Vitamin D deficiency is shockingly prevalent in the U.S. A whopping 29% of adults are deficient in vitamin D, and another 41% are insufficient in the critical nutrient. Neurologists, such as Dr. Dale Bredesen, routinely see severely low vitamin D levels in patients with early memory changes and emphasize correcting this as a key part of cognitive health programs.

Isn't it just for bone health?

Vitamin D3 does far more than support bones; it acts like a hormone, influencing hundreds of processes throughout the body, including those essential for brain, mood, and nervous system health. Deficiency is extremely common, especially in older adults and those with autoimmunity. Research has repeatedly linked low vitamin D levels with forgetfulness, slower mental processing, mood changes, and a higher risk of cognitive decline. A 2025 review in Nutrients found that individuals with low vitamin D often have reduced levels of BDNF (brain-derived neurotrophic factor), a key protein that protects and repairs neurons, while supplementation may support clearer thinking and improved mood (Skoczek-Rubińska et al., 2025).  

What are optimal levels?

Maintaining optimal vitamin D levels, often 55–80 ng/mL in functional medicine, may offer powerful long-term neuroprotective benefits. Vitamin D3 helps regulate genes involved in inflammation, immune balance, and neuronal repair, all of which contribute to a more resilient brain as we age. Genetics also play a meaningful role: variations in the vitamin D receptor (VDR) gene, such as Fok1, Bsm1, Taq1, and Apa1, can affect how efficiently the body uses vitamin D. Some people with these variants may require higher levels of Vitamin D3 to achieve the same cognitive and immune support. Chronically low levels increase vulnerability to oxidative stress, inflammation, and cognitive decline over time.

Vitamin D3 supplementation appears most effective in individuals who begin with deficiency or insufficiency. The same Nutrients review reported consistent improvements in mood, cognition, and BDNF activity in adults with low baseline levels who corrected their vitamin D status (Skoczek-Rubińska et al., 2025). Although vitamin D alone is not a treatment for dementia, supporting healthy levels early on may help slow the progression of mild memory concerns and contribute to healthier cognitive aging. Functional nutrition and nutrigenomics approaches often combine vitamin D3 with synergistic nutrients and personalize dosing using information about VDR gene variants, an approach I use with clients through SNIP Nutrigenomics to ensure efficient utilization based on their DNA.

How much do I need to take?

A typical functional range for daily Vitamin D3 supplementation is 2,000–5,000 IU, though some individuals, especially those with VDR variants or very low baseline levels, may require more to reach the optimal serum target of 55–80 ng/mL. With autoimmune disease or cancers, Dr. Steven Gundry targets 70-110 ng/ML. Combining Vitamin D3 with Vitamin K2 may support balanced calcium metabolism while improving overall utilization. Because each person metabolizes vitamin D differently, regular serum testing through functional ranges is the safest and most effective way to determine the ideal dose for both cognitive and whole-body health.

How does it work in the body?

Vitamin D3 is first converted in the liver to 25-hydroxyvitamin D, the form measured on blood tests, and then into its active hormone form within the kidneys and brain tissues. Once activated, vitamin D crosses the blood–brain barrier and binds to receptors on neurons and glial cells, regulating gene pathways involved in neuroprotection, antioxidant activity, inflammation control, and cellular repair. The 2025 Nutrients review underscores that deficiency is linked with impaired mood, lower BDNF levels, and decreased cognitive performance, while correcting low vitamin D improves several key markers of brain resilience. Left unaddressed, long-term deficiency increases the risk of inflammation, oxidative stress, and accelerated cognitive decline.

What's my next step?

Do NOT think for one second that you have to live with your chronic gut-brain-immune symptoms forever. My Tier 2 online program is designed for you to address your root causes and includes the lab kit to test your VDR genes. It's designed as an evergreen course, so pop in at any time! 

References 

Skoczek-Rubińska, A., Cisek-Woźniak, A., Molska, M., Heyser, M., Trocholepsza, M., Pietrzak, S., & Mruczyk, K. (2025). Impact of Vitamin D Status and Supplementation on Brain-Derived Neurotrophic Factor and Mood–Cognitive Outcomes: A Structured Narrative Review. Nutrients, 17(16), 2655.

SNIP Nutrition. (n.d.). Unlocking the Secrets of Vitamin D: The Role of VDR SNPs in Health and Wellness. https://snipnutrition.com/blog/unlocking-the-secrets-of-vitamin-d-the-role-of-vdr-snps-in-health-and-wellness/ 

Apollo Health. (n.d.). A Neurologist’s Top Tip for Preventing Alzheimer’s: Healthy Vitamin D Levels. https://www.apollohealthco.com/a-neurologists-top-tip-for-prevening-alzheimers-healthy-vitamin-d-levels/ 

mindbodygreen. (n.d.). Low Vitamin D Levels Can Lead to Alzheimer’s, Says Neurologist. https://www.mindbodygreen.com/articles/low-vitamin-d-levels-can-lead-to-alzheimers-says-neurologist-tips-for-preventing

goop (n.d.). Can Vitamin D3 Heal Autoimmune Diseases?. https://goop.com/wellness/health/can-vitamin-d3-heal-autoimmune-diseases/?srsltid=AfmBOopS9jSOtVeFx1Ml_4V0dkUkasoqFZHpuGWc2_z0GE2jsvi6R3Ps