Vitamin D is a nutrient essential for bone health, promoting calcium and phosphate absorption. It also supports muscle function and the immune system. While diet and supplements provide vitamin D, sunlight is a primary natural source.
The Sun’s Vitamin D: Cholecalciferol
The specific form of vitamin D produced in human skin from sun exposure is vitamin D3, or cholecalciferol. This form is naturally synthesized in animals, including humans. It differs from vitamin D2 (ergocalciferol), which comes from plants and fungi exposed to ultraviolet (UV) radiation. While both D2 and D3 increase vitamin D levels, D3 is generally more effective at raising and sustaining them in the bloodstream.
How Sunlight Creates Vitamin D in Your Skin
The production of vitamin D3 in the skin begins when ultraviolet B (UVB) radiation from sunlight penetrates the epidermis, the outer layer of skin. Within skin cells, a cholesterol precursor called 7-dehydrocholesterol (7-DHC) absorbs these UVB photons. This absorption triggers a photochemical reaction, converting 7-DHC into previtamin D3. Following its formation, previtamin D3 undergoes a heat-dependent transformation to become vitamin D3 (cholecalciferol). This newly formed vitamin D3 then diffuses from the skin into the bloodstream. Only UVB rays are effective in initiating this synthesis, while ultraviolet A (UVA) rays do not contribute to vitamin D production.
Optimizing Sun Exposure for Vitamin D
Several factors influence the skin’s ability to produce vitamin D from sunlight, including geographical location, time of day, season, and skin pigmentation. The intensity of UVB radiation is highest around midday, making this the most efficient time for vitamin D synthesis. For many, short periods of sun exposure (5 to 30 minutes to the face, arms, and hands, a few times per week) can be sufficient. Darker skin tones have higher melanin, which acts as a natural sunscreen, reducing vitamin D production efficiency; they may require longer exposure times. Sunscreen use also significantly reduces vitamin D synthesis (an SPF as low as 8 decreases production by over 95%), so balancing sun exposure with skin protection is a practical consideration.
Other Sources of Vitamin D
Beyond sun exposure, individuals can obtain vitamin D from dietary sources and supplements. Fatty fish, such as salmon, mackerel, and tuna, are among the best natural dietary sources of vitamin D3. Egg yolks, beef liver, and mushrooms also provide smaller amounts. Many foods are fortified with vitamin D, such as milk, plant-based milk alternatives, breakfast cereals, and some orange juice. These fortified options often contain vitamin D3, although some may use vitamin D2. When sun exposure is limited, dietary intake and vitamin D supplements become important. Supplements are available in both D2 and D3 forms, with D3 generally considered more effective at raising blood levels.