Vitamin D3 is made in three ways: your skin produces it when exposed to sunlight, animals accumulate it naturally in their tissues, and manufacturers extract or synthesize it for supplements. The most fascinating source is your own body, which runs a small chemical factory powered by ultraviolet light.
How Your Skin Makes Vitamin D3
Your skin contains a cholesterol-related compound called 7-dehydrocholesterol, concentrated mainly in the epidermis (your outermost skin layer). When UVB radiation in the 285 to 315 nanometer wavelength range hits your skin, it triggers a chemical reaction in the keratinocytes, the cells that make up most of the epidermis. The UVB energy breaks open a ring in the cholesterol molecule, transforming it into a precursor called previtamin D3. Body heat then slowly converts previtamin D3 into cholecalciferol, the form we call vitamin D3.
This isn’t the final product your body actually uses. Cholecalciferol travels through your bloodstream to the liver, where it’s converted into a circulating form called 25-hydroxyvitamin D. That’s what doctors measure when they check your vitamin D levels. Your kidneys then convert it again into the fully active hormone that regulates calcium absorption and dozens of other processes.
The whole chain, from sunlight hitting skin to active hormone circulating in blood, takes roughly a week. About 10 to 30 minutes of midday sun exposure on bare arms and legs, a few times per week, is enough for most lighter-skinned people to maintain adequate levels during summer months. But several factors change that equation significantly.
What Affects Your Skin’s Production
Melanin, the pigment that determines skin color, competes with 7-dehydrocholesterol for UVB photons. Research comparing the lightest and darkest skin types (type II versus type VI) found a melanin inhibition factor of roughly 1.3 to 1.4, meaning people with very dark skin need about 30 to 40 percent more UVB exposure to produce the same amount of vitamin D3. That’s a meaningful difference, but smaller than older estimates once suggested.
Age matters more dramatically. As you get older, the concentration of 7-dehydrocholesterol in your skin declines. A person in their 70s may produce less than half the vitamin D3 that a 20-year-old makes from the same sun exposure. This is one reason the recommended daily intake rises from 600 IU for adults under 70 to 800 IU for those over 70.
Geography and season also play major roles. Above roughly 37 degrees latitude (think San Francisco, Athens, or Seoul), winter UVB intensity drops so low that your skin produces little to no vitamin D3 for several months. Sunscreen with SPF 30 blocks about 97 percent of UVB. Glass blocks it almost entirely, so sitting by a sunny window does nothing for your vitamin D levels.
How Supplements Are Manufactured
Most vitamin D3 supplements come from lanolin, the waxy grease in sheep’s wool. Manufacturers extract 7-dehydrocholesterol from lanolin, then expose it to UVB radiation in a controlled industrial setting, mimicking exactly what happens in human skin. The resulting cholecalciferol is purified, concentrated, and formulated into capsules, tablets, drops, or gummies.
This lanolin-based process dominates the global supplement market because sheep produce enormous quantities of wool grease, making it cheap and scalable. The 7-dehydrocholesterol extracted from lanolin is chemically identical to the compound in your skin, so the end product is the same molecule your body would make on its own.
For people who avoid animal products, a plant-based alternative exists. Certain species of lichen, a composite organism made of fungi and algae, naturally produce cholecalciferol. Lichen-derived D3 supplements are the only vegan source of true vitamin D3. (Vitamin D2, a slightly different molecule made by irradiating yeast or mushrooms, is also vegan but is less effective at raising blood levels.)
Vitamin D3 in Food
Very few foods contain meaningful amounts of vitamin D3 naturally. The richest sources are fatty fish like salmon, mackerel, sardines, and trout, which accumulate D3 through the aquatic food chain starting with plankton that produce it in response to sunlight. Cod liver oil is particularly concentrated. Egg yolks contain smaller amounts, and the vitamin D content varies depending on what the hens ate and whether they had sun exposure.
Because natural food sources are so limited, many countries fortify staple foods. In the United States, most milk is fortified with about 120 IU per cup. Many orange juices, cereals, and plant-based milks are also fortified. These fortified products typically use the same lanolin-derived cholecalciferol found in supplements.
Vitamin D3 is fat-soluble, meaning it dissolves in fat rather than water. Having some fat in your meal improves absorption when you take a supplement or eat fortified food. That said, the National Institutes of Health notes that some vitamin D is absorbed even without dietary fat, so you don’t need to plan elaborate fatty meals around your supplement.
How Much You Need and Upper Limits
The recommended daily allowance is 600 IU for adults up to age 70 and 800 IU for those older than 70. Many physicians recommend higher doses for people with documented deficiency, and doses of 1,000 to 2,000 IU daily are common.
Toxicity from vitamin D3 is rare but real, and it only happens from supplements or fortified foods, never from sun exposure (your skin has a self-regulating mechanism that breaks down excess previtamin D3). Blood levels above 80 ng/mL are the lowest threshold associated with possible toxicity, according to Mayo Clinic Laboratories, though most patients who develop actual symptoms have levels above 150 ng/mL. Symptoms of toxicity include nausea, vomiting, weakness, and dangerously high blood calcium, which can damage the kidneys and heart.
For context, a typical healthy person with adequate vitamin D status has blood levels between 30 and 50 ng/mL. Getting from there to the toxicity range requires sustained intake of very high doses, usually well above 10,000 IU daily for extended periods. Standard supplementation at 1,000 to 2,000 IU per day carries essentially no toxicity risk for most people.