Systematic Review: PubMed was searched for papers on vitamin D and dementia. The literature revealed that although vitamin D deficiency has been associated with higher risk of dementia, the role of supplementation remains in equipoise. Thus, we explored longitudinal associations between vitamin D supplementation and incident dementia in a sample of 12,388 dementia‐free older adults.

Interpretation: Exposure to vitamin D supplementation was associated with a 40% lower dementia incidence rate than no exposure, providing strong support for supplementation. The results were consistent across three vitamin D formulations. The effect of vitamin D exposure on the rate of incident dementia differed significantly across the strata of sex, cognitive status, and apolipoprotein E (APOE) ε4 status.

Future Directions: Future trials should include a more ethnoracially diverse sample, assess baseline vitamin D levels, and account for sun exposure, in addition to sex, baseline cognitive status, and APOE genotype.