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June 7, 2009 at 9:11 AMComments: 1 Faves: 0

Turning Sunlight into Vitamin D: How the Conversion Happens

By Katie from SLN More Blogs by This Author

Featured on #100daysofhealthy

You've probably heard talk in the media and health circles about the surge of interest in vitamin D. Most sources are very clear about the ways to obtain it and the benefits it has on bone and immune health. But do you know what happens in between?

How exactly does the human body turn sunlight into vitamin D? It's actually a pretty straight-forward process. Vitamin D is a fat-soluble vitamin that is difficult to get in sufficient quantities through diet. Because of this, exposure to the sun's ultraviolet light (UV) rays is the best way to obtain vitamin D.

How it Works:

  1. When UV rays occur at about 300 nanometers, and your bare skin is exposed to them without the inhibition of sunscreen, vitamin D synthesis can take place.
  2. The UV rays penetrate the epidermal layer of your skin, which contains 7-dehydrocholesterol. This substance exists in large amounts in the epidermal layer of most vertebrates. The reaction of the UV rays with the 7-dehydrocholesterol converts it to previtamin D.
  3. Previtamin D spontaneously changes into vitamin D. At room temperature, the alteration can take up to 12 days.
  4. After vitamin D is produced (or eaten), the liver and kidneys convert it to calcitriol, which is released into your blood circulation. The calcitriol then binds to carrier proteins and is transported to various organs, tissues, and bones.


There are a few factors that act as obstacles to vitamin D synthesis. Northern latitude locations, cloudy atmospheres, darker seasons, and pollution can all get in the way of direct sunlight. Additionally, people with higher concentrations of melanin in their skin (darker skin) need more time to absorb enough sunlight for vitamin D synthesis. This is because melanin acts as a light filter, undoubtedly as an evolutionary result of dark skinned people tending to live in warmer and sunnier climates. The light filter makes it harder for the UV rays to penetrate the epidermal layer, thereby allowing for less reaction with 7-dehydrocholesterol.

Of course, there are more in-depth details about the essential process of vitamin D synthesis, but we hope that this basic explanation is enough to give you sense of the importance of sunlight for your bone and immune health. Remember not to underestimate regulating your time in the sun, as too much UV is not necessarily a good thing. But let yourself enjoy and benefit from the warmth of our favorite star in the summer months!


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1 Comment

  • I talked to a pain doctor who raved about the benefits of Vitamin D3. However, as much as he is a proponent of getting the D from sunlight he makes a point that at our somewhat northern latitude we need to catch the rays between 10:00AM and 2:00PM for optimal absorption.

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