There are five numbered forms of vitamin D (D1 through D5), but only two matter for human health: vitamin D2 and vitamin D3. These are the forms found in food, supplements, and your body. The other three are chemically distinct compounds that play no meaningful role in nutrition or medicine.
The Five Forms, Explained
When scientists first began isolating vitamin D in the early 1900s, they numbered each new compound they discovered. That’s how the list grew to five. But the story quickly simplified.
Vitamin D1 was the first compound identified, isolated from irradiated plant sterols. Researchers later realized it wasn’t a single vitamin at all. It was a mixture of vitamin D2 and another compound called lumisterol. Once that became clear, the term “vitamin D1” was dropped entirely.
Vitamin D2 (ergocalciferol) comes from fungi and yeast. It’s produced commercially by exposing a compound in yeast called ergosterol to ultraviolet light. Mushrooms grown under UV light are one of the few natural plant-based sources.
Vitamin D3 (cholecalciferol) is the form your skin makes when exposed to sunlight. It’s also found in animal-based foods like fatty fish, egg yolks, and liver. Most D3 supplements are made by irradiating lanolin from sheep’s wool, though a vegan version sourced from lichen also exists.
Vitamins D4 and D5 are real chemical compounds, but they don’t appear in food, aren’t sold as supplements, and have no established role in human nutrition. For all practical purposes, they’re footnotes in chemistry textbooks.
D2 vs. D3: Which One Works Better
Both D2 and D3 are absorbed well in the gut and go through nearly identical steps once inside your body. Both can prevent and treat rickets. But they are not equally potent.
Vitamin D3 raises blood levels of vitamin D significantly more than D2, and it keeps those levels elevated longer. A study published in The Journal of Clinical Endocrinology & Metabolism found that D3 was roughly three times more potent than D2 at raising total blood levels over 28 days. When researchers extended the analysis further, the gap widened even more, with D3 showing up to nine times the potency of D2. The study estimated that a 50,000 IU dose of D2 delivers the equivalent of no more than 15,000 IU of D3, and possibly as little as 5,000 IU. Earlier research using daily 4,000 IU doses for two weeks found D3 raised blood levels 70% more than D2.
This is why most doctors and dietitians recommend D3 when you’re choosing a supplement. D2 still works, and it’s the form most commonly used in prescription-strength vitamin D in the United States, but D3 gives you more benefit per unit.
Where Each Form Comes From
Your body produces D3 on its own when ultraviolet B rays from the sun hit your skin. This is the single largest natural source for most people, though the amount you make depends on latitude, skin tone, time of day, and how much skin is exposed.
In food, D3 shows up primarily in animal products: salmon, mackerel, sardines, cod liver oil, egg yolks, and fortified dairy. D2 is found in UV-exposed mushrooms and is the form added to many fortified plant milks and cereals. If you follow a vegan diet and want D3 specifically, lichen-sourced supplements are an option.
How Much You Need
The recommended daily amount of vitamin D (from all sources combined, whether D2 or D3) varies by age:
- Infants up to 12 months: 400 IU (10 mcg)
- Children and teens 1 to 18: 600 IU (15 mcg)
- Adults 19 to 70: 600 IU (15 mcg)
- Adults over 70: 800 IU (20 mcg)
- Pregnant or breastfeeding women: 600 IU (15 mcg)
These numbers represent the amount considered sufficient for bone health in most people. Many researchers argue that optimal levels, particularly for immune function and other non-bone outcomes, may require higher intakes, but official guidelines haven’t shifted yet.
What This Means for Supplement Labels
When you’re shopping for vitamin D, the label will say either D2 or D3. Now you know the difference: D3 is more effective at raising and maintaining your blood levels, and it’s the same form your body produces naturally. D2 is a reasonable alternative, especially for people avoiding animal-derived products, but you may need a higher dose to get the same result. If a label simply says “vitamin D” without specifying, check the ingredient list for “cholecalciferol” (that’s D3) or “ergocalciferol” (that’s D2).