Immune Support · Dr. Alina Reyes, PharmD · March 22, 2026 · 6 min read
Vitamin D is produced in the skin when UVB radiation reaches it. Between late fall and early spring across most of the continental United States, the sun's angle simply doesn't deliver enough UVB to sustain production — which is why serum levels drift downward for millions of Americans every winter.
The research on supplementation is nuanced. Meta-analyses consistently show the greatest benefit in people who begin with insufficient levels, and much smaller effects in people who are already replete. That's why we recommend testing rather than guessing: a simple 25-hydroxyvitamin D panel gives you a real baseline.
For most adults, a daily maintenance dose in the 1,000–4,000 IU range is well tolerated. Pairing D3 with vitamin K2 and magnesium supports normal calcium handling — which is why nutrient form and delivery matter as much as dose.
This article is educational and is not medical advice. These statements have not been evaluated by the Food and Drug Administration. Consult your healthcare provider before starting any supplement.