Vitamin D and D3 are not exactly the same thing. “Vitamin D” is an umbrella term that covers two forms: vitamin D2 (ergocalciferol) and vitamin D3 (cholecalciferol). When people say “vitamin D” casually, they often mean D3, since it’s the form your body makes naturally and the type found in most supplements. But technically, vitamin D includes both forms.
Two Forms of Vitamin D
Vitamin D2 comes from plant sources, mainly fungi and yeast exposed to ultraviolet light. Vitamin D3 comes from animal sources and is also the form your skin produces when exposed to UVB rays from the sun. The chemical difference between the two is small: D2 has an extra double bond and an extra methyl group on its molecular chain. That small structural difference, though, changes how your body processes each one.
Your liver converts both forms into a compound called 25(OH)D, which is the marker doctors measure in a blood test. But the liver enzyme responsible for this conversion, along with the protein that transports vitamin D through the bloodstream, both have a stronger affinity for D3 than D2. D2 also breaks down faster in the body, giving it a shorter half-life. The practical result: D3 raises your blood levels roughly twice as high as D2 at the same dose and keeps them elevated longer.
D3 Raises Blood Levels More Effectively
A 2024 meta-analysis of 20 clinical studies confirmed that D3 consistently produces a greater increase in blood vitamin D levels compared to D2 when taken as a daily supplement. The difference held across most populations, with one notable exception: in people with a BMI above 25 (the standard cutoff for overweight), D2 and D3 performed similarly. Excess body fat appears to blunt the potency advantage D3 normally has.
Animal research supports this pattern at a deeper level. In vitamin D-depleted rats, D3 improved calcium absorption in the intestine more efficiently than D2, restored bone strength and quality more effectively, and better normalized gene activity in the kidneys related to vitamin D metabolism.
How Your Body Makes D3
Your skin contains a cholesterol compound called 7-dehydrocholesterol. When UVB rays hit your skin, they convert this compound into previtamin D3, which then transforms into vitamin D3 over hours. From there, the liver and kidneys convert it into the active hormone your body actually uses. This is why vitamin D is sometimes called the “sunshine vitamin,” though the term is slightly misleading since your body only makes the D3 form through sun exposure, never D2.
Where Each Form Comes From
If you’re choosing a supplement, the label will usually specify D2 or D3. Here’s how they differ by source:
- Vitamin D2: Derived from irradiated yeast or mushrooms. It’s plant-based by default, which made it the traditional choice for vegans.
- Vitamin D3 (standard): Most D3 supplements are made from lanolin, a waxy substance extracted from sheep’s wool. This makes standard D3 an animal product.
- Vitamin D3 (vegan): Vegan D3 supplements now exist, made from lichen, a plant-like organism. These use species such as Cladina arbuscula and provide the same D3 molecule without animal-derived ingredients.
Prescription vitamin D is almost always D2 (ergocalciferol), while over-the-counter supplements are more commonly D3. If your bottle just says “vitamin D” without specifying, check the ingredient list for either cholecalciferol (D3) or ergocalciferol (D2).
What Counts as a Healthy Blood Level
Regardless of which form you take, the goal is the same: reaching adequate levels of 25(OH)D in your blood. The NIH provides these general thresholds:
- Below 12 ng/mL: Deficient. Associated with rickets in children and bone softening in adults.
- 12 to 19 ng/mL: Generally considered inadequate for bone and overall health.
- 20 ng/mL or above: Generally considered adequate for most healthy people.
- Above 50 ng/mL: Linked to potential adverse effects, particularly above 60 ng/mL.
The Endocrine Society’s updated 2024 guideline, however, recommends against routine blood testing for vitamin D in the general population. The panel concluded that the specific blood level needed for optimal health outcomes hasn’t been pinned down by clinical trials, so testing and targeting a number may not be as useful as previously thought.
Safety Limits
Vitamin D is fat-soluble, meaning excess amounts are stored in body fat rather than flushed out. This makes toxicity possible, though rare at normal supplement doses. The NIH sets the tolerable upper intake level for adults at 4,000 IU per day from all sources combined (food, drinks, and supplements). For children, the limit is lower: 1,000 IU for infants under 6 months, scaling up to 3,000 IU for children ages 4 to 8.
Blood levels above 150 ng/mL can cause nausea, vomiting, muscle weakness, confusion, excessive thirst, and kidney stones. At extreme levels, vitamin D toxicity can lead to kidney failure and irregular heartbeat. You cannot reach toxic levels from sun exposure alone, because your skin self-regulates production. Toxicity comes from supplements.
Which Form Should You Take
For most people, D3 is the better choice. It raises blood levels higher, maintains them longer, and is the form your body naturally produces. If you follow a vegan diet, lichen-based D3 supplements give you the same molecule without animal ingredients, making D2 no longer the only plant-friendly option. D2 still works and is still prescribed, but you may need higher or more frequent doses to achieve the same blood levels as D3.
Because vitamin D is fat-soluble, taking your supplement with a meal that contains some fat improves absorption. This applies equally to both D2 and D3.