Most vitamin D supplements come from sheep’s wool. Specifically, the waxy grease coating raw wool, called lanolin, contains a cholesterol compound that can be converted into vitamin D3 through ultraviolet light exposure. This is the same basic reaction that happens in your skin when you step into sunlight, just recreated in a factory. Vitamin D2 supplements take a different path, starting from yeast or mushrooms instead of animal sources.
How Lanolin Becomes Vitamin D3
Sheep produce a thick, oily substance called lanolin that waterproofs their fleece. When wool is sheared and washed, this grease is collected as a byproduct. Lanolin is rich in a compound called 7-dehydrocholesterol, the same molecule sitting in the outer layers of your skin right now, waiting for sun exposure to kick off vitamin D production.
In manufacturing, 7-dehydrocholesterol is extracted from lanolin and then hit with UV-B light at wavelengths between 285 and 315 nanometers. The reaction is run at low temperatures in an oxygen-free environment to prevent the compound from breaking down into unwanted byproducts. This UV exposure creates a precursor molecule, which then converts into vitamin D3 (cholecalciferol) through heat. The final product is purified using laboratory filtration techniques to meet pharmaceutical-grade standards. Wavelengths above 315 nm produce little to no vitamin D, which is why the process requires precise UV-B exposure rather than just any light source.
Because lanolin comes from sheep, this form of D3 is technically an animal product, though no sheep are harmed in the process. The grease is simply a byproduct of wool production. Lanolin used in supplements must meet United States Pharmacopoeia or European Pharmacopoeia standards, which set limits for pesticide residues (no more than 1 part per million for any individual pesticide, and no more than 3 ppm total). Pesticides historically used to protect sheep fleece from parasites, including organochlorine and pyrethroid insecticides, are specifically screened for during testing.
Where Vitamin D2 Comes From
Vitamin D2 (ergocalciferol) starts with a different raw material: ergosterol, a compound found naturally in fungi. Commercially, ergosterol is sourced from yeast or mushrooms and exposed to UV-B radiation at 290 to 320 nm, which triggers a photochemical conversion into vitamin D2. Cultivated mushrooms typically contain very little vitamin D on their own because they’re grown indoors without sunlight, but they’re loaded with ergosterol, making them ideal starting material.
The conversion works through a two-step process. First, UV light transforms ergosterol into a precursor compound. Then heat causes it to rearrange into the final vitamin D2 molecule. UV-B is the most effective wavelength range for this conversion, and the same principle is used when commercial mushroom growers expose their harvested mushrooms to UV light to boost vitamin D content before selling them.
D2 vs. D3: Does the Source Matter?
Your body handles these two forms slightly differently. Once absorbed, both D2 and D3 are converted in the liver into a circulating form that doctors measure in blood tests. However, the D3 version stays in your bloodstream longer. Research published in the Journal of Clinical Endocrinology and Metabolism found the circulating form of D2 has a half-life of about 13.9 days, compared to 15.1 days for D3. That roughly one-day difference means D3 maintains your blood levels more effectively over time, which is why most doctors and supplement manufacturers favor it.
The practical takeaway: D3 supplements raise and sustain your vitamin D levels more reliably than D2 at the same dose. If you’re taking D2 for dietary reasons, it still works, but you may need to take it more consistently to maintain the same blood levels.
Vegan Vitamin D3 Options
For people who avoid animal products, lanolin-derived D3 is off the table. The main vegan alternative comes from lichen, a composite organism made of fungi and algae that grows on trees and rocks. Several lichen species naturally produce vitamin D3. Research has found that species including Bryoria fuscescens, Evernia prunastri, and Pseudevernia furfuracea contain the highest concentrations, ranging from 0.6 to 20.33 micrograms per gram. The vitamin D3 is typically extracted using olive oil-based soaking methods, which have proven more efficient than other industrial techniques.
A newer source gaining attention is the microalga Nannochloropsis oceanica, which produces vitamin D3 when exposed to UV-B radiation after being grown in culture. This approach is still being optimized for commercial scale, but it represents another non-animal pathway to D3 production. Most vegan D3 supplements currently on the market use lichen rather than algae.
Fish Liver Oil: The Original Source
Before lanolin-based manufacturing took over, fish liver oil was the primary dietary source of supplemental vitamin D3. Cod liver oil remains available and naturally contains vitamin D3 at concentrations typically between 40 and 200 IU per milliliter, depending on the species and extraction method. A standard tablespoon of cod liver oil provides roughly 1,060 IU. Some shark liver oils contain significantly higher concentrations, with one study finding over 1,000 IU per milliliter in extracted oil from certain species.
Fish liver oil is less common in modern supplements because lanolin-derived D3 is cheaper to produce at scale and easier to standardize into precise doses. You’ll still find fish liver oil sold on its own, often marketed for its combination of vitamin D, vitamin A, and omega-3 fatty acids.
What Else Is in a Vitamin D Supplement
Vitamin D is fat-soluble, meaning it needs some dietary fat to be absorbed properly. Softgel capsules typically dissolve the vitamin D in a carrier oil. Common options include soybean oil, olive oil, sunflower oil, and medium-chain triglyceride (MCT) oil derived from coconut. Some brands use avocado oil or flaxseed oil. The choice of carrier oil varies by manufacturer but generally doesn’t affect how well the vitamin D works, as long as some fat is present during digestion.
If you take a dry tablet or capsule form rather than a softgel, taking it with a meal that contains some fat will improve absorption. The vitamin D molecule itself is identical regardless of whether the supplement is a softgel, tablet, gummy, or liquid drop.