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Information on the latest vitamin D news and research.

Find out more information on deficiency, supplementation, sun exposure, and how vitamin D relates to your health.

Sun exposure: Is just a little bit enough?

Some experts opine that brief sun exposure will supply all the vitamin D that one needs. However, there has never been a study of how much vitamin D such exposure generates.

Recently, researchers in Korea decided to find out. Dr. Sang-Hoon Lee and colleagues from the Ajou University School of Medicine in South Korea studied the effect of brief sun exposure on vitamin D levels in 20 young women for four weeks.

Lee SH, Park SJ, Kim KM, Lee DJ, Kim WJ, Park RW, Joo NS. Effect of sunlight exposure on serum 25-hydroxyvitamin d concentration in women with vitamin D deficiency: using ambulatory lux meter and sunlight exposure questionnaire. Korean J Fam Med. 2012 Nov;33(6):381-9.

The study was conducted between October and November at latitude 37 degrees north, about the latitude of Washington DC. Initial mean 25(OH)D levels were 11 ng/ml and no woman had levels greater than 20 ng/ml to begin the study. The women were told to get 20 minutes of midday sun exposure on their hands, forearms and face every weekday for four weeks. Facial sunblock and sunglasses were permitted.

Guess how much 25(OH)D levels increased after a month of daily sun exposure? Vitamin D levels did not increase at all; in fact, they were a little lower than when the study began!

Why? I think four reasons may explain the finding.

Perhaps the women did not go outside as often as required. Second is the latitude; I think less UVB is available in October and November than many people think at such latitudes. Perhaps the vitamin D winter begins in the fall at latitude 37 degrees. Third, perhaps forearms, face and hands are just not enough skin surface to make meaningful amount of vitamin D. And four, such brief sun exposure may not be long enough to make meaningful amount of vitamin D.

Our hunter-gatherer equatorial ancestors had very dark skin but wore no or little clothing and were outside most or all of the day. Recent studies indicate such people had vitamin D levels around 50 ng/ml. This indicates a vitamin D input of about 5,000 to 10,000 IU/day.

Vitamin D status in indigenous populations: Part 1. Posted on August 27, 2012 by John Cannell, MD

That’s why the Vitamin D Council recommends full body sunbathing, not incidental sun exposure. Make sure your shadow is shorter than you are so you know you are making vitamin D. Also, since most people can’t sunbath every day, and because the vitamin D winter is so severe, we recommend 5,000 IU/day on the days you don’t sunbathe.

  About: John Cannell, MD

Dr. John Cannell is founder of the Vitamin D Council. He has written many peer-reviewed papers on vitamin D and speaks frequently across the United States on the subject. Dr. Cannell holds an M.D. and has served the medical field as a general practitioner, emergency physician, and psychiatrist.

11 Responses to Sun exposure: Is just a little bit enough?

  1. kenmerrimanmd says:

    would be an interesting study to repeat in the summer with a bit more skin exposed

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  2. Rita and Misty says:

    Hi Dr. Cannell,

    It appears that the study participants wore sunglasses….

    A while back, I remember reading a theory that sunlight had to be acquired via eyes for full Vitamin D benefits….Of course, I can’t find my source now…I will keep searching.

    I did however find the attached link, which is based upon interpretation of the article “Why Light Matters” by Colleen Huber, NMD



    Rita :)

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  3. Rita and Misty says:

    PS….I would recommend that readers disregard the above link’s advice to acquire Vitamin D thru Cod Liver Oil (CLO) supplementation if exposure to sunlight isn’t possible….

    IMO–CLO contains WAY TOO MUCH Vitamin A…and this can be very detrimental to one’s health (again in my opinion).


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  4. Kate Saley says:


    I agree, a similar study conducted during summer months would be interesting. I’m sure the participants would be much more comfortable! Also, a study comparing vitamin D production at different latitudes would be fascinating.



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  5. Rebecca Oshiro says:

    This past summer I received at least two, and often three, hours per week of full body sun exposure at midday. At the end of August, after three months of this regimen, my 25(OH)D level was 46. This experiment was conducted in Seattle.

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  6. The women did go outside enough, the title says that they recorded how much light each women got. The most likely reasons that the vitamin D levels dropped was that the trial was held in the fall, and there was substantially less sun/vitamin D at the end of the trial,

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  7. “Sun exposure: Is just a little bit enough?”

    Yes it is, according to the Danish Cancer-society (I’m Danish), and according to them, does people who are using sun-screen have more vitamin D than people who don’t !

