Dear Dr. Cannell:
I’m an engineer and I want to know how much vitamin D I make from sunbathing.
Can I get vitamin D from sunshine anytime I want, as long as the sun is high in the sky (above 50° elevation)?
I calculate one needs 22 minutes to generate 1,000 IUs when the sun is at 50° above the horizon. But, when the sun is higher in the sky, say at 67° sun elevation, would I only need 12 minutes to generate 1,000 IUs from sunbathing (half the time)?
Awaiting your answer,
George Roberts, M.Sc.
New York City
Good questions. As a general rule, the higher the sun is in the sky (solar zenith angle), the more vitamin D is made in the skin. By controlling for solar zenith angle, as you wisely did, you also controlled for factors such as latitude, season and time of day. However, multiple other factors are at play, such as:
- cloud cover
- skin pigmentation
- baseline 25(OH)D
- clothes worn
- depth of the ozone column
- oil and water content of skin
- reflective surfaces around you (albedo)
- amount of vitamin D precursors in the skin
There are other factors just being discovered, factors that are difficult for people to gauge. For example, presence or absence of common “mutations” called single nucleotide polymorphism (SNPs) of the vitamin D metabolic machinery.
More distressing, even when one controls for these factors, enormous variations in 25(OH)D responses to sunbathing occur. For example, major person to person variation in both the total amount of induced 25(OH)D (range 35 – 85 ng) and the increase in 25(OH)D (range 0.5 to 55 ng) to standardized UVB administration exist. We really don’t know how much intrapersonal variation occurs in the same person if they check their level 3-4 times in a day,
I’d advise a more artistic approach to sunbathing rather than an engineering one! When you can, sunbathe for half the time it takes for your skin to begin to turn slightly flush.
John Cannell, MD