Home / Skin Cancer  / Vitamin D Holiday?

We all know that we aren’t going to see a lot of sun in Seattle after October.  It sometimes feels like it will be July (or August) before it comes back.  Since exposure to sunlight is one way that our body makes Vitamin D, it is worth talking about what this means for us in Seattle.

Why does Vitamin D matter?

In 2010 the Institutes of Medicine looked at 1000 different studies on the relationship of Vitamin D and health.  The main findings were:

As we’ve known for a long time, Vitamin D is essential for maintaining healthy bones.  This is particularly critical for women.  Supporting bone health with adequate levels of Vitamin D helps prevent osteoporosis or brittle bones, leading to fewer fractures in post-menopausal women.  Since hip fractures are a major cause of mortality in elderly women, this is a big deal.

No conclusions about other health effects could be made.  This was a major diapppointment.  Various studies have suggested that low levels of Vitamin D are related to all kinds of internal malignancies like ovarian and colon cancer, depression, heat disease, and stroke.  Unfortunately, these studies have not been well done nor large enough in scale to be conclusive.   There is current a well designed, randomized controlled trial (the type of study that is most likely to actually prove something) involving 20,000 people that hopes to determine whether Vitamin D supplementations affects cancer, heart disease, and stroke.

Background: How Vitamin D is made

There are two major sources of Vitamin D knows as D2 and D3.  These are converted by our liver into something called 25-hydroxyvitamin D and then converted by our kidneys into 1,25-dihydroxyvitamin D3.  This is the stuff that actually does something in our body.

Vitamin D2 and D3 can be found in some animal products, mostly oily fish like salmon and cod.  Beef liver and egg yolks have a little bit as well.  If you are a vegetarian, there are few natural sources of Vitamin D, except for shitake mushrooms.  Here are some Vitamin D contents of different foods:

Fresh wild salmon                             3.5 oz     600-1000 IU

Fresh farmed salmon                        3.5 oz     100-250 IU

Canned salmon                                   3.5 oz     300-600 IU

Cod Liver Oil                                        1 tsp       400-1000 IU

Fresh shitake mushrooms               3.5 oz    100 IU

Sun-dried shitake mushrooms       3.5 oz    1600 IU

Egg Yolk                                                1             20 IU

8 oz glass of milk                                               100 IU

Our skin can make Vitamin D3 when it is exposed to ultraviolet radiation.  The ideal wavelength is from 290-320.  Sun exposure of the whole body leading to a sunburn can generate about 10-25,000 IU per day.  A more realistic scenario showed that exposing 25% of the body to the sun at noon for 3-8 minutes creates about 400 IU of Vitamin D.

Limits to Vitamin D production in the skin

Time:  The initial exposure of the skin to sun leads to a burst of Vitamin D production, but after awhile, and certainly by the time a sunburn occurs, Vitamin D starts to be destroyed in the skin.

Skin color.  Anyone with darker skin will make much less Vitamin D from the sun.  This is because the melanin pigment in the skin absorbs a lot of the ultraviolet radiation needed for Vitamin D production.

Angle of the Sun.  The UV index has to be > 3 for Vitamin D to be made.  The UV index is a measure of solar intensity, or how strong the sun’s rays are.  This is influenced by the season and latitude.  When the sun is directly overhead, the solar intensity is much greater, but when it is lower in the horizon, it tends to remain quite low.  Between the tropics, there will be a high solar intensity year round.  But up here in Seattle, the solar index is low all winter long.   A study done in Boston which is at 42.2 deg N showed that people cannot make ANY Vitamin D from Nov to Feb.  In Edmonton which is at 52 deg N, the Vitamin D winter lasts from Oct to Mar. Seattle is at 47.3 degrees, meaning our Vitamin D holiday is going to be similar to these cities.

Sunlight , Vitamin D, and Cancer

For a dermatologist, the main conundrum with Vitamin D is that the wavelengths of ultraviolet light that cause its production are also the most carcinogenic.  For this reason, we don’t recommend using sun exposure to get your Vitamin D and instead push sunscreen.  This has led to concerns that our rabid sunscreen recommendations are leading to an epidemic of Vitamin D deficiency.

In an initial study, it was found that whole body application of sunscreen could lead to  a 95% decrease in Vitamin D production.  This caused a lot of concern, but hasn’t been reproduced in real world conditions.  This is likely because the laboratory study of regular, uniformly thick application of sunscreen is just not really all that relevant.  Maybe in the stone age when people wore a lot less clothing, it might be relevant, but except for the Solstice Day Parade, I rarely see naked people outside in Seattle.

Another study showed that wearing long sleaved pants and shirts does have a measurable impact.  This I think, is realistic since most people do wear clothes most of the time.  The parts of us that are exposed, our hands, neck, and face represents only a small amount of skin, not enough to produce a lot of Vitamin D, yet still the prime areas to develop skin cancer.  Real world studies between regular sunscreen users and sunscreen avoiders haven’t shown statistically significant difeerent levels of Vitamin D, probably because the skin just isn’t that effective at maintaining Vitamin D levels.

How to get your Vitamin D.

From the sun?

I think this is a really bad choice for several reasons:

  1. It doesn’t work at all in Seattle all winter long no matter how much time you are outside.  So if you depend on the sun for your Vitamin D in Seattle, you will be deficient for at least half the year.
  2. It doesn’t work very well.  In studies of Vitamin D deficiency, sun exposure has failed to lead to correction .


It doesn’t matter whether its from a tanning bed or from the sun, the outcome of intentional sun exposure is the same:  more skin cancer and rapid aging of the skin.  Regular, high intensity tanning probably will increase your Vitamin D levels, but it still doesn’t justify carcinogenic exposure.  If Vitamin D were in cigarettes, would you start smoking?


Really, the ideal way to get Vitamin D is with a pill.  A year’s supply of 2000 IU pills is usually about $30.  This is simple, and reliable.  The question is how much.

Not knowing whether Vitamin D does anything else important, current recommendations are based on what it takes to prevent osteoporosis since we know this matters.  The current recommendations are for supplmenation of 600 IU per day for people < 70 yo and 800 IU per day for people over the age of 70.

Many recommend higher amounts, including me.  I make this recommendation primarily based on 2 factors:

  1. It seems really hard to get to toxic levels.  Doses < 4000 IU per day seem very safe, and one study looked at 10,000 IU per day and didn’t find any problems.  At high levels of Vitamin D peole get nausea, vomiting, poor appetite, and other symptoms.
  2. Its likely that everyone in Seattle is deficient unless you do take a supplement since we don’t make any all winter long, and even though our summers are beautiful, there isn’t a lot of bikini weather around here.

So, I recommend taking 2000 IU per day.

I seldom test Vitamin D levels in patients, primarily because its very likely that insurance would fail to cover the test if I order it and patients would be stuck with the bill.  It’s a reasonable test to order, but I think that talking to your primary care doctor and checking it along with your other preventative health screening tests is in your interest.  I think its especially worth checking if you are Asian, Latino, or African American because its very likely that you are deficient since your skin pigmentation further hinders summertime Vitamin D production, you can’t make any in the winter, and because the incidence of lactose intolerance is much higher so there is less opportunity to get Vitamin D from fortified foods.

No Comments
Post a Comment

Keep in Touch

Join our mailing list to be notified via email of our latest specials, new products and services, and more.

Schedule an Appointment

We are taking virtual and emergency appointments during the covid-19 quarantine. Visit our schedule page for more info. You can also schedule via phone, at (425) 654-1275.