THE CONUNDRUM OF VITAMIN D, SUNLIGHT EXPOSURE AND SPF
By David Muir, PH.D.
An article posted this week in the Spoonbill Courier (The Bright Tale of the Sunshine Vitamin; June 11, 2025) presented the history, production and importance of vitamin D in child bone development and general good health. An important aspect of vitamin D is that it is produced in skin exposed to direct sunlight. However, presently the pervasive recommendation is to minimize exposure to sunlight to avoid skin damage and cancer. What’s a person to do? Let’s shine some light on this conundrum with hard facts and concluding AI recommendations.
THE CONUNDRUM
(Nerd Warning! Skip to the Bottom for the David’s Takeaway)
THE GOOD: Sunlight exposure is the primary source of vitamin D for most people. Solar ultraviolet-B radiation (UVB; wavelengths of 290 to 315 nanometers) stimulates the production of vitamin D3 from 7-dehydrocholesterol (7-DHC) in the epidermis of the skin.
THE BAD: Covering exposed skin or using sunscreen whenever outside are risk factors for vitamin D deficiency. The consequences of vitamin D deficiency primarily concern bone health. In vitamin D deficiency, calcium absorption cannot be increased enough to satisfy the body’s calcium needs.
THE UGLY: Sunlight contains UVB radiation, which is harmful to skin cells. UV rays damage the DNA in skin cells, leading to abnormal cell growth. Over time, this damage can build up, increasing the risk of skin cancer.
Numerous variables affect skin synthesis of vitamin D, including latitude, season, time of day, degree of skin pigmentation, age, amount of skin exposed, and sunscreen use. Latitude, season, and time of day create the solar zenith angle, which determines the intensity of sunlight. People living in higher latitudes are more at risk for vitamin D deficiency compared to those living in more equatorial latitudes because the sunlight intensity is lower.
For those residing in temperate latitudes, time of year influences the ability to generate vitamin D3 in skin. In latitudes around 40 degrees north or 40 degrees south (Boston is 42 degrees north), there is insufficient UVB radiation available for vitamin D synthesis from November to early March. Ten degrees farther north (Edmonton, Canada) or south the “vitamin D winter” extends from October to April. Time of day also influences the ability to generate vitamin D in skin, with midday solar radiation being the most intense. Melanin, the tanning pigment in skin, competes with 7-DHC for the absorption of UV light and thus acts as a natural sunscreen, reducing the effectiveness of vitamin D production in skin. Therefore, individuals with dark-colored skin require more time (up to ten times as long) to synthesize the same amount of vitamin D3 in skin as those with fair skin.
The consensus is, sensible sun exposure, i.e., exposing bare arms and legs to midday sun (between 10 am and 3 pm) for 10-30 minutes twice weekly, may be sufficient to meet vitamin D requirements. But use of sunscreen effectively blocks UVB absorption and therefore vitamin D synthesis in skin: application of sunscreen with a sun protection factor (SPF) of 8 reduces production of vitamin D in skin by more than 95%.
So what exactly is the Sun Protection Factor?
Sun Protection Factor or SPF, and the number beside it indicates how well the sunscreen protects skin against sunburn. It is not an indicator of how long you can stay out in the sun, rather, it indicates how much longer it takes untanned skin to start to redden with sunscreen applied compared to how long it takes to start reddening without it. While this might vary depending on skin color, on average it takes 15 minutes of exposure to detect a sunburn on unprotected, untanned skin.
To work out the SPF of a sunscreen, laboratory tests are carried out on an untanned patch of skin (such as the buttocks) of human volunteers. Sunscreen is applied liberally to the skin, which is then exposed to simulated sunlight via UV lamps. Measurements are taken of how long it takes the skin to get a minimal burn when covered with sunscreen, and how long it takes to get the same minimal redness without it.
The question is, if not from UV exposure, how can you obtain enough vitamin D?
It’s pretty straightforward. You can acquire vitamin D from a combination of diet and supplements. Fatty fish such as salmon, mackerel and tuna are especially good sources. Small amounts are also present in egg yolks, beef liver and cheese. And many common foods such as milk and orange juice are fortified with vitamin D. Also, if you’re willing to be a throwback to the past, just hold your nose and down a tablespoon of cod liver oil, which has more than twice the daily recommended intake of vitamin D.
The solution to the sunlight exposure conundrum – provided by ChatGTP AI:
The best way to produce enough vitamin D through sunlight while minimizing skin damage is to strike a careful balance. Here’s a safe and effective strategy:
✅ 1. Get Short, Regular Sun Exposure
- Duration: 5–15 minutes of midday sun (10 a.m. – 2 p.m.) a few times per week.
- Skin area: Expose face, arms, and legs (or back) — more skin = less time needed.
- Skin tone matters: Lighter skin synthesizes vitamin D faster. Darker skin may need longer exposure.
⛔️ Avoid tanning or burning. Redness means UV damage — once skin turns pink, the risk outweighs benefit.
✅ 2. Choose the Right Time and Conditions
- Midday sun (solar noon) is most efficient — the UVB rays needed for vitamin D are strongest.
- Spring to early fall is better in temperate zones; winter sun in high latitudes may not work at all.
✅ 3. Don’t Rely Only on Sun
- If you’re at high risk for skin cancer, live in northern latitudes, or have dark skin, combine modest sun with:
- Vitamin D–rich foods: fatty fish, fortified dairy/plant milks, egg yolks
- Supplements: 600–800 IU/day (or higher if blood levels are low, under medical guidance)
✅ 4. Use Sunscreen Strategically
- Brief unprotected sun exposure is okay, but for longer time outdoors, use sunscreen after the initial few minutes to prevent damage.
- SPF 15+ blocks most UVB needed for D, but you still get enough if time is limited and exposure is partial.
✅ 5. Monitor Your Levels
- Ask your doctor to test your serum 25(OH)D (vitamin D) levels if you:
- Have low sun exposure or cover skin often
- Are over 65
- Have darker skin or conditions like obesity or malabsorption
Summary Table
| Factor | Recommendation |
| Time of Day | Midday (10 a.m. to 2 p.m.) |
| Frequency | 2–4 times/week |
| Duration | 5–15 minutes, depending on skin tone |
| Skin Exposure | Arms/legs/face |
| Sunscreen Use | After initial brief exposure |
| Supplementation | If needed, especially in winter or high-risk groups |
Let me [ChatGPT] know your latitude, skin tone, or health status if you’d like personalized sun exposure timing.
Primary Sources: ChatGTP; “What does the SPF really mean?”, Australian Academy of Science website; “Vitamin D and skin health”, Linus Pauling Institute website
