
• 2 small randomized controlled trials on the effects of vitamin D in combination with other drugs in people with dementia
• 1 non-randomized clinical trial testing the effects of vitamin D on cognitive function in healthy elderly people
• 6 observational studies examining the relationship between vitamin D levels and cognitive function
• Numerous preclinical studies
People with low levels or low dietary intake of vitamin D appear to be more likely to develop mild cognitive impairment (MCI) or dementia [3-7], but no clinical research has yet tested whether treatment with vitamin D can protect from this risk. In a small non-randomized clinical trial, elderly people receiving vitamin D3 supplements had better cognitive function compared to untreated people, with particular improvement in executive function , but the study was not controlled or designed to look at the risk of cognitive decline.
Clinical trials are underway to examine the effects of vitamin D on cognitive function in older adults who have low vitamin D levels , memory complaints , mild cognitive impairment , and type 2 diabetes , as well as those in good health . These studies are scheduled to be completed in late 2016 to mid-2019. Another trial is testing whether vitamin D can reduce the risk of cancer, heart disease, and stroke in 20,000 men and women , with a subgroup undergoing testing for cognitive decline and dementia (scheduled to complete by late 2017) .
Research on the benefits of vitamin D for dementia patients is very limited and has produced mixed results. In a small, six-month pilot study, Alzheimer’s patients who were treated with memantine plus vitamin D improved their cognitive scores, whereas those taking memantine alone or vitamin D alone remained the same . A larger trial testing the effects of vitamin D in combination with memantine was scheduled to be completed in 2013 , but the results have not been published. A small randomized trial of Parkinson's disease patients suggested that vitamin D supplementation stabilized the disease, possibly by improving strength and balance .
Vitamin D supplementation is usually safe when used as directed but it can be harmful at artificially high levels . While it is impossible for the human skin to produce too much vitamin D, excessive unprotected sunlight exposure can increase risks of skin cancer. Recommended daily dietary intake of vitamin D is 600 IU for adults and 800 IU for people over age 70 . Supplementation up to 4000 IU is likely to be safe . Taking too much vitamin D can elevate blood calcium levels and cause toxicity with symptoms such as nausea, vomiting, constipation, confusion, and heart rhythm abnormalities. Excess levels can also increase the risk of bone fractures, urinary tract infections, weight loss, digestive problems, and certain types of cancer . People with overactive parathyroids, kidney and liver disease, or certain immune disorders should take extra caution, as should people with diabetes and hypotensive disorders. Some common drugs can change the way that your body manages vitamin D .
NOTE: This is not a comprehensive safety evaluation or complete list of potentially harmful drug interactions. It is important to discuss safety issues with your physician before taking any new supplement or medication.
There are two forms of vitamin D: D2 and D3. While vitamin D2 can be obtained from some plants and mushrooms, a significant percentage of vitamin D is produced in our skin as vitamin D3 when stimulated by the sun . However, the skin’s ability to produce vitamin D3 declines with age and vitamin D deficiency is more common in older adults than previously thought . Dietary sources of vitamin D3 include salmon, sardines, and fortified milk. Vitamin D is also a part of many multivitamins and is available separately as a dietary supplement in both liquid and pill forms. Most vitamin D supplements contain 400 IU (10 µg), either as D2 or D3. Although both D2 and D3 from supplements or diet can treat rickets (a disease caused by vitamin D deficiency), some evidence suggests that supplements with D3 are more potent than D2  and have better evidence for long-term health benefits .
More information on doses, side effects, and drug interactions with vitamin D supplementation can be found at the NIH Office of Dietary Supplements.
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