Vitamin D and vitamin D3 are not exactly the same thing. “Vitamin D” is an umbrella term that covers two forms: vitamin D2 and vitamin D3. When people say “vitamin D” casually, they often mean D3, and most supplements labeled simply “vitamin D” do contain D3. But the distinction matters because these two forms come from different sources, behave differently in your body, and aren’t equally effective at raising your blood levels.
Two Forms Under One Name
Vitamin D2 comes from plant and fungal sources. It’s naturally found in mushrooms and yeast, and it’s the form typically added to fortified foods like cereal, orange juice, and plant-based milks. Some mushrooms sold in grocery stores have been treated with UV light specifically to boost their D2 content.
Vitamin D3 is the form your skin produces when exposed to sunlight, and it’s found in animal-based foods: fatty fish like salmon and sardines, egg yolks, beef liver, and cheese. Most over-the-counter vitamin D supplements use D3, often derived from lanolin (a substance in sheep’s wool). Vegan D3 supplements sourced from lichen are also available and absorb similarly to animal-derived D3.
Both D2 and D3 are biologically inactive on their own. They’re precursors that your body has to convert before they do anything useful.
How Your Body Activates Vitamin D
Whether you swallow D2 or D3, or your skin makes D3 from sunlight, the molecule travels through your bloodstream to the liver. There, liver cells add a chemical group to convert it into a compound called 25-hydroxyvitamin D. This is the form doctors measure when they check your vitamin D levels with a blood test.
From the liver, the molecule travels to the kidneys, where a second conversion produces the fully active form of vitamin D. This active form is what regulates calcium absorption, supports bone health, and plays roles in immune function. Some tissues outside the kidneys, including skin, bone cells, and immune cells, can also perform that final activation step locally.
D3 Raises Blood Levels More Effectively
This is the practical difference that matters most. D3 is significantly better than D2 at raising and maintaining your blood levels of 25-hydroxyvitamin D, the marker your doctor tracks.
The reason comes down to how tightly each form binds to the protein that carries vitamin D through your bloodstream. D3’s converted form binds more strongly to this transport protein than D2’s does, which gives it a longer circulating half-life. D2 is cleared from the blood much more rapidly. In one study published in The Journal of Clinical Endocrinology & Metabolism, researchers observed a substantially faster decline in blood levels among people given D2 compared to those given D3, starting just three days after supplementation. The study’s conclusion was blunt: vitamin D2 is much less effective than vitamin D3 in humans.
This doesn’t mean D2 is useless. It still raises your levels, just not as efficiently or for as long. If you’re taking a prescription vitamin D supplement, check the label. Prescription formulations have historically used D2, while most over-the-counter options use D3.
Where You’ll Find Each Form
Knowing which form is in your food or supplement helps you make better choices:
- Vitamin D3 sources: salmon, sardines, egg yolks, beef liver, cheese, sunlight exposure, most over-the-counter supplements, lichen-based vegan supplements
- Vitamin D2 sources: UV-treated mushrooms, fortified cereals, fortified orange juice, fortified plant-based milks, some prescription supplements
If you eat a varied diet that includes fatty fish or eggs, you’re likely getting some D3 from food. If you follow a vegan diet and want D3 specifically, lichen-derived supplements are the main option. Otherwise, fortified plant foods and mushrooms will give you D2.
Which One to Choose in a Supplement
For most people, D3 is the better pick. It raises blood levels more effectively, stays in circulation longer, and is widely available at low cost. When you see a supplement labeled “vitamin D” without specifying D2 or D3, flip it over and check the ingredients. You’ll usually see “cholecalciferol” (that’s D3) or “ergocalciferol” (that’s D2).
Vitamin D is fat-soluble, so taking it with a meal that contains some fat improves absorption regardless of which form you choose. And because it is fat-soluble, it can accumulate in the body over time, so sticking to a consistent daily amount rather than sporadic large doses helps maintain steady blood levels without the risk of taking too much.
The bottom line: vitamin D3 is one specific type of vitamin D, not a synonym for it. But it’s the type your body makes naturally, the type that works best in supplements, and the type most worth seeking out.