How Much Sunshine does it take to make enough Vitamin D??

[frankie]

How much sunshine does it take to make enough vitamin D? Probably more than you think!

POSTED BY: Monica Reinagel, M.S., LD/N | August 10, 2009 | 2:07 PM

There's been a lot of hand-wringing lately about people not getting enough vitamin D. Deficiency is quite common--especially among kids, the elderly, and those with dark skin. And a growing list of diseases and conditions are being linked with vitamin D deficiency.
Regular sun exposure, without sunscreen, causes your skin to produce vitamin D naturally.
How much sun does it take to satisfy your vitamin D requirements?
If you've read anything about this, you've probably seen some vague guidelines, recommending "a few minutes every day." But these recommendations are far too general to be useful. The amount of sun you need to meet your vitamin D requirements varies hugely depending on your location, your skin type, the time of year, the time of day, and even the atmospheric conditions.
A Cool Tool: The vitamin D/UV calculator
Scientists at the Norwegian Institute for Air Research have devised a calculator that will take all those factors into consideration and estimate how many minutes of exposure you need for your skin to produce 25 mcg (the equivalent of 1,000 International Units) of vitamin D.
It's a very cool tool and the best I've seen so far. Be forewarned: It's not the most user-friendly interface and its very easy to enter the wrong information, which makes your results misleading. But once you get past the technicalities, it's very interesting to see how much the answers change when you vary the input.
Here are some tips to help you get good results.
1. Unless you live in one of a few European or African cities listed as options, you'll need to determine your latitude and longitude. A GPS device will tell you your lat/long. You can also find this information on Google Earth. Here's also a list of lat/longs for lots of American cities. VERY IMPORTANT: The program reads all latitudes as N and all longitudes as E. If your latitude is 39 S, enter -39. If your longitude is 76 W, enter -76. (It took me forever to figure out this quirk!) Also, be sure to click the radio button next to your latitude and longitude entries. It is not automatically selected when you fill in these values.
2. You also need to estimate your complexion, and the options include both "blond" and "pale," which is a little confusing. (Maybe it makes more sense to Norwegians). Basically, the six categories are arranged from lightest (pale) to darkest (black). Make your best guess based on that. I have dark eyes and hair so I chose "darker caucasian."
3. Enter the time of day you are going out in the sun, expressed as UTC (aka Greenwich Mean Time). Here is a converter that will convert local time into UTC. The calculator uses a 24 hour clock, so hours from 1pm to midnight are expressed as 13 to 24. There is also an option that lets you choose "about midday." If you enter an actual time, be sure to click the radio button next to your start time. It doesn't move automatically when you fill in these values.
4. Next, enter how much cloud cover there is. If you choose "overcast," you can also enter in the UV Index from your local newspaper's weather page and the program will estimate the thickness of the cloud deck. Again, be sure to click the radio button if you choose this option. (Disclaimer: I played around with this and couldn't see that changing the UV index had any effect...)
5. You're almost done! Next, the calculator wants to know the thickness of the ozone layer. I have to admit, I was unable to find a simple resource to help with this one. Unless you live under the ozone hole or have some other way of knowing that the ozone is particularly thick or thin where you are, I suggest just setting this one to medium.
6. Find your altitude. Some GPS devices can tell you your altitude, as can Google Earth or topographical maps. But the easiest way may be to simply google "altitude of [your town]". Remember to convert it to kilometers. One kilometer = about 3300 feet.
7. Choose your surface.

[Don]

Or, you can do it the easy way and buy one of Solarmeter Steve's Vitamin D meters and get the calculation spreadsheet that Stan Pope designed.

[solarmeter®]

I posted this on their comments board:

There is a lot easier way than that to find real-time D3 IU: The Solartech Model 6.4 Solarmeter® as seen on Welcome to Solartech .
It comes with an interactive excel spreadsheet to adjust for skin type, age, tan level, and percent body exposure.
The calculator from Norway appears too conservative. At this time of year mid latitude USA near solar noon it would take about 15 minutes (not 5) to synthesize 1000 IU on 10% body (face, hands, forearms) for type 2 skin.



Posted by: Steve | August 11, 2009 at 09:09 AM

[stan@sunsys]

Ola Engelsen uses correct methods, but also makes mistakes. For starters, their Vitamin D action spectrum is not very accurate, compared to either the original Holick one, or the current CIE one. Secondly, he attempts to equate Holick's "ballpark" MED requirements to an actual J/m^3 energy requirement. In doing so, he underestimates the solar UV required for a given skin type. Third, he fails to address the issue of human body geometry. His calculations assume a horizontal surface, our bodies are not horizontal surfaces. For this reason alone, actual solar UV exposure times are more than twice as long as currently estimated.

