The most significant source of Vitamin D for most humans is not food, but the sun. The human body has an elegant and ancient mechanism for manufacturing its own supply through a photochemical process in the skin. This endogenous production provides the majority of the Vitamin D required for health, but its efficiency is governed by a complex interplay of environmental, geographical, and personal factors. 1
The Photochemical Process: Crafting Vitamin D3 from Light
The synthesis of Vitamin D in the skin is a multi-step process initiated by a specific spectrum of solar energy.
1. UVB Absorption: The process begins when solar Ultraviolet B (UVB) radiation, specifically within the wavelength range of 290–315 nanometers, penetrates the outer layers of the skin, primarily the epidermis.1
2. Conversion to Previtamin D3: This UVB energy is absorbed by a precursor molecule called 7-dehydrocholesterol (7-DHC), which is abundant in the skin’s keratinocytes and fibroblasts. The absorption of UVB photons causes the chemical bonds in 7-DHC to break and rearrange, transforming it into previtamin D3. 2
3. Isomerization to Vitamin D3: Previtamin D3 is thermally unstable and, over the course of several hours at body temperature, undergoes a heat-dependent rearrangement (isomerization) to become the stable form, Vitamin D3, also known as cholecalciferol. 3
From the skin, Vitamin D3 enters the bloodstream and travels to the liver, where it is converted into 25-hydroxyvitamin D, the primary circulating form of Vitamin D and the compound measured in blood tests to assess a person’s status. The kidneys then convert 25(OH)D into the biologically active hormone, 1,25-dihydroxyvitamin D, or calcitriol. 2
Crucially, the body has a built-in safety mechanism that prevents Vitamin D toxicity from prolonged sun exposure. The same UVB rays that initiate the synthesis of previtamin D3 also act to degrade both previtamin D3 and Vitamin D3 into biologically inactive photoproducts. This self-regulating feedback loop ensures that once an optimal amount has been produced, further sun exposure will not lead to an overaccumulation of the vitamin. 3
Factors Modulating Synthesis: A Complex and Variable Equation
While sunlight is a powerful source, its ability to generate Vitamin D is highly variable and unreliable for many individuals. Numerous factors can dramatically influence or even completely halt this vital process.
Geographic and Environmental Factors
The quantity and quality of UVB radiation reaching the Earth’s surface is the single most important determinant of Vitamin D synthesis potential. 1
1. Latitude and Season: The angle of the sun in the sky, known as the solar zenith angle, is critical. During winter months at latitudes above approximately 35° North or South (a line that runs through cities like Atlanta, Los Angeles, and Tokyo), the sun’s rays strike the Earth at too oblique an angle. This causes the atmosphere to absorb or scatter the vast majority of UVB photons before they can reach the ground. This phenomenon creates a “Vitamin D winter,” a period of several months where it is biologically impossible to produce any Vitamin D from sun exposure, regardless of the time spent outdoors. 3
2. Time of Day: UVB radiation is most intense when the sun is highest in the sky, typically between 10 a.m. and 3 p.m. Sun exposure during the early morning or late afternoon is far less effective for Vitamin D synthesis.
3. Altitude: At higher altitudes, there is less atmosphere to filter the sun’s rays. Consequently, UVB radiation is more intense, and Vitamin D production is more efficient. For example, studies have shown that at the same latitude, Vitamin D synthesis can be two to four times greater at an altitude of 3,400 meters compared to near sea level. 3
4. Atmospheric Conditions: Cloud cover can significantly reduce UVB penetration, with thick clouds capable of blocking over 90% of UVB rays. Similarly, air pollution and stratospheric ozone levels can absorb and scatter UVB, diminishing the amount that reaches the skin. 1
Personal and Behavioral Factors
Individual characteristics and modern lifestyle choices also play a major role in determining Vitamin D production.
