Vitamin D3 (cholecalciferol) comes from three main places: your own skin when it’s exposed to sunlight, a handful of animal-based foods, and supplements typically derived from sheep’s wool. Unlike vitamin D2, which comes from plants and fungi, D3 is produced almost exclusively by animals or through UV exposure.
Your Skin Makes It From Sunlight
The primary source of vitamin D3 for most people is their own body. Your skin contains a cholesterol compound called 7-dehydrocholesterol, concentrated in the upper layers. When UVB rays from sunlight hit this compound, they break open part of its molecular ring, creating a precursor molecule. Body heat then converts that precursor into cholecalciferol, the form of D3 that enters your bloodstream.
This process takes roughly 30 minutes of sun exposure to get going, though the amount of D3 you actually produce depends heavily on your skin tone, where you live, and the time of year. Under clear skies, people with lighter skin need about 3 to 15 minutes of exposure depending on latitude. People with darker skin need significantly more time at every latitude, sometimes exceeding an hour at higher latitudes, because melanin absorbs UVB before it can reach 7-dehydrocholesterol.
Cloud cover matters too. Clouds extend the required exposure time by about 15% near the equator and up to 60% at higher latitudes. Geography creates an even bigger limitation: in much of the northern hemisphere, UVB rays are too weak from roughly October through March to produce meaningful D3 regardless of how long you spend outside. A study from Estonia (latitude 59°N) found that only a third of the population had sufficient vitamin D levels during winter, with average blood levels dropping from their September peak to a February low.
Foods That Naturally Contain D3
Very few foods contain vitamin D3 naturally, and the amounts vary dramatically. Fatty fish are by far the richest source. A 3-ounce serving of farmed rainbow trout delivers about 645 IU, and the same amount of cooked sockeye salmon provides around 570 IU. Those numbers are significant when you consider that most adults need 600 to 800 IU daily.
Beyond fatty fish, the numbers drop off quickly:
- Sardines (2 canned sardines): 46 IU
- Egg yolk (1 large egg): 44 IU
- Beef liver (3 ounces, braised): 42 IU
- Canned tuna (3 ounces): 40 IU
So unless you eat fatty fish several times a week, food alone is unlikely to meet your D3 needs. The vitamin D in egg yolks is entirely in the yolk, not the white, which is worth knowing if you typically eat egg-white omelets.
D3 in Fortified Foods
Because natural food sources are so limited, manufacturers add D3 to everyday staples. Cow’s milk is the most common example, typically fortified to around 400 IU per quart, with some enhanced varieties allowed up to 800 IU per quart under FDA regulations. Breakfast cereals can be fortified with up to 560 IU per 100 grams. Many plant-based milks, yogurts, and orange juices are also fortified, though the specific amounts vary by brand.
The D3 used in fortification and most supplements comes from the same industrial process: lanolin extracted from sheep’s wool. After sheep are shorn, the wool is washed in hot water with detergents to pull out this waxy, oil-like substance. Lanolin contains the same cholesterol precursor (7-dehydrocholesterol) found in human skin. Manufacturers then expose it to UV light, mimicking what sunlight does to your skin, and purify the resulting cholecalciferol for use in pills, drops, and fortified foods.
Vegan Sources of D3
For people who avoid animal products, D3 has traditionally been a problem. Vitamin D2 (ergocalciferol) comes from mushrooms and fungi exposed to UV light, but D3 was considered exclusively animal-derived. That changed with the discovery that certain species of lichen, a slow-growing organism that’s part fungus and part algae, naturally contain 7-dehydrocholesterol. When exposed to UV light at controlled temperatures below 16°C, lichen produces genuine D3 through the same chemical conversion that happens in human skin.
Lichen-sourced D3 supplements are now widely available and marketed as vegan. The lichen is typically wild-harvested, and the resulting D3 is chemically identical to what your body makes or what comes from lanolin.
How Your Body Activates D3
Regardless of whether D3 enters your body through skin, food, or a supplement, it arrives in an inactive form. It needs two chemical conversions before it can do anything useful. The first happens in your liver, which adds a hydroxyl group to create calcidiol. This is what doctors measure when they check your vitamin D levels through a blood test.
The second conversion happens primarily in your kidneys, which transform calcidiol into calcitriol, the fully active hormone. Your body tightly regulates this second step based on calcium levels and signals from the parathyroid gland. Some other tissues, including breast, colon, and lung cells, can also perform this activation locally, which is part of why researchers have linked vitamin D to functions well beyond bone health.
Why D3, Not D2
Vitamin D comes in two forms: D2 from plants and fungi, and D3 from sunlight and animal sources. Both go through the same liver and kidney activation steps, but they aren’t equally effective. D3 raises and maintains blood levels of vitamin D more efficiently than D2, which is why most supplements and fortified foods now use the D3 form. If you see “vitamin D” on a nutrition label without a number, check the ingredients. D3 (cholecalciferol) and D2 (ergocalciferol) are different compounds with different origins and slightly different potency in your body.