FYI, by my calculations, the time requirement for ~1000 IU of D3 from 10% (area) exposure today in Detroit around solar noon is 20 minutes for a horizontal surface, but 50 minutes for an actual human.

25% (what Engelsen uses with their FastRT tool) actaully requires 10 minutes for a horizontal surface and 20 minutes for a real person (typically ~type 3).

[Don]

Regarding the original question. How much sunlight exposure does it take to stimulate an optimal blood level of vitamin D (25-OH-D). First of all, the words "how much" and "sunlight exposure" must be clarified.

Common sense tells us that IF only 5 - 15 minutes of sunlight exposure (see below) once or twice each week was adequate, we wouldn't have 70% of the public vitamin D "insufficient" in America. And keep in mind that the 70% is calculated using an "adequate" level of 70 nmol/L (28 ng/ml) is used. If a much more realistic minimum level of 120 nmol/L (48 ng/ml) is used, over 90% of the American public have an insufficient level (especially teenagers who are the "target" of the anti-tanning nonsense). Therefore, "common sense" tells us that it takes more (much more) sunlight exposure than 5-15 minutes.

Regarding sunlight exposure, the anti-UVR folks almost never "define" the exposure parameters. Noon? 8:00 am? 3:30 pm? Needless to say, the time of day, the season of the year, and the ambient conditions make a huge difference.

So here is the bottom line (based upon 10 years of research I have done measuring sunlight here in Tucson). At this time of year, it takes 15 minutes PER SIDE (remember, sunlight only hits one side of the body at a time) at noon (here in Tucson) for each exposure session. How many exposure sessions (and how often) are required in order to increase your 25-OH-D level to 120-150 nmol/L depends upon (a) your "baseline" 25-OH-D level, (b) the amount of 7DHC in your skin, (c) your skin type/subtype, (d) your age (old farts like me require about twice the exposure time) and, (e) whether or not you have any disease that would affect the synthesis/metabolism of 25-OH-D.

SO, my calculations show that one or two sessions per week for 4-5 weeks is enough for an individual under 50; two to three sessions per week for 5-7 weeks for someone under 70; and 4-6 sessions per week for 7-10 weeks (at noon in August) will be sufficient.

OR, you can go in a tanning bed for 10-20 minutes once or twice per week (under 50) or 2-3 times per week (over 50) for 3 - 5 weeks and accomplish the same level (120-150 nmol/L). And, you can "maintain" that level with1-2 sessions per week (depending on age).

Finally, there is NO SCIENTIFIC EVIDENCE proving that a 25-OH-D level of 120-150 nmol/L achieved by ingesting a supplement has the same "biological activity" as does the same level stimulated by UVR.

[solarmeter®]

exposure today in Detroit around solar noon is 20 minutes for a horizontal surface

Yeah - I pulled that 15 minutes out of my rear end rather than running the spreadsheet. You're right - at today's UVI of about 7-8 it would be 20 min. If UVI was 10 it would be 14 minutes.

but 50 minutes for an actual human.

Well - maybe if they are standing up like a stick facing east or west and not moving or swinging arms or facing toward then 45° then 90º to the sun - like on a tennis court. Remember... about half the irradiance is diffuse... hitting you from all angles not just toward the sun. Plus the Deff is higher than Eeff near solar noon... so it kind of all balances out closer to 20 (or maybe 30) minutes imo.

[stan@sunsys]

Steve, the "Geometry Conversion..." paper will be published soon. From all those crazy formulas, integrals, vectors, and matrices you will see that for the average irradiance it doesn't really matter what posture you are in, or how much you are moving (for typical behavior). At any given time part of our exposed skin is facing toward the sun, away from the sun, facing up, facing down, and everything inbetween. You'll bee able to see the tables and graphs once it becomes public.

If, at solar noon, you were standing in your back yard just staring at the boating beauties on the lake though, you are going to get more like 30% of the horizontal dose. But how often do you do that... wait, nevermind. LOL

[Don]

If you go back and look at the papers by Matsuoka, et al, you will see that (1) she was using a 360 degree booth (which irradiates BOTH sides of the body), and, (2) she was using FS series sunlamps (which have SIGNIFICANTLY more vitamin D-effective energy). Unfortunately, everyone has forgotten/ignored/failed to appreciate these two facts and, therefore, have underestimated the time it takes to stimulate vitamin D in sunlight or a sunbed.

That's why/where the "erroneous" 5-15 minute numbers came about.