1. Skin Pigmentation: Melanin, the pigment that determines skin color, is a highly effective natural sunblock. It competes with 7-DHC for the absorption of UVB photons. As a result, individuals with darker skin (higher melanin content) require significantly longer sun exposure to produce the same amount of Vitamin D as individuals with lighter skin. The necessary exposure time can be three to five times longer, placing populations with darker skin at a much higher risk of deficiency, especially when living in higher latitudes. 4
2. Age: The concentration of the precursor 7-DHC in the skin declines with age. An individual over the age of 70 may produce only 25% of the Vitamin D from the same amount of sun exposure as a young adult. This physiological decline is a major contributor to the high rates of Vitamin D insufficiency seen in the elderly. 4
3. Sunscreen Use: Sunscreens are specifically designed to block UVB radiation to prevent sunburn and reduce the risk of skin cancer. When applied correctly and liberally, a sunscreen with an SPF of 15 or higher can reduce Vitamin D synthesis by more than 95%. 3
4. Clothing and Lifestyle: Modern life is predominantly lived indoors. Office work, indoor recreation, and cultural or religious clothing practices that cover most of the skin surface area severely limit the opportunity for UVB exposure, contributing significantly to the global deficiency problem 5
The science of cutaneous Vitamin D synthesis reveals a significant public health paradox. Dermatological organizations rightfully advocate for sun protection measures, including seeking shade, wearing protective clothing, and using sunscreen, to mitigate the well-established risks of skin cancer. 1 However, the very UVB radiation that poses this risk is also the essential catalyst for Vitamin D production. 1 Consequently, individuals who diligently follow sun safety guidelines are inadvertently placing themselves at a substantially higher risk of Vitamin D deficiency. This inherent conflict suggests that public health messaging cannot be one-dimensional. A more nuanced approach that balances the risks and benefits of sun exposure is needed, but for many, this paradox underscores the necessity of turning to dietary sources as a more reliable and controllable strategy.
Furthermore, the data on geography reveals a profound reality: for a large portion of the world’s population, a person’s location on the planet is a non-negotiable risk factor for Vitamin D deficiency. The existence of the “Vitamin D winter” in regions north of 35° latitude means that for millions of people in Canada, the northern United States, and most of Europe, obtaining sufficient Vitamin D from the sun is physically impossible for three to five months of the year. 3 This elevates the issue beyond individual lifestyle choices to a population-level health challenge dictated by planetary physics. For these populations, dietary and supplemental Vitamin D are not merely helpful additions; they are essential tools to prevent a predictable, seasonal decline into deficiency and its associated health consequences, such as an increased risk of falls and fractures in older adults during winter months. 6
Sources
1. THE VITAMIN D HORMONE: A MULTITUDE OF ACTIONS …, accessed October 19, 2025 https://pmc.ncbi.nlm.nih.gov/articles/PMC4384440/
2. Vitamin D Deficiency: Causes, Symptoms & Treatment – Cleveland Clinic, accessed October 19, 2025, https://my.clevelandclinic.org/health/diseases/15050-vitamin-d-vitamin-d-deficiency
3. Recognition and Management of Vitamin D Deficiency | AAFP, accessed October 19, 2025, https://www.aafp.org/pubs/afp/issues/2009/1015/p841.html
4. sources, physiological role, biokinetics, deficiency, therapeutic use, toxicity, and overview of analytical methods for detection of vitamin D and its metabolites – PubMed, accessed October 19, 2025, https://pubmed.ncbi.nlm.nih.gov/35575431/
5. Vitamin D: Beyond Traditional Roles—Insights into Its Biochemical Pathways and Physiological Impacts – MDPI, accessed October 19, 2025, https://www.mdpi.com/2072-6643/17/5/803
6. Global prevalence and disease burden of vitamin D deficiency: a roadmap for action in low- and middle-income countries – CDC Stacks, accessed October 19, 2025, https://stacks.cdc.gov/view/cdc/89677
7. [Vitamin D2 or vitamin D3?] – PubMed, accessed October 19, 2025, https://pubmed.ncbi.nlm.nih.gov/18406498/
8. Physical Determinants of Vitamin D Photosynthesis: A Review – PMC, accessed October 19, 2025, https://pmc.ncbi.nlm.nih.gov/articles/PMC7839826/
9. Vitamin D and skin physiology: a D-lightful story – PubMed, accessed October 19, 2025, https://pubmed.ncbi.nlm.nih.gov/18290718/
10. Sunlight and Vitamin D: A global perspective for health – PMC – PubMed Central, accessed October 19, 2025, https://pmc.ncbi.nlm.nih.gov/articles/PMC3897598/
11. Factors Influencing Vitamin D Status | HTML | Acta Dermato-Venereologica, accessed October 19, 2025, https://www.medicaljournals.se/acta/content/html/10.2340/00015555-0980
12. Ask the Doctors – How much sunshine do I need for enough vitamin D? – UCLA Health, accessed October 19, 2025, https://www.uclahealth.org/news/article/ask-the-doctors-round-sun-exposure-vital-to-vitamin-d-production
13. CDC’s Second Nutrition Report: Vitamin D Factsheet, accessed October 19, 2025, https://www.cdc.gov/nutrition-report/media/2nd-Nutrition-Report-Vitamin-D-Factsheet.pdf
14. Physical Determinants of Vitamin D Photosynthesis: A Review – Oxford Academic, accessed October 19, 2025, https://academic.oup.com/jbmrplus/article/5/1/e10460/7486276
15. od.nih.gov, accessed October 19, 2025, https://ods.od.nih.gov/factsheets/VitaminD-HealthProfessional/#:~:text=Vitamin%20D%20is%20found%20naturally,most%20of%20the%20vitamin%20D.
