Vitamin D is the umbrella term for a group of related nutrients, while vitamin D3 is one specific type within that group. When people say “vitamin D,” they’re usually referring to either vitamin D2 or vitamin D3, the two forms that matter most for human health. The practical difference comes down to where they come from, how well your body uses them, and which one you’ll find in most supplements.
Vitamin D Is the Category, D3 Is the Star Player
Think of “vitamin D” like saying “fruit.” It’s a broad label that covers multiple forms. The two you’ll actually encounter are vitamin D2 and vitamin D3. Vitamin D2 comes primarily from plants and fungi. Vitamin D3 comes from animal sources and, importantly, is the form your own skin produces when exposed to sunlight.
The molecules are nearly identical but differ in the shape of their side chains. D2 has an extra branch and double bond in its structure that D3 lacks. That small structural difference changes how your body processes each one, which is why the distinction matters beyond chemistry.
How Your Body Makes and Activates Vitamin D
Your skin contains a cholesterol compound called 7-dehydrocholesterol. When UVB rays from the sun (wavelengths between 295 and 315 nanometers) hit your skin, they convert that compound into previtamin D3, which then transforms into vitamin D3 through body heat. This is the only form of vitamin D your body naturally produces. D2 never enters the picture unless you eat it.
Neither D2 nor D3 is active right away. Both go through two conversion steps before your body can use them. First, the liver adds a chemical group to create a storage form (this is what blood tests measure). Then the kidneys convert that storage form into the fully active hormone that regulates calcium absorption, bone health, immune function, and dozens of other processes. Both D2 and D3 follow this same activation pathway, but they don’t follow it with equal efficiency.
Why D3 Is Generally Preferred
Vitamin D3 raises and sustains blood levels of the storage form more effectively than D2. Several comparison studies have shown that taking the same dose of D3 produces higher, longer-lasting blood levels than an equivalent dose of D2. The binding proteins that carry vitamin D through your bloodstream appear to have a stronger affinity for D3, which means it stays in circulation longer rather than being broken down.
D2 still works. Prescription vitamin D supplements have historically used D2 (ergocalciferol), and they can correct deficiency. But if the goal is maintaining healthy levels over time, D3 is the more efficient option, and most over-the-counter supplements now use it.
Where Each Form Shows Up in Food
Vitamin D3 is found almost exclusively in animal-based foods. The richest sources are fatty fish: a 3-ounce serving of rainbow trout provides about 645 IU, salmon ranges from 383 to 570 IU, and even canned light tuna delivers around 231 IU. Sardines, herring, tilapia, and flounder are also solid sources. Egg yolks contain small amounts of D3 as well.
Vitamin D2 comes from fungi. Mushrooms exposed to UV light can contain impressive amounts, with some varieties providing up to 1,110 IU per cup. However, the vitamin D content in mushrooms varies enormously depending on whether they were grown in sunlight or darkness, so the range extends all the way down to nearly zero.
Many everyday foods are fortified with vitamin D, including milk (about 117 IU per cup), yogurt (116 IU per 8 ounces), fortified orange juice (100 IU per cup), and plant-based milks like soy, almond, and rice beverages (100 to 119 IU per cup). These fortified products may use either D2 or D3 depending on the manufacturer.
Supplements: What to Look For
Most vitamin D3 supplements are made from lanolin, a waxy substance extracted from sheep’s wool. The lanolin contains the same precursor compound found in human skin, and manufacturers use UV exposure to convert it into D3. Fish oil is another common source.
If you follow a vegan diet, D2 supplements have traditionally been the default choice since they come from fungi. But vegan D3 now exists, extracted from lichen, a composite organism made of algae and fungi. Certain lichen species naturally contain the same precursor your skin uses, and when exposed to light, they produce D3 that is chemically identical to the animal-derived version. Vegan D3 from lichen is considered equally effective at raising blood levels.
Both forms are sensitive to heat, air exposure, and UV radiation. In storage tests on fortified foods, vitamin D2 and D3 retained nearly all their potency for the first day but gradually degraded over time, dropping to roughly 72 to 79 percent of their original levels by day five. For supplements, keeping them in a cool, dark place with the lid sealed preserves potency.
How Much You Need and Safe Upper Limits
The recommended daily intake for most adults is 600 IU (15 mcg), rising to 800 IU (20 mcg) for adults over 70. These numbers apply whether you’re getting D2, D3, or a mix from food and supplements.
The tolerable upper intake level for adults 19 and older is 4,000 IU (100 mcg) per day from all sources combined. For children, the limits are lower:
- Birth to 6 months: 1,000 IU
- 7 to 12 months: 1,500 IU
- 1 to 3 years: 2,500 IU
- 4 to 8 years: 3,000 IU
- 9 to 18 years: 4,000 IU
Signs of toxicity are unlikely below 10,000 IU daily, but long-term intakes even below the upper limit could potentially cause problems, which is why the ceiling is set conservatively at 4,000 IU. Vitamin D toxicity causes calcium to build up in the blood, leading to nausea, weakness, and in severe cases, kidney damage.
What Blood Tests Actually Measure
When your doctor checks your vitamin D level, they’re measuring 25-hydroxyvitamin D in your blood. This is the storage form produced by the liver, and it reflects your total vitamin D status from sunlight, food, and supplements combined. The test captures both D2 and D3 contributions in a single number.
Most labs define deficiency as a blood level below 20 ng/mL (50 nmol/L). Levels between 20 and 29 ng/mL are often labeled insufficient, and 30 ng/mL or above is generally considered adequate for bone and overall health. Because D3 sustains blood levels more effectively than D2, people who supplement with D3 tend to reach and maintain sufficient levels more easily.
The Bottom Line on Choosing Between Them
If you see a supplement labeled simply “vitamin D” without specifying a type, check the ingredient list. It could be either D2 or D3. For most people, D3 is the better choice because it more effectively raises blood levels and is the same form your body produces naturally. D2 remains a reasonable option for people who prefer a plant-derived supplement, though vegan D3 from lichen has largely closed that gap. Whichever form you choose, the activation pathway in your body is the same, and both ultimately serve the same functions once converted to the active hormone.