What is Vitamin D?

Vitamin D can be categorized as both a vitamin and a hormone.  It can be consumed from your diet the way a vitamin can (in this case, mostly through fatty fish).  It can also be made by the body, the way a hormone can.  When your skin is exposed to UVB rays from sunlight, it triggers a precursor molecule (7-dehydrocholesterol) which is then turned to Vitamin D.

What does Vitamin D do?

  • Vitamin D facilitates the absorption of calcium and helps metabolize phosphorous, in order to maintain healthy bones.
    • Without it, our bones can become brittle, weak, thin and misshapen.
  • Vitamin D, along with calcium, helps protect against osteoporosis.
  • Vitamin D, has in more recent years, been found to have additional functions including:
    • Neuromuscular and immune function
    • Cell growth
    • Reduction of inflammation

Note: As Vitamin D receptors are being found in more parts of the body, it is likely that it plays other, yet currently unknown, roles in the human body.  

Why are so many deficient, if it’s the “Sunshine Vitamin”?  

  • Peak sun exposure is estimated to be between 10am and 3pm.  However, most people today are indoors or work indoors during the day.  
  • Cloud cover can reduce the UVB rays necessary for Vitamin D formation by 50%.
  • Air pollution can reduce UVB rays by 60%.
  • Windows and glass prevent UVB rays from penetrating.
  • Sunscreen with an SPF of 8 or more, block UVB rays.
  • Other factors that affect the amount of UVB ray exposure are:
    • Seasons
    • Climate
    • Altitude

Sources of Vitamin D

  • Vitamin D comes primarily from sun exposure and a few food sources:
    • Sun exposure: researchers believe that with 5-30 minutes of unprotected sun exposure 2 x per week, one should get sufficient levels.  However, for the reasons mentioned above, many people do not get enough sunshine for adequate levels of Vitamin D.
    • Food sources: fatty fish (e.g. salmon, tuna, mackerel), small amounts are found in organ meats such as beef liver, egg yolks, and some foods are fortified with Vitamin D like milk and yogurt.   


KILIM, H. P., & ROSEN, H. (2018). Optimizing calcium and vitamin D intake through diet and supplements. Cleveland Clinic Journal of Medicine, 85(7), 543-550. doi:10.3949/ccjm.85a.17106

Makhni, E. (2018). The Effect of Abnormal Vitamin D Levels in Athletes. The Permanente Journal. doi:10.7812/tpp/17-216

Office of Dietary Supplements – Vitamin D. (2018, March 2). Retrieved from https://ods.od.nih.gov/factsheets/VitaminD-HealthProfessional/

Daniel Yinh

Daniel Yinh


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