16. Appendix E-3.3: Meeting Vitamin D Recommended Intakes in USDA …, accessed October 19, 2025, https://health.gov/sites/default/files/2019-09/Appendix-E-3.3.pdf
17. Appendix E-3.3: Meeting Vitamin D Recommended Intakes in USDA Food Patterns, accessed October 19, 2025, https://odphp.health.gov/our-work/nutrition-physical-activity/dietary-guidelines/previous-dietary-guidelines/2015/advisory-report/appendix-e-3/appendix-e-33
18. Vitamin D2 vs. D3: Differences, benefits, sources – Medical News Today, accessed October 19, 2025, https://www.medicalnewstoday.com/articles/vitamin-d2-vs-d3
19. Comparison of vitamin D2 and vitamin D3 supplementation in …, accessed October 19, 2025, https://pmc.ncbi.nlm.nih.gov/articles/PMC3349454/
20. An Evaluation of the Vitamin D3 Content in Fish: Is the Vitamin D …, accessed October 19, 2025, https://pmc.ncbi.nlm.nih.gov/articles/PMC2698592/
21. Foods High in Vitamin D3 – WebMD, accessed October 19, 2025, https://www.webmd.com/diet/foods-high-in-vitamin-d3
22. vitamin D content – USDA National Nutrient Database for Standard ReferenceRelease 28, accessed October 19, 2025, https://ods.od.nih.gov/pubs/usdandb/VitaminD-Content.pdf
23. Which foods are high in vitamin D? – British Heart Foundation, accessed October 19, 2025, https://www.bhf.org.uk/informationsupport/heart-matters-magazine/nutrition/ask-the-expert/foods-high-in-vitamin-d
24. Farmed and wild fish in the prevention of cardiovascular diseases: assessing possible differences in lipid nutritional values – PubMed, accessed October 19, 2025, https://pubmed.ncbi.nlm.nih.gov/15053162/
25. healthline.com, accessed October 19, 2025, https://www.healthline.com/nutrition/9-foods-high-in-vitamin-d#:~:text=5.-,Eggs,or%205%25%20of%20the%20DV.
26. Appendix E-3.3: Meeting Vitamin D Recommended Intakes in USDA Food Patterns, accessed October 19, 2025, https://odphp.health.gov/sites/default/files/2019-09/Appendix-E-3.3.pdf
27. Nutrient Data for Whole, Large Eggs from a USDA … – USDA ARS, accessed October 19, 2025, https://www.ars.usda.gov/ARSUserFiles/80400525/Articles/EB11_Egg.pdf
28. cnet.com, accessed October 19, 2025, https://www.cnet.com/health/nutrition/11-vitamin-d-rich-foods-no-supplements-needed/#:~:text=Liver%20is%20a%20love%2Dit,D%20in%20a%20single%20slice.
29. A Review of Mushrooms as a Potential Source of Dietary Vitamin D …, accessed October 19, 2025, https://pmc.ncbi.nlm.nih.gov/articles/PMC6213178/
30. vitamin D mushrooms (UV-exposed) are a feasible and acceptable way to increase vitamin D intake in a residential aged care facility – PMC, accessed October 19, 2025, https://pmc.ncbi.nlm.nih.gov/articles/PMC12213422/
31. Food Sources of Vitamin D Standard Portions – Dietary Guidelines for Americans, accessed October 19, 2025, https://www.dietaryguidelines.gov/sites/default/files/2024-08/Food-Sources-Vitamin-D-Standard-508C.pdf
32. The mushrooms on the menu (MOM) study: vitamin D mushrooms (UV-exposed) are a feasible and acceptable way to increase vitamin D intake in a residential aged care facility – PubMed, accessed October 19, 2025, https://pubmed.ncbi.nlm.nih.gov/40606102/
33. Vitamin D • The Nutrition Source, accessed October 19, 2025, https://nutritionsource.hsph.harvard.edu/vitamin-d/