    I’ve written to them and suggested, that either the sun-screen didn’t work or people who didn’t use it, stayed out of the sun, the never answered me back :(

    Just ask the Danes, and the over-population will be solved in no time, the uninformed will go first :) ..

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  8. Jim Larsen says:

    The mean 25(OH)D was 11.01 ng/ml. The mean personal sunlight exposure recorded by ambulatory lux meter, 292.6 lux/s. Study ran for 4 weeks.

    Half of each arm would be 4.5% of skin area. Head is 4.5%, but sunblock was allowed. So, somewhere around 9%- 13.5% of skin area exposed. The study calculated skin exposure.


    Full study at: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3526721/

    I assume Koreans would be a Fitzpatrick scale score of 3-4?


    a. All participants were D ‘deficient.’ Therefore they would all have substrate starvation (see Holick). 4 weeks with minimal UVB exposure would be inadequate to raise D levels.

    b. UVB exposure was not measured (a lux meter only measures light). UVB varies widely by “Weather, latitude, altitude, air pollution, age, exposed skin area, and use of sunblock may influence vitamin D synthesis in human skin.” And…” serum vitamin D concentration significantly correlated with the score of the sunlight exposure questionnaire, but was not associated with ambulatory lux meter results.”

    c. Holick usually talks to arms & legs exposed. That would be 9+9+18+18 = 54%. Some authors suggest 40% of skin exposed.

    Therefore, the conclusion is (maybe) that exposure of 9-13.5% of D deficient Ss skin for 4 weeks to an unknown amount of UVB results in no significant 25(OH)D changes.

    See also http://www.scribd.com/doc/45847911/D-in-BCT. In this case, Ss would only expose forearms and faces (but wore hats). No sunglasses, but eyeglasses (glass blocks UVB)(have to check plastic lenses). 8-9 weeks intermittent exposure in SC. So, a fair bet that faces and arms is inadequate skin area to get enough UVB.

    “Nude at noon” may be right?

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  9. eewright says:

    Based on my best research, time of the year and hour of the day will give you a pretty close to accurate time to create D on your skin outdoors. Use the Navy chart:


    put in your location and time of year and then look at the altitude column. If it does not show 50 or above you are not at the time of year/or time of the day to have D created on your skin.

    I live at the 38th latitude and between mid-April to Mid-Sept close to noon are when my latitude’s “altitude numbers get to 50 or higher”. That’s when I know I’m making D. Anything under 50 you simply are not creating D on your skin. There is no guessing on this. It’s based on pure science.

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  10. According to the calculator here (http://aa.usno.navy.mil/data/docs/AltAz.php) and using a latitude of 37.28 and longitude of 127.05 and 9 hours east of GMT for Ajou, South Korea, in 2010 the sun was last higher than 50 degrees above the horizon on September 30th. Thus during October and November 2010 it was impossible to manufacture any vitamin D by exposing your skin to the sun in Ajou, South Korea, ie there could not have been any effect of sunlight exposure on serum 25(OH)D levels in these women during these months. This paper should have been rejected by the reviewers. Peter

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  11. GHamilton says:

    My patients often asked me about the basis of my Vit D supplement recommendations, given that I would prescribe amounts much larger than recommended in the medical literature. It was mostly based on observing the changes through lab monitoring, as there is really no good documentation in the literature.

    I felt obliged to do a some “real life” testing, so I did a little experiment a few years ago on my favorite guinea pigs – myself, 2 of my daughters (teens), and one of my nurses. We live in Wisconsin, so no incidental natural sun exposure capable of inducing vit D production for us! We are all Caucasian and of normal weight.

    My nurse and my daughters had no Vit D supplementation for several weeks, then were able to have significant sun exposure during spring break – I do not have baseline values for the girls, but my nurse had a level of about 20. All three had levels in the 60-75 range within days of returning (minimal sunscreen usage).

    My baseline level while taking 10,000 IU daily in the same time period was 46. I discontinued Vit D for a couple of weeks prior to our trip, and likewise spent a week in the Florida sun w/minimal use of sunscreen. My level upon return was 62.

    I concluded that a “normal” physiologic level is around that 60 to 70 mark, achieved by natural exposure w/o supplementation. I also found it interesting that despite taking more than the “recommended” amount of Vit D supplement, my level was only 46. I likewise have found that most of my patients are unable to achieve a level >50 w/supplementation alone (with a few exceptions of course), most ending up in the low to mid 40 range as well (albeit much better than the 10-20 they were at previously).

    Would love to be able to carry this out further using different times of year, tanning beds vs natural sunlight, etc!

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