The Sunshine Supplement: Understanding Vitamin D and the Sun

March 6, 2023
15 MIN

Typically, we get vitamin D from three sources – from food, from sunshine, and from nutritional supplements. Achieving an adequate amount of vitamin D from our diet is not an easy task, because most of us don’t consume enough of the necessary foods that can provide us with an optimal amount of vitamin D.

Foods naturally containing good amounts of vitamin D3:

  • Fatty fish – salmon, tuna, mackerel, sardines, herring, halibut
  • Beef liver
  • Cod liver oil
  • Cheese
  • Eggs
  • Mushrooms (vitamin D2)

Food often fortified with vitamin D (but read the label):

  • Milk
  • Orange juice
  • Soy milk
  • Cereals

There is a lot of misconception about sunlight and vitamin D. Spending a few minutes in the sun doesn’t mean your body will produce sufficient vitamin D. There are several factors that impact vitamin D production from the sun, and it’s important we understand how our body actually produces the vitamin.

How our bodies synthesize vitamin D from the sun

To begin synthesizing vitamin D, your skin needs to be exposed to UV rays from the sun. Although the process of absorbing the sun’s rays to make vitamin D sounds simple enough, there is a lot going on behind the scenes.

The body specifically requires UVB rays to begin the process. When UVB rays come into contact with your skin, it starts a chemical reaction.

The energy from the UVB rays turns a natural substance found in our skin (7-dehydrocholesterol) into a precursor to vitamin D3 (called cholecalciferol).

This pre-vitamin D is sent through the bloodstream to the liver. In the liver, the body converts cholecalciferol into 25-hydroxyvitamin D3 (25-OHD). From the liver, 25-OHD is sent to the kidneys where it is converted into active 1,25-dihydroxyvitamin D3 that is ready to be used by the body.

Although it sounds simple enough, there are several factors that can limit our body effectiveness at producing an optimal amount of vitamin D.

Factors that inhibit vitamin D production

First, in order to get the sun’s UVB rays, you need to actually spend time outdoors. Sun exposure from sitting near an office window or through a closed car window does not count. Glass windows block UVB rays, so your body will not get any of its benefits.

Another factor is skin tone. Darker skin tones have higher amounts of melanin, and melanin does a really good job of absorbing UVB rays.

That’s good for overall skin protection, but unfortunately it inhibits the chemical reaction that creates vitamin D. So, the darker your skin tone, the more likely you could have a vitamin D deficiency.

Where you live is an important factor because season length, altitude, and air pollution all impact how much sun you get.

Although most individuals should get the amount of sunlight they need to make adequate vitamin D during the summer months just by spending 10-20 minutes outside with face and arms exposed, winter is a different story.

Depending on where you live and your lifestyle, sunlight can be difficult to come by. Even if you do spend time outdoors during the winter, the further north you live, the fewer the UVB rays that reach the earth’s surface.

So if you live anywhere in the northern half of the United States, then you probably aren’t getting enough sun exposure during the winter to make enough vitamin D. For example, if you live in Boston, which is at 42 degrees latitude, you won’t make vitamin D between November and February.

Altitude is also important for sunlight absorption. Because the atmosphere is thinner at higher altitudes, less sunlight is blocked and more UVB rays will reach your skin. For example, at the base camp of Mount Everest, about five times more vitamin D is synthesized in the skin compared to someone at sea level at the same latitude.

Air pollution is another inhibitor of vitamin D synthesis because high levels of air pollution block the sun’s UVB rays. So even if you live in a city that typically gets a lot of sun, if it has a high rate of air pollution, then you will get less UVB ray exposure and likely less vitamin D.

Other factors that inhibit the skin’s production of vitamin D include the use of sunscreen, aging, and wearing clothing that covers the majority of the skin. These are all reasons why it is currently estimated that 42% of U.S. adults have a sub-optimal level of vitamin D.

Checking for vitamin D deficiency

Having an optimal vitamin D level is important for overall health because vitamin D provides support for a variety of bodily functions. These functions include the body’s capacity to support calcium absorption and bone health, as well as vitamin D’s role in supporting immune function, a healthy inflammatory response, and muscle function.

On the flip side, low levels of vitamin D are associated with a higher risk of various chronic health concerns, including heart disease, diabetes, cancer, and osteoporosis.

If one of the following applies to you, then it is a good idea to check your vitamin D level. You should check your vitamin D level if you:

  • Do not get daily sun exposure
  • Live in a northern latitude
  • Do not eat an adequate amount of vitamin D-containing foods
  • Follow a strict vegan diet
  • Have dark-pigmented skin
  • Have issues with small intestine absorption or kidney function
  • Are an athlete
  • Are overweight or obese
  • Are elderly

How to test vitamin D levels from the comfort of your home

If you are worried because it’s winter and you haven’t seen the sun in days or because the other factors might put you at risk for a vitamin D deficiency, then consider testing your vitamin D level.

Thorne’s at home vitamin D test kit is a simple at-home test that will measure your total vitamin D level to ensure it’s at an optimal level for good health.

Collect a bio sample via a simple finger-stick blood test and return it with prepaid shipping. In about 8-10 days, you will have your results available online, along with personalized lifestyle recommendations on what you can do to maintain or improve your vitamin D level.

Pro Tip:
Order any Thorne product(s) from the links on this page and receive 5% OFF and FREE SHIPPING on all future orders. (When you create your account you will be prompted to confirm RYSE Chiropractic as your referral. Discount applied in cart after account created. View Professional Connection on your Thorne Profile page.) You can also visit our dispensary here.
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The information, including but not limited to, text, graphics, images and other material contained on this website are for informational purposes only. No material on this site is intended to be a substitute for professional medical advice, diagnosis or treatment. Always seek the advice of your physician or other qualified health care provider with any questions you may have regarding a medical condition or treatment and before undertaking a new health care regimen, and never disregard professional medical advice or delay in seeking it because of something you have read on this website.
* These statements have not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure, or prevent any disease.

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Guest Author
Guest Author
Guest Contributor
Originally published in
Thorne's Take 5 magazone
contributed by
Jacob Terranova
Orignial Publication Date:
March 15, 2019
This post was last updated on:
March 6, 2023


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