Vitamin D and Brain Health
Summer is officially over — which makes this the perfect time to start thinking about vitamin D and your brain.
Most of us know vitamin D for its role in maintaining strong bones, but it’s actually a hormone with receptors throughout the body — including in the brain. In fact, studies show that patients with low vitamin D levels have a significantly increased risk of dementia, depression, and diseases like multiple sclerosis.
Sunlight and Vitamin D Production
Our bodies make vitamin D in a frankly miraculous way: when sunlight’s UVB rays hit the skin and transform 7-dehydrocholesterol (a pre cholesterol molecule) into pre-vitamin D3. This then converts into vitamin D3, which is processed in the liver and kidneys to form its active hormone, calcitriol. How much vitamin D you make depends on sun exposure, but also on factors like skin color, age, air quality, ozone index as well as the presence of cofactors such as magnesium that are required to produce active calcitriol.
In the summer, this might mean as little as 10 minutes outdoors if you’re fair-skinned, or about 30 minutes if you’re dark-skinned.
In the winter, you may need 30 minutes if you’re fair-skinned, or up to two hours if you’re dark-skinned.
The process to convert 7-dehydrocholesterol in the skin to the active Vitamin D hormone can take about 21 days from the time of sun exposure.
(These estimates assume some skin exposure — arms and legs in summer, face and hands in winter.)
What about “sun lamps” often used for Seasonal Affective Disorder? While they can be tremendously helpful for mood and circadian rhythm regulation, they don’t boost vitamin D production because most (appropriately) filter out UV light.
Food Sources of Vitamin D
Since most of us spend much of our time indoors and use sunblock when outside, dietary sources matter. Fatty fish and fortified dairy are the best options. Personally, I recommend sardines, anchovies, and wild salmon — not only rich in vitamin D but also in omega-3s. For those on a vegan diet, fortified foods are necessary (cereals, juices). Fun fact: Gilled mushrooms exposed to sunlight also produce their own vitamin D but most commercial mushrooms are grown in the dark and I don’t have a sunroom just for my mushrooms (much less myself).
Supplementation: What the Evidence Says
Because of our largely indoor lifestyles, regular sunscreen use, and even migration patterns that limit natural sun exposure (especially for darker skinned individuals), most people can’t rely on diet alone to maintain healthy vitamin D levels. That’s where supplementation comes in — but what does the evidence actually say
A large prospective study published in 2023 followed more than 12,000 people over 10 years. Those who used vitamin D supplementation (including calcium–vitamin D, cholecalciferol, and ergocalciferol) had a 40% lower risk of developing dementia compared with non-users. Benefits were greater in women than men, and in people without the ApoE4 gene variant than in carriers.
This is a remarkable potential benefit for a relatively low-risk intervention.
Dosing and Safety
That said, the study didn’t answer the question of optimal dosing. The Endocrine Society suggests:
600 IU/day for adults 19–50
700 IU/day for adults 50–70
800 IU/day for adults over 70
The Endocrine Society’s guidelines do not recommend supplementation above standard levels for most people, and they advise against routine testing. Yet, because dementia was not considered as an endpoint in the extensive number of studies informing these guidelines, there’s an important opportunity to expand and refine future recommendations.
In my practice I take an individualized approach. I typically check levels at least once a year during the peak (early Fall, now!) and supplement accordingly, often at least through the winter. If you're low in September, trust me, you’re probably going to be really low by March. Importantly: more is not always better. Too much vitamin D can lead to hypercalcemia, which can harm the kidneys, brain, and blood vessels.
Final Thoughts
Vitamin D is more than just a bone-health nutrient — it’s important for brain health too. As days grow shorter (and darker and colder where I am), it’s worth talking to your doctor, checking your levels and rethinking your habits to considering whether food or supplementation might benefit you.
Has this post prompted any changes in how you’ll approach vitamin D this season? I’d love to hear from you.
