Vitamin D — The Hormone Your Skin Makes from Sunlight
Vitamin D is essential for calcium absorption, immune function, and maintaining muscle strength — but it's nearly impossible to get enough from food alone. Up to 80–90% of your body's vitamin D comes from UVB-driven skin synthesis. How long you need to be in the sun depends heavily on season, skin type, and how much skin is exposed.
In summer at midday, a light-skinned person with arms exposed can synthesize roughly 1,000 IU in 10–15 minutes. But in winter at latitudes above 35°N (which includes most of the continental US), UVB is so weak that sun exposure alone can't meet your vitamin D needs regardless of duration.
Tips to Maximize Vitamin D Synthesis
- Midday sun (10 AM – 2 PM) is the most efficient — UVB intensity peaks
- UVB doesn't pass through glass — step outside, don't rely on window light
- More skin exposed = more synthesis in the same time
- In winter or with dark skin, use food sources or supplements (1,000–2,000 IU/day)
Frequently Asked Questions
In summer at midday, a light-skinned person with arms exposed can make ~1,000 IU in 10–15 minutes. Darker skin or winter requires longer exposure; winter often can't provide enough at all.
SPF 30+ blocks over 90% of UVB. These estimates assume no sunscreen. Consider a brief unprotected exposure before applying sunscreen, or get vitamin D from food and supplements.
At latitudes above 35°N, UVB is minimal November–February. Sun exposure alone is insufficient — use food or vitamin D3 supplements during these months.