How Vitamin D Levels Change With the Seasons


Vitamin D3, also known as cholecalciferol, is synthesized in human skin upon exposure to sunlight, when ultraviolet (UV) rays convert 7-dehydrocholesterol to previtamin D3 (which is subsequently modified in the liver and kidneys to form the biologically active hormone). The amounts of vitamin D3 produced, then, becomes a function of the amount of sunlight that comes in contact with the skin and how well the skin is able to synthesize the vitamin precursor. Cloud cover, shade, seasonal climates, and the clothes that cover skin (which often depend on climate) all become responsible for promoting or impeding endogenous vitamin D production.

Related: Vitamin D and Race

Is More Always Better?

Greater exposure to sunlight, especially in locations or during times when intensity is highest, will necessarily lead to greater vitamin D production. In this case, “more” is better than its alternative but should also be experienced in moderation.

Residing in a region that offers exposure to the sun is significantly better for preventing vitamin D deficiency than residing in a region where sun exposure is rare. However, while sunlight is necessary for vitamin D production, it also exposes individuals to UV radiation, a carcinogen known to promote DNA damage and increase the risk of skin cancer. For this reason, the National Institutes of Health (NIH) recommend limiting exposure to sunlight as a preventative measure against skin cancer, and further recommend wearing protective clothing or applying a sunscreen with an SPF (Sun Protecting Factor) rating of 8 or higher when out in the sun for more than several minutes. According to the National Library of Medicine (NLM), just 10 – 15 minutes a day of casual sun exposure (in the face and hands), for three days/week, is enough to provide adequate amounts of vitamin D.

Those who avoid sunlight or reside in areas where sunlight is limited should be sure to include vitamin D in their diets or take supplements. Additionally, those who live in high latitude areas (further from the equator, such as North America), will likely get diminished sunshine, especially during the winter. During that time, a well balanced diet and multivitamin supplementation can provide adequate amounts of the nutrient.

  • Header Image: Angelo Juan Ramos (Flickr)
  • Vitamin D – Office of Dietary Supplements of The National Institutes of Health
  • Vitamin D – National Library of Medicine of the National Institutes of Health