The Verdict on Vitamin D: Why You Might Not Be Getting Enough

Imagine it’s a sunny summer afternoon. You decide to go outside to get your dose of vitamin D. Maybe the sun doesn’t impact your skin much, or you seem to burn within minutes of sun exposure. Maybe you fall between those extremes. Getting out in the sun can positively impact vitamin D levels, but spending time outside won’t necessarily result in optimal vitamin D levels. Individuals that get outside might think the dose of sun they receive eliminates the need for a vitamin D supplement, but understanding the complexities of Vitamin D production is critical to deciding whether a supplement is necessary.

Vitamin D is an essential vitamin directly and indirectly involved in the regulation of up to 1,250 genes.¹ It helps the body maintain appropriate serum calcium levels, balance phosphorus levels, aids in the specialized function of cells through cellular differentiation, and supports the immune system, among other functions. Low serum levels of vitamin D create a host of issues including rickets, a bone disease in children, or osteomalacia (weakened bones) in adults.² Muscle weakness and pain are also consequences associated with insufficient vitamin D levels.¹

Vitamin D is found in a very limited number of foods, which includes some fish, fish liver oils, and mushrooms exposed to ultraviolet light.¹ Dairy products, cereal, and fruit juices may also be fortified with some amount of vitamin D.¹ It may also be obtained through supplementation, but keep in mind there are two main forms of vitamin D supplements - vitamin D2 and vitamin D3. Vitamin D3 is also known as cholecalciferol and these supplements are made by extracting and irradiating lanolin from the wool of sheep. A plant-based version of D3 is also available, which is derived from lichen.²

On the contrary, vitamin D2, known as ergocalciferol, is another plant-based formula sometimes obtained from alfalfa.³ You'll be able to identify the plant-based version if you see ergocalciferol on the label of your supplement, or if you verify the source of vitamin D with the manufacturer. When choosing a supplement, opting for vitamin D3 is preferred. Even though both D2 and D3 can increased serum vitamin D levels, D3 is more effective at increasing serum vitamin D levels when compared with vitamin D2.⁴

Depending on your geographical location, you may be overestimating the vitamin D you are producing. The angle of the sun is an essential factor to ponder when considering whether you get enough vitamin D from sun exposure. Vitamin D is only produced by the skin during certain times of the year, which depends on location. Northern latitudes do not receive the optimal sun angle for vitamin D production for many months of the year. So despite getting sun exposure during non-optimal times of the year, an individual might still need to supplement with vitamin D.

Vitamin D is produced in the skin through a biochemical reaction where sunlight interacts with molecules in the epidermis.1 However, it’s not as simple as making time to get out in the sun. The skin can only synthesize vitamin D with exposure to certain wavelengths of solar ultraviolet-B radiation, which is impacted by geographical location and latitude.¹ For example, in latitudes around 40 degrees North and 40 degrees South, the sun’s UVB wavelengths are not sufficient to promote vitamin D production in the skin between November and early March. A latitude around 50 degrees North, such as in Edmonton, Canada, shortens that interval, where the sun does not provide sufficient radiation between mid-October and mid-March.¹

When the time of year for optimal vitamin D production finally comes, determining how much time to spend in the sun can be tricky. Data indicates that if an individual in Boston, Massachusetts exposes approximately 25% of their skin to the sun for 3 to 8 minutes, they would produce around 400 IUs of vitamin D. Someone in Miami, Florida would need to spend 3 to 6 minutes in the sun to produce the same amount. The degree of vitamin D production is also dependant on skin type, where the previously referenced timeframes are applicable to people with Fitzpatrick Type III skin, and individuals with lighter or darker skin would need less or more time in the sun, respectively.⁵ It may be difficult to determine the precise timing of sun exposure based on specific latitude, skin type, and weather patterns. However, some experts estimate that 5 to 30 minutes of sun exposure between 10 am and 4 pm at least twice a week is correlated with sufficient vitamin D levels.² Individuals might also use personal indicators to determine how much time to spend in the sun for vitamin D production. The amount of sunlight that produces a slight pinkness of the skin is estimated to be equivalent to 10,000 to 25,000 IU of oral vitamin D intake.¹

