“Wrong” Sunlight Can Lower Your Vitamin D Levels


 As a regular reader of our daily alerts, you will know that vitamin D is one of the vitamins that has been labelled a “Wonder Vitamin” and it most certainly is one of the few that can actually live up to the expectation.

You will also know that this fat-soluble vitamin is produced by our bodies when the skin is exposed to UV radiation from sunlight. In theory a few minutes exposure to sun each day should generate sufficient vitamin D.

But what would you say if you were told that the “wrong” kind of sunlight can actually lower your vitamin D levels?

Let the sun shine – but not through your window

First, we need to distinguish between the two primary forms of ultraviolet radiation from the sun: UVA, and UVB. These have different wavelengths and impact your body in different ways.

UVA has a wavelength of about 320 to 400 nanometres, and UVB has a wavelength of about 280 to 320 nanometres.( A wavelength is simply the distance a wave has to travel before it starts repeating itself, and a nanometre is a millionth of a meter.)

UVB forms the precursor to vitamin D in the skin, but too much UVB causes sunburn and damages the skin surface. By contrast, UVA provides nearly no benefits while making the skin wrinkled and leathery over time (and it gives that” lovely”brown tan). This damage can also lead to melanoma, the most deadly form of skin cancer.

And UVA isn’t just a summer thing. Unlike UVB, it’s prominent all year round – even on most cloudy days – and UVA passes right through most windows.

And that is where the tricky part comes in.

Window glass will effectively filter out the majority of UVB radiation, but it minimally filters out UVAs.

Why should we care about this? Vitamin D is formed from exposure to UVB rays, whereas UVA radiation actually destroys vitamin D.

Clever trick

That’s just typical! I hear you say.

Well it is indeed typical of how amazing our bodies are: The UVB/UVA ratio in natural sunlight helps keep your body in balance. See it as protective mechanisms your body has to avoid overdosing on vitamin D when you’re outside. However, when you’re exposed to sunlight through windows – in your office, your home or your car – you get the UVA but virtually none of the beneficial UVB.

Overexposure to UVA can lead to significant health problems, because in addition to destroying vitamin D, it also increases oxidative stress. UVA is one of the primary culprits behind skin cancer, and it increases photo-ageing of the skin (photo-ageing is the “nasty” word for tanning). You can actually get vitamin D without significantly darkening your skin, because the UVB wavelength does not stimulate the melanin pigment to produce a tan.

Of course, when you get tanned from outdoor sun exposure you’re getting both UVA and UVB at the same time, so it’s not a problem. But when you are indoors and expose yourself to sunlight filtered through window glass, you are increasing your risk of a variety of conditions, primarily skin cancer, because the UVAs are effectively destroying your vitamin D levels while you’re getting none of the benefits from UVB. Don’t think for one minute sun bedding will factor these risks out. In fact, when you lie under a UVA sunbed (and most of them emit predominantly UVA radiation with small amounts of UVB radiation compared to the ratio in natural sun radiation) you effectively eliminate the vitamin D in your body in a very short period of time… Not good.

Typical! I hear you say again…

Baby’s out with the bathwater 

When UVB strikes the surface of your skin, your skin converts a cholesterol derivative in your skin into vitamin D3. The vitamin D3 that is formed is on the surface of your skin does not immediately penetrate into your bloodstream. It actually needs to be absorbed from the surface of your skin into your bloodstream.

New evidence shows it takes up to 48 hours before you absorb the majority of the vitamin D that was generated by exposing your skin to the sun. So, taking a bath or shower with soap will simply wash away much of the vitamin D3 your skin generated, and decrease the benefits of your sun exposure. So to optimize your vitamin D level, you need to delay washing your body with soap for about two full days after sun exposure.

I am probably not wrong in thinking that there is not many of us that will go without a shower or a bath for two days.

The simple way around this, is to only soap the areas of your body that really needs it: underneath your arms and your groin area.

Keeping it simple

The benefits of vitamin D are staggering, and the optimal way to get your vitamin D is through safe exposure to the sun. But as you can see optimising the benefits of this amazing vitamin gets a bit tricky and there are a lot of pieces to this puzzle.

The key points to keep in mind are:

To optimize your vitamin D levels, get safe outdoor sun exposure or use a safe tanning bed, being careful not to get sunburned.

  • If you choose to avoid the sun or tanning beds completely, then you need to supplement with oral or sublingual vitamin D3. This will also eliminate the whole issue of absorbing the vitamin D3 from your skin. Remember to have your vitamin D levels checked regularly to avoid toxicity.
  • Try not to shower or bath with soap for at least two days, after spending time outdoors in the sun, to maximize the absorption of vitamin D in your skin.

Disclaimer: Bear in mind the material contained in this article is provided for information purposes only. We are not addressing anyone’s personal situation. Please consult with your own physician before acting on any recommendations contained herein.


Tan or No Tan? The Vitamin D Question, by Gailon Totheroh, published online 25.06.08, cbn.com

Sunlight, Vitamin D, and the Innate Immune Defences of the Human Skin , by Michael Zasloff, published online, Journal of Investigative Dermatology (2005) 125, xvi–xvii; doi:10.1111/j.0022-202X.2005.23924.x

Shocking Update – Sunshine Can Actually Decrease Your Vitamin D Levels, by Dr. Mercola, published online 12.05.09, articles.mercola.com

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