Vitamin D is a fat-soluble compound that your body builds from a cholesterol precursor in your skin, absorbs from a handful of foods, or gets from supplements that typically contain just one or two active ingredients plus a carrier oil. Despite being called a vitamin, it functions more like a hormone, and understanding what’s actually in it, both chemically and in the bottle, helps you make sense of labels, food sources, and how your body uses it.
The Molecule Itself: D2 vs. D3
There are two main forms of vitamin D. Vitamin D2 (ergocalciferol) comes from plants and fungi. Vitamin D3 (cholecalciferol) comes from animal sources and is also the form your skin produces. Both are structurally similar, built on a backbone derived from cholesterol, but D3 is generally more effective at raising and maintaining blood levels of active vitamin D.
Neither form is active on its own. After you swallow a supplement or your skin manufactures D3, the molecule travels to your liver, where it’s converted into a circulating form called calcidiol. Your kidneys then convert calcidiol into calcitriol, the fully active hormonal form that regulates calcium absorption, bone health, and immune function. So “vitamin D” is really a raw material your body processes through two steps before it can use it.
How Your Body Makes It From Sunlight
Your skin contains a cholesterol-related compound called 7-dehydrocholesterol. When UVB rays from sunlight hit your skin, they break open that molecule’s ring structure, creating a precursor that gradually rearranges into vitamin D3. From there, the liver and kidney conversions described above finish the job. This entire chain, from sun exposure to active hormone, is why vitamin D is sometimes called the “sunshine vitamin.” It’s the only vitamin your body can manufacture in meaningful amounts just from light exposure.
What’s in Vitamin D Supplements
If you flip over a vitamin D bottle, the ingredient list is usually short. The active ingredient is either cholecalciferol (D3) or ergocalciferol (D2), measured in micrograms or international units (IU). Most over-the-counter supplements use D3 because it matches what your body naturally produces.
Beyond the active ingredient, softgel capsules typically contain a carrier oil to help absorption. Soybean oil is one of the most common. Other brands use olive oil, coconut oil, or medium-chain triglyceride (MCT) oil. Because vitamin D dissolves in fat rather than water, these oils serve a real purpose: they help the vitamin D pass through your intestinal wall and into your bloodstream. Taking vitamin D with a meal that contains some fat improves absorption for the same reason.
Tablets and gummies may include additional inactive ingredients like cellulose fillers, gelatin (for the capsule shell), and sometimes colorings or sweeteners. For people avoiding animal products, vegan D3 supplements sourced from lichen are available, replacing the sheep lanolin that most D3 is traditionally extracted from. Vitamin D2 supplements are also inherently vegan since they’re derived from irradiated mushrooms or yeast.
Vitamin D in Food
Very few foods naturally contain significant vitamin D. Fatty fish dominates the list. Three ounces of cooked swordfish delivers about 566 IU, while the same amount of canned pink salmon provides around 493 IU. A fillet of farmed rainbow trout comes in at roughly 539 IU. These are the richest natural sources by a wide margin.
After fish, the numbers drop fast. A single large egg contains about 41 IU, almost all of it in the yolk. A cup of raw chanterelle mushrooms has around 114 IU. Cheddar cheese provides about 32 IU per cup, diced.
Most of the vitamin D in a typical diet comes from fortified foods rather than naturally rich ones. A cup of fortified milk (any fat level) provides roughly 98 IU. Fortified orange juice, almond milk, and certain cereals each add about 100 IU per serving. These are useful contributors, but none of them come close to what a serving of fatty fish offers, and all of them fall well short of the 600 IU daily recommendation for most adults.
How Much You Need and Upper Limits
The recommended daily amount of vitamin D is 600 IU (15 mcg) for everyone from age 1 through 70, and 800 IU (20 mcg) for adults over 70. Infants need 400 IU (10 mcg). These numbers assume minimal sun exposure.
The tolerable upper intake level, the maximum considered safe for long-term daily use, is 4,000 IU (100 mcg) for anyone age 9 and older, including pregnant and breastfeeding women. Children ages 4 to 8 have a slightly lower ceiling of 3,000 IU, and toddlers ages 1 to 3 should stay under 2,500 IU. Signs of toxicity from vitamin D are unlikely below 10,000 IU per day, but the NIH cautions that even amounts below that threshold could cause problems over time. Excess vitamin D forces your body to absorb too much calcium, which can damage your kidneys, harden soft tissues, and disrupt heart rhythm.
Why Fat Matters for Absorption
Vitamin D is fat-soluble, which shapes everything about how your body handles it. It dissolves into dietary fat in your gut, gets packaged into fat-carrying particles, and is absorbed alongside those fats through your intestinal lining. If you take a vitamin D supplement on an empty stomach or with a fat-free meal, less of it makes it into your bloodstream. This is also why people with conditions that impair fat absorption, like celiac disease, Crohn’s disease, or after certain weight-loss surgeries, are at higher risk for vitamin D deficiency even when their intake looks adequate on paper.
Once absorbed, vitamin D is stored in your body fat and released slowly. This means levels build up gradually and also decline gradually, which is why a single missed dose doesn’t matter much but months of low intake do.