Vitamin D levels are linked to having sufficient access to sunlight and promote bone health. Over the last decade or so, vitamin D has increasingly been studied as a potential neuroprotective agent and people are interested in whether it may prevent or slow Alzheimer’s disease and other forms of dementia. However, conflicting results from clinical research into its potential benefits makes it difficult to answer whether patients should be taking a supplement or not.
According to the National Institutes of Health, vitamin D, which can be obtained both from an individual’s diet and from sun exposure, plays a prominent role in promoting intestinal calcium absorption, maintaining bone health and preventing osteoporosis. The vitamin is naturally found in only a few foods, such as the fatty components of salmon and tuna, fish liver oils, cheese, and egg yolks. However, many products, particularly milk, are fortified with vitamin D to ensure sufficient consumption of the vitamin. Additionally, exposure to ultraviolet (UV B) radiation from sunlight triggers vitamin D synthesis in the skin, though the amount of synthesis is dependent on factors such as time of day, cloud cover, shade, and age. As people age, their skin has a reduced ability to efficiently synthesize vitamin D. This, combined with the fact that older individuals generally spend less time outdoors and are less likely to consume an adequate amount of the vitamin in their diets, contributes to the tendency of older adults to develop vitamin D deficiencies (Office of Dietary Supplements, 2019). Beyond the issues with bone health associated with a deficiency in this vitamin, however, growing evidence suggests that inadequate levels of vitamin D in elderly individuals may also contribute to the development of dementia and mild cognitive impairment.
Many recent studies have examined the potential association between vitamin D levels and the development of Alzheimer’s disease. For instance, a 2014 study published in the journal Neurology tested 25-hydroxy-vitamin D concentrations in blood samples from healthy older adults. At a follow-up assessment approximately five years later, the researchers reassessed the participants’ vitamin D levels and evaluated whether they had developed dementia. They found a correlation between vitamin D deficiency and a higher risk of developing dementia (Littlejohns et al., 2014). A similar study by Oudshoorn et al. examined the relationship between vitamin D levels of elderly patients suspected to have Alzheimer’s and their performance on the Mini-Mental State Exam (MMSE), a test of cognitive function. They found that patients with sufficient levels of vitamin D scored significantly higher on the MMSE than patients with vitamin D deficiencies, implying a correlation between vitamin D levels and cognitive performance (Oudshoorn et al., 2008). These correlative studies do not prove that vitamin D deficiency caused dementia, nor do they explain the mechanism by which vitamin D might help prevent Alzheimer’s or other forms of dementia. A 2017 study found that that vitamin D decreased the production of toxic β-amyloid and increased its degradation in cultured neurons. They also found that vitamin D (and similar compounds) reduced the activity of β- and ?-secretase, two enzymes critical for the production of β-amyloid. Additionally, vitamin D was found to reduce the production of some inflammatory cytokines in this model, so it may work by suppressing some of the neuroinflammation which has been associated with Alzheimer’s disease (Grimm et al., 2017).
While there is fairly convincing evidence that vitamin D deficiency is not good for your brain, there is no convincing evidence that simply taking a supplementary pill will improve brain function. A 2018 meta-analysis comparing 73 studies that investigated the link between vitamin D and neurodegenerative disease, for instance, found no significant evidence of a neuroprotective benefit of vitamin D (Iacopetta et al., 2018). Again, while much of the available research does find an association between this vitamin and cognitive decline, this is correlational and does not clearly identify whether lower levels of vitamin D cause dementia or whether they are simply a symptom of it.
Nevertheless, avoiding vitamin D deficiency is reasonable as there are various health benefits beyond the prevention of Alzheimer’s disease and other forms of dementia and there is some potential for benefit related to cognition. We strongly encourage our patients to spend an adequate amount of time in outdoor activities. The added physical activity, social interaction and daylight exposure is unequivocally beneficial for the clear majority of patients. It is very easy as Alzheimer’s disease and dementia symptoms progress for people to become housebound, so for many patients and families, getting out of doors needs to be made a specific priority. For older adults, particularly those who do not spend ample time outdoors or consume a sufficient amount of vitamin D in their diets, vitamin D supplements may be beneficial. According to the NIH, the Recommended Daily Allowances for vitamin D are 600 International Units (IU) for most individuals under 70 years and 800 IU for adults over age 70. As a reference, one cup of fortified milk contains approximately 100 IU (Office of Dietary Supplements, 2019). While the data about the impact of this vitamin on Alzheimer’s disease remains incomplete, maintaining sufficient levels of vitamin D will, at minimum, support healthy bone development and promote brain health.
References
- Grimm, M., Thiel, A., Lauer, A. A., Winkler, J., Lehmann, J., Regner, L., … Hartmann, T. (2017). Vitamin D and Its Analogues Decrease Amyloid-β (Aβ) Formation and Increase Aβ-Degradation. International journal of molecular sciences , 18 (12), 2764. doi:10.3390/ijms18122764
- Littlejohns, T. J., Henley, W. E., Lang, I. A., Annweiler, C., Beauchet, O., Chaves, P. H., … Llewellyn, D. J. (2014). Vitamin D and the risk of dementia and Alzheimer disease. Neurology , 83 (10), 920–928. doi:10.1212/WNL.0000000000000755
- Iacopetta, K., Collins-Praino, L.E., Buisman-Pijlman, F.T.A., Liu, J., Hutchinson, A.D., & Hutchinson, M.R. (2018). Are the Protective Benefits of Vitamin D in Neurodegenerative Disease Dependent on Route of Administration? A Systematic Review. Nutritional Neuroscience, doi: 10.1080/1028415X.2018.1493807
- Oudshoorn, C., Mattace-Raso, F. U. S., van der Velde, N., Colin, E. M., & van der Cammen, T. J. M. (2008). Higher Serum Vitamin D3 Levels Are Associated with Better Cognitive Test Performance in Patients with Alzheimer’s Disease. Dementia and Geriatric Cognitive Disorders. doi: https://doi.org/10.1159/000134382
- Office of Dietary Supplements. (2019, August 7). Vitamin D: Fact Sheet for Health Professionals. Retrieved from https://ods.od.nih.gov/factsheets/VitaminD-HealthProfessional/.