Readers of this blog are well aware than many (if not most) Americans have insufficient to deficient levels of vitamin D. Other than prescribing oral vitamin D or vitamin D-containing foods, we doctors were left with prescribing a little sunshine. But, we know that exposing your skin to unprotected UVA or UVB light can increase your risk of skin cancer. And, there has been controversy about exactly how much sunlight one might need to avoid vitamin D supplements. Now, I may have an answer for you.
But, first a few basics. Vitamin D is essential for bone mineralization and may have a wide variety of other health benefits. Here are just a few I’ve blogged about:

• Vitamin D Supplementation and Cancer Prevention
• Vitamin D helps fend off flu and asthma attacks
• Increasing vitamin D levels may cut heart disease risk
• Vitamin D Supplementation Helps Prevent Falls in Older Adults
• Vitamin D Linked to Lower Heart Risk
• Can Vitamin D Ease Fibromyalgia Pain?
• In Lab Tests Vitamin D Shrinks Breast Cancer Cells
• Vitamin D May Lower Colon Cancer Risk
• Daily Calcium Plus Vitamin D Supplements May Reduce Fracture Risk
• Low levels of vitamin D may be linked to greater asthma severity
• Heart patients lacking vitamin D more likely to be depressed
• More reasons to consider having your vitamin D level checked – you may think better and have less arthritis
• Vitamin D tests soar as deficiency, diseases linked

Experts disagree on the serum vitamin D level necessary to maintain health. Some recommend concentrations above 30 ng/mL and consider the range between 20 and 30 ng/mL insufficient and concentrations lower than 20 ng/mL deficient. In our area, most experts are recommending level of 50 ng/mL (and, indeed, we are supplementing to this level).
By this reckoning, many, perhaps most, Americans are vitamin D insufficient or deficient.
Because it is difficult to obtain enough vitamin D from food intake, oral supplements and sunlight have been recommended for individuals with low serum D levels.
The suggested dose for supplements is 400 to 1000 IU/day.
It has also been suggested that a few minutes of sunlight each day to the face, neck, hands, and arms are all that is necessary to restore vitamin D sufficiency, but the amount of sunlight required for photoconversion of 7-dehydrocholesterol to pre–vitamin D varies considerably depending on a person’s age, Fitzpatrick sun-reactive skin type, geographic location, and season.
The six Fitzpatrick skin types classify sensitivity to ultraviolet light; skin type I is fair skin that always burns, never tans; type III is darker white skin that burns and tans; type V is brown skin that rarely burns, tans easily.
Investigators in a new study employed the FastRT computational tool to predict the length of daily exposure required to obtain the sunlight equivalent of 400 and 1000 IU oral vitamin D supplementation.
At noon in Miami, someone with Fitzpatrick skin type III would require 6 minutes to synthesize 1000 IU of vitamin D in the summer and 15 minutes in the winter.
Someone with skin type V would need 15 and 29 minutes, respectively.
At noon in the summer in Boston, necessary exposure times approximate those in Miami, but in winter, it would take about 1 hour for type III skin and 2 hours for type V skin to synthesize 1000 IU of D.
After 2 PM in the winter in Boston, it is impossible for even someone with Fitzpatrick type I skin to receive enough sun to equal even 400 IU of vitamin D.
About this study, Craig A. Elmets, MD, writes, “These findings raise serious questions about the recommendation that a ‘little bit’ of outdoor sun exposure is sufficient to maintain adequate vitamin D levels.
“Moreover, predictions of the time required to achieve adequate vitamin D photosynthesis are probably underestimates, because it is unlikely that people would walk around Boston for an hour or two in the winter with face, neck, and arms exposed.
“These findings corroborate another study that casts doubt on sun exposure as a way to prevent vitamin D deficiency.”
The bottom line is that it looks like oral vitamin D supplements are going to end up being shown to be the safest and most effective to gain adequate vitamin D levels.