After sun exposure, a vitamin D precursor forms within the epidermal layer of the skin. This substance must be activated. The precursor to active vitamin D, called pre D3, gets converted to D3 (cholecalciferol) and is absorbed into the body. Interestingly, pre D3 also gets metabolized within the skin to other constituents that can be converted back to pre D3 in the dark.⁷ After the molecule is absorbed, the liver modifies the precursor to 25-hydroxylated-D (25(OH)D). The kidneys later provide the final edits that cause the vitamin D to become the active form, 1,25-hydroxylated vitamin D (calcitriol).^1,7 Those with kidney or liver disease may need more frequent monitoring of vitamin D levels, or a specialized vitamin D supplement, due to the involvement of the kidney and liver in the activation of vitamin D. Liver and kidney disorders have been associated with insufficient vitamin D levels.^2,8

Understanding how vitamin D is produced can help you make decisions on the need for vitamin D testing or supplementation. Using your location, health status, and skin type as a reference, you can make better decisions about when to supplement vitamin D during the year. Whether you choose to supplement with vitamin D only during the winter months or year-round, your body will thank you for adequately supporting its need for vitamin D.

• References

  1. Linus Pauling Institute. (n.d.). Vitamin D. Oregon State University. Retrieved April 16, 2025, from https://lpi.oregonstate.edu/mic/vitamins/vitamin-D

  2. Office of Dietary Supplements. (2020, March). Vitamin D: Health professional fact sheet. National Institutes of Health. https://ods.od.nih.gov/factsheets/VitaminD-HealthProfessional/

  3. National Center for Biotechnology Information (2025). PubChem Compound Summary for CID 5280793, Ergocalciferol. Retrieved April 17, 2025 from https://pubchem.ncbi.nlm.nih.gov/compound/Ergocalciferol.

  4. van den Heuvel, E. G., Lips, P., Schoonmade, L. J., Lanham-New, S. A., & van Schoor, N. M. (2024). Comparison of the Effect of Daily Vitamin D2 and Vitamin D3 Supplementation on Serum 25-Hydroxyvitamin D Concentration (Total 25(OH)D, 25(OH)D2, and 25(OH)D3) and Importance of Body Mass Index: A Systematic Review and Meta-Analysis. Advances in nutrition (Bethesda, Md.), 15(1), 100133. https://doi.org/10.1016/j.advnut.2023.09.016

  5. Terushkin, V., Bender, A., Psaty, E. L., Engelsen, O., Wang, S. Q., & Halpern, A. C. (2010). Estimated equivalency of vitamin D production from natural sun exposure versus oral vitamin D supplementation across seasons at two US latitudes. Journal of the American Academy of Dermatology, 62(6), 929.e1–929.e9299. https://doi.org/10.1016/j.jaad.2009.07.028

  6. Savoye, I., Olsen, C. M., Whiteman, D. C., Bijon, A., Wald, L., Dartois, L., Clavel-Chapelon, F., Boutron-Ruault, M. C., & Kvaskoff, M. (2018). Patterns of Ultraviolet Radiation Exposure and Skin Cancer Risk: the E3N-SunExp Study. Journal of epidemiology, 28(1), 27–33. https://doi.org/10.2188/jea.JE20160166

  7. Bikle DD. Vitamin D: Production, Metabolism and Mechanisms of Action. [Updated 2021 Dec 31]. In: Feingold KR, Ahmed SF, Anawalt B, et al., editors. Endotext [Internet]. South Dartmouth (MA): MDText.com, Inc.; 2000-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK278935/

  8. Iruzubieta, P., Terán, Á., Crespo, J., & Fábrega, E. (2014). Vitamin D deficiency in chronic liver disease. World journal of hepatology, 6(12), 901–915. https://doi.org/10.4254/wjh.v6.i12.901

Created April 2025.

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