“Vitamin D” is an umbrella term that covers two forms: vitamin D2 (ergocalciferol) and vitamin D3 (cholecalciferol). So when you see a bottle labeled “vitamin D3,” you’re looking at one specific type of vitamin D, not something separate from it. The distinction matters because the two forms come from different sources, and D3 is generally more effective at raising your blood levels.
D2 vs. D3: What’s Actually Different
Vitamin D2 comes from plant-based sources, primarily mushrooms and fungi that have been exposed to UV light. Vitamin D3 is the form your own skin produces when sunlight hits it, and it’s also found in animal products like fatty fish, egg yolks, and liver. Both forms are biologically inactive on their own. They need to be converted by your liver and then your kidneys before your body can actually use them.
The conversion process is the same for both. First, the vitamin travels to the liver, where enzymes add a chemical group to create a halfway-active compound. That compound then moves to the kidneys, where a second enzyme converts it into the fully active hormone your body uses for calcium absorption, bone maintenance, and immune function.
Why D3 Is Usually the Better Choice
Although D2 and D3 follow the same activation pathway, D3 is more effective at raising and sustaining your blood levels of vitamin D over time. Some research puts D3 at roughly 80% more effective than D2 at boosting levels. This is partly because D2 is less stable: it breaks down more quickly when exposed to humidity and temperature changes, while D3 holds up better under the same conditions. D2 also appears to have a shorter duration of action in the body, meaning blood levels drop off faster after you stop taking it.
For these reasons, most over-the-counter supplements use D3, and it’s the form most clinicians prefer for correcting a deficiency.
Where Each Form Comes From
Your body makes D3 when UVB radiation (wavelengths between roughly 295 and 315 nanometers) reaches a cholesterol compound in your skin called 7-dehydrocholesterol. Sunlight converts it into a precursor that your body then reshapes into vitamin D3. This is the primary way most people get their vitamin D, with food and supplements filling in the gaps.
In supplement form, D3 has traditionally been sourced from lanolin (sheep’s wool oil) or fish oil, making it an animal-derived product. That made D2 the default for vegans for a long time. Today, though, vegan D3 supplements extracted from lichen are widely available. Lichen is a naturally rich source: some species contain 67 to 204 micrograms of D3 per 100 grams of dry material. These plant-based D3 supplements offer the same bioavailability advantages as animal-derived D3, so vegans no longer need to settle for the less effective D2 form.
How Much You Need
The recommended daily amount of vitamin D (in either form) varies by age:
- Infants (0 to 12 months): 400 IU
- Children and adults (1 to 70 years): 600 IU
- Adults over 70: 800 IU
The tolerable upper limit for adults is 4,000 IU per day. Going above that without medical supervision raises the risk of toxicity, which can cause dangerously high calcium levels. Vitamin D is fat-soluble, so taking your supplement with a meal that contains some fat helps your body absorb it more efficiently.
What to Look for on the Label
If a supplement simply says “vitamin D” without specifying D2 or D3, check the ingredients list. You’ll see either “ergocalciferol” (D2) or “cholecalciferol” (D3). Most standalone supplements sold today use D3, but multivitamins and fortified foods sometimes use D2 because it’s cheaper to produce. When you have the choice, D3 is the stronger option for maintaining healthy blood levels regardless of whether it comes from an animal or lichen-based source.