About 90% of vitamin D your body makes itself — in the skin under sunlight, only 10% comes from food. But the same sun that "makes" the vitamin also causes burns and photoaging. Let's get the benefit without the harm.
How Sun Produces Vitamin D
UVB rays (280–315 nm) hit 7-dehydrocholesterol in skin and convert it to vitamin D3. So synthesis depends directly on UV index, not temperature or brightness: on a cold sunny day production is active, on a hot cloudy one — almost none.
At What UV Index Synthesis Starts
Synthesis effectively starts at UV index 3 and above. At 0–2 it's practically zero. That's why from November to February in northern and mid-latitudes vitamin D isn't produced even on sunny days — the sun is too low.
How Much Sun: Numbers
| UV Index | Time (arms and legs exposed) | Result |
|---|---|---|
| 3–5 | 15–20 minutes | daily D requirement |
| 6–7 | 10–15 minutes | daily D requirement |
| 8–10 | 5–10 minutes | requirement met, but high burn risk |
For fair skin (I–II) take the lower bound; for darker skin (IV–VI) 2–3× longer: melanin "competes" with UVB for the radiation.
Rules for "Vitamin" Sun
- Expose arms, legs, back — bigger surface area means more D;
- best time is around midday when UVB peaks (but above 8 the risk outweighs benefit);
- don't soap-wash for 2–3 hours after — vitamin "matures" on the skin surface;
- glass blocks UVB — tanning through a window is useless for vitamin D.
Winter: Why Sun Doesn't Work
Above ~35° latitude from November to March UVB is absorbed by the atmosphere. Check your city's UV index in winter: if it's below 3, synthesis doesn't happen and you need D from food (fatty fish, eggs, liver) or supplements.
Does SPF Block Vitamin D Production?
Theoretically yes: SPF 30 blocks ~97% of UVB. But studies show that in real use, D levels stay normal — nobody applies it perfectly evenly. If worried: 10–15 minutes without sunscreen at UV 3–5 is enough, then apply SPF as usual.
Bottom Line
Sun is the cheapest source of vitamin D, but only at UV index 3–7 and short sessions. At 8+ risks outweigh benefits. Check current UV in your city — Miami, Madrid, Rome — and catch the "vitamin window" safely.