Researchers analyzed participants of the Rush Memory and Aging Project (MAP)—an ongoing longitudinal study that aims to identify risk factors for Alzheimer’s disease and other cognitive decline disorders—before and after death to see how their vitamin D levels affected cognitive function in their later years.

Free of known dementia at the time of enrollment, all MAP participants agreed to participate in annual evaluations and organ donation when they died. In this study, the average age of participants was 92 at the time of death.

Total serum vitamin D levels [25(OH)D] and global cognitive function were assessed antemortem, while vitamin D3, 25(OH)D3, and 1,25(OH)D3 (the active form of vitamin D3) were measured in four brain regions (the mid-temporal cortex, mid-frontal cortex, cerebellum, and anterior watershed white matter) post-mortem.

The main form of vitamin D3 found in the brain (and thus, the form researchers focused on in their analysis) was 25(OH)D3. It’s worth noting that there are two types of vitamin D—D2 and D3—and brain concentrations of vitamin D2 (the form found in most fortified food sources) were not measured in this study.

Vice president of scientific affairs at mindbodygreen Ashley Jordan Ferira, Ph.D., RDN, elaborates on this limitation: “Vitamin D3 is found in animal sources and key algae and lichen, while vitamin D2 comes from plant sources like yeast and irradiated mushrooms. If your health care provider accidentally measured serum 25(OH)D3, but you were knocking back irradiated mushrooms or a vitamin D2 supplement, your lab results wouldn’t reflect your intake. Serum total 25(OH)D is best to capture the full picture.”

While the results of this study are still pertinent to dementia research, it’s important to keep this discrepancy in mind as you read the results.