The Vitamin D Winter
"Vitamin D winter" is the months during which UVB radiation reaching the ground at your latitude is too weak to trigger cutaneous vitamin D3 synthesis, regardless of how much time you spend outside. It's driven by solar zenith angle — the sun's altitude above the horizon at solar noon.
Why UVB disappears in winter
UVB photons (280–315 nm) that reach the ground must pass through the atmosphere. In winter at temperate latitudes the sun sits low in the sky; sunlight travels a much longer path through the atmosphere, and the stratospheric ozone layer selectively absorbs almost all the UVB. The typical rule from Webb et al.: cutaneous vitamin D3 synthesis effectively stops when the solar zenith angle exceeds ~60° — i.e., when the sun is less than about 30° above the horizon.
This is why the same latitude that produces plenty of vitamin D at noon in June yields essentially zero at noon in December.
Vitamin D winter by latitude band
| Latitude | Effective UVB season |
|---|---|
| <35° | Year-round |
| 35–42° | March–October |
| 42–50° | April–September |
| 50–55° | May–August |
| 55–60° | Late-May–mid-August |
| >60° | June–early August |
Based on modelled UVB flux (Fioletov et al. 2010) and empirical measurements at multiple NDACC monitoring sites. Local variations arise from altitude, ozone anomalies, and persistent cloud cover.
How your body copes
Because cholecalciferol is fat-soluble and partitions into adipose tissue, humans store a reservoir that sustains circulating 25(OH)D through winter. In lean adults, serum 25(OH)D decays with an effective terminal half-life of ~60 days from a summer peak — enough to hold sufficiency for 2–3 months into winter before dropping into the insufficient range. Obese individuals show more sequestration and slower release, giving a longer terminal half-life but a lower peak. Use our reserves calculator to model your own decay curve.
What to do
- Bank as much vitamin D as safely possible during summer through short, sensible sun exposures — 10–30 minutes of midday sun on arms and legs, 2–3 times a week, without burning.
- Through the vitamin D winter, supplement with 1,000–2,000 IU/day cholecalciferol; higher doses for known insufficiency. See our dosage guide.
- Eat vitamin D–rich foods when possible: fatty fish, UV-exposed mushrooms, egg yolks, and fortified dairy or plant milks.
- Consider testing serum 25(OH)D at end of winter (February–March in the Northern Hemisphere) to see the seasonal nadir.