Vitamin D and the Brain

Vitamin D is commonly known as the "bone vitamin." Scientists have discovered that vitamin D acts as a neurosteroid hormone, with receptors widely distributed throughout brain regions involved in memory and cognition, particularly the prefrontal cortex and hippocampus. The active form of vitamin D, calcitriol (1,25(OH)₂D₃), is involved in regulating at least five important mechanisms that protect nerve cells. These include reducing neuroinflammation, limiting the accumulation of amyloid-beta, an enzyme responsible for producing the amyloid plaques characteristic of Alzheimer's disease, reducing abnormal phosphorylation, supporting neurotrophic factors, regulating intracellular calcium and protecting mitochondria. Together, these mechanisms explain why vitamin D deficiency may facilitate the progression of dementia, both theoretically and, increasingly, through experimental evidence.

Epidemiological Evidence: A Consistent Association

Epidemiological evidence of the association between vitamin D and dementia is becoming increasingly robust and consistent. A 2024 meta-analysis published in the Journal of Alzheimer's Disease, which pooled findings from 23 prospective cohort studies, found that vitamin D deficiency was associated with a 1.42-fold increased risk of dementia and a 1.57-fold increased risk of Alzheimer's disease.

Another meta-analysis, updated through October 2024 and incorporating data from 22 studies involving 53,122 participants, demonstrated a linear association: individuals with the lowest serum vitamin D concentrations had a 49% higher risk of dementia than those with the highest concentrations. A dose-response analysis found that each 10 nmol/L increase in serum vitamin D concentration corresponded to a 1.2% reduction in dementia risk. Earlier research examining the severity of vitamin D deficiency also found that severe deficiency (below 10 ng/mL) was more strongly associated with Alzheimer's disease risk than moderate deficiency (10–20 ng/mL).

Latest Evidence on Vitamin D Supplementation

A large-scale cohort study using data from the US National Alzheimer's Coordinating Center database found that individuals taking vitamin D supplements had a 40% lower risk of developing dementia than those who did not take supplements, after adjustment for confounding factors. The benefits were particularly evident among women, individuals who did not carry the APOE ε4 gene variant, and those who began supplementation before the onset of cognitive impairment.

A study published in the journal Neurology (2026) followed 793 adults from the Framingham Heart Study. Researchers measured vitamin D concentrations at a mean age of 39 and performed brain positron emission tomography (PET) scans an average of 16 years later. They found that higher vitamin D concentrations in midlife were associated with lower protein levels detected on PET scans, an important biomarker of preclinical brain changes associated with Alzheimer's disease.

Vitamin D Deficiency Among Older Adults

Vitamin D deficiency is a global nutritional problem, and older adults are a particularly high-risk group, especially those living in nursing homes, receiving limited sunlight exposure or residing in regions with long winters.

In Vietnam, despite its tropical climate and abundant sunshine, several local and regional studies have reported considerable rates of vitamin D deficiency among older adults, particularly those who spend most of their time indoors and consume few vitamin D-rich foods. This is a concerning warning as Vietnam's population is rapidly aging.

Factors that increase the risk of vitamin D deficiency among older adults include:

• Reduced vitamin D synthesis in the skin: Aging reduces the skin's capacity to synthesize vitamin D₃ from sunlight by 70–75% compared with younger individuals, due to decreased levels of the precursor 7-dehydrocholesterol in the skin.
• Reduced intestinal absorption: Declining digestive function, reduced bile production and impaired fat absorption decrease the absorption of dietary vitamin D.

• Declining liver and kidney metabolism: Vitamin D must undergo two hydroxylation steps in the liver and kidneys to become biologically active. The function of both organs declines with age.

• Limited sunlight exposure: Older adults often spend more time indoors and wear clothing that covers much of their skin. Concerns about skin cancer also lead many to avoid sunlight entirely.
• Low consumption of vitamin D-rich foods: These include fatty fish, eggs and vitamin D-fortified milk.

Can Vitamin D Supplementation Prevent Dementia?

This is the most important question, and the current answer is: promising, but not yet conclusive. Evidence from intervention studies presents a complex picture and depends heavily on the timing of intervention, the population studied and the degree of vitamin D deficiency before supplementation begins.

How Can Vitamin D Help Protect Brain Health?

Sunlight remains the most effective and economical source of vitamin D. For older adults in Vietnam, 15–30 minutes of daily sunlight exposure (on the arms, legs and face, while avoiding intense sunlight between 10 a.m. and 2 p.m.) can make a significant contribution to meeting vitamin D requirements. An important point to remember is that sun exposure does not need to continue until the skin becomes red. Sunlight passing through window glass does not count as effective sunlight exposure because window glass blocks UV-B radiation.

Increase the consumption of vitamin D-rich foods, including:

• Fatty fish: Salmon (containing approximately 600–1,000 IU of vitamin D per 100 g), mackerel, herring and tuna should be consumed at least 2–3 times per week.
• Egg yolks: Each egg provides approximately 40 IU of vitamin D. Eggs should be consumed regularly unless contraindicated due to high blood lipid levels.
• Vitamin D-fortified milk and dairy products: These are recommended options for older adults who rarely eat fish.
• Fish liver oil: A concentrated source of vitamin D and omega-3 fatty acids, suitable when fresh fish is difficult to obtain.

Conclusion

Accumulating evidence from dozens of large-scale studies, particularly those conducted between 2022 and 2025, increasingly supports the conclusion that vitamin D deficiency is a modifiable risk factor in the progression toward dementia.

Vitamin D is not yet a "vaccine" against Alzheimer's disease, and it would be a serious mistake to expect it to be one. However, within the multifactorial approach to dementia prevention, ensuring adequate vitamin D intake is an important, practical and low-cost measure, particularly for older Vietnamese adults facing the fastest rate of population aging in Southeast Asia.

Make morning sunlight exposure a daily habit. Eat fish regularly. And if you or an older family member show signs of memory decline, ask a doctor to check vitamin D levels today. It may be a simple but meaningful step toward protecting brain health.