Vitamins · 5 min read
Vitamin D in winter: how much is actually enough?
Latitude, skin tone, and time of year all matter. Here's a framework, not a prescription.
Vitamin D is produced in your skin when UVB hits it. Above roughly 37° latitude, from November through March, there isn't enough UVB in sunlight to make any meaningful amount.
That's why serum 25(OH)D drops in winter for most people, even those who spend time outside. The question isn't whether to supplement — it's how much.
The RDA is 600–800 IU. That's a floor set to prevent deficiency in a healthy population, not a target for optimal levels. Most clinical labs consider 30–50 ng/mL a reasonable range.
A practical starting point: 1,000–2,000 IU daily with a fat-containing meal, tested after 12 weeks. Adjust based on the number, not the marketing.
Pair with vitamin K2 if you're supplementing long-term. And test — this is one of the few nutrients where a $30 blood test removes all the guessing.
