Vitamin D2, also called ergocalciferol, is one of two main forms of vitamin D used by the human body. It comes from fungi, yeast, and certain plants, setting it apart from vitamin D3, which is produced in human skin and found in animal-based foods. D2 is the form most often found in prescription-strength vitamin D supplements and in fortified plant-based foods, making it particularly relevant for people on vegan or vegetarian diets.
Where Vitamin D2 Comes From
Vitamin D2 doesn’t exist in large amounts in nature until ultraviolet light gets involved. Fungi and yeast contain a compound called ergosterol, which sits in their cell membranes. When UV-B light (280 to 315 nm) hits ergosterol, it converts into vitamin D2. This is the same basic principle behind how your skin makes D3 from sunlight, just with a different starting molecule.
This UV conversion is the basis for most commercial vitamin D2 production. Manufacturers expose yeast or mushrooms to UV light to generate D2 in bulk. You can even do a version of this at home: placing store-bought button mushrooms in direct sunlight for one to two hours can raise their vitamin D2 content to roughly 10 micrograms per 100 grams of fresh weight, enough to make a meaningful dent in your daily needs. Mushrooms that are deliberately exposed to stronger UV sources after harvest can reach levels as high as 320 micrograms per 100 grams, though this declines over time on the shelf.
How Your Body Activates It
Vitamin D2 on its own is biologically inactive. It has to go through two chemical conversions before your body can use it. The first happens in the liver, where an enzyme converts D2 into a compound called 25-hydroxyvitamin D2. This is the form that circulates in your blood and the one measured in standard vitamin D blood tests. The liver enzyme responsible handles both D2 and D3 with comparable efficiency.
The second conversion takes place in the kidneys, where another enzyme transforms 25-hydroxyvitamin D2 into its fully active hormonal form. This active form is what regulates calcium absorption in your gut, maintains bone mineralization, and supports immune function. The kidneys are the primary source of this circulating active hormone, and the process is tightly regulated based on your body’s calcium and phosphorus needs.
D2 vs. D3: Key Differences
Both D2 and D3 go through the same activation steps and perform the same functions once converted. The practical difference comes down to how long they last in your body. Research measuring the half-life of each form found that the circulating form of D2 clears from the blood faster than D3: about 13.9 days on average for D2 compared to 15.1 days for D3. That gap widens in some populations. In a study comparing participants in the UK and The Gambia, Gambian participants showed an even larger difference (12.8 days vs. 14.7 days).
The reason for this shorter half-life is that D2 binds less tightly to the protein that carries vitamin D through the bloodstream. Weaker binding means more of it is exposed and available for breakdown at any given time. In practical terms, this means equal doses of D2 and D3 don’t maintain blood levels equally well over time. D3 tends to raise and sustain total vitamin D levels more effectively, which is why many clinicians and over-the-counter supplements favor D3.
That said, D2 still works. It raises blood levels of vitamin D, supports bone health, and corrects deficiency. It just may need to be taken more frequently or at slightly higher doses to match D3’s staying power.
Food Sources of Vitamin D2
Mushrooms are the only widely available whole food that naturally contains significant vitamin D2, and only when they’ve been exposed to UV light. Most commercially grown mushrooms are raised in the dark, so they contain very little unless they’ve been UV-treated after harvest. Look for packaging that specifically mentions UV exposure or lists a vitamin D content on the nutrition label.
Beyond mushrooms, most dietary vitamin D2 comes from fortified foods. Plant-based milks (soy, almond, oat), some orange juices, and certain breakfast cereals are commonly fortified with D2 rather than D3, since D2 is plant-derived and suitable for vegan products. Yeast-based supplements are another source.
Because vitamin D is fat-soluble, it absorbs best when eaten alongside some dietary fat. Both D2 and D3 are well absorbed in the small intestine, but absorption depends on your gut’s ability to process fat. Taking a supplement with a meal that contains fat, even a small amount, improves uptake. Some vitamin D absorbs even without fat, but you get more from it when fat is present.
Prescription Use
Vitamin D2 is the form available in high-dose prescription capsules, typically at 50,000 IU. These are used to treat specific medical conditions rather than routine supplementation. The approved indications include hypoparathyroidism (underactive parathyroid glands), vitamin D-resistant rickets, and familial hypophosphatemia, a genetic condition that causes low phosphorus levels.
Prescription dosing for these conditions varies enormously, from 12,000 IU to as high as 500,000 IU daily for resistant rickets. The gap between a therapeutic dose and a toxic dose is narrow at these levels, so treatment requires regular blood monitoring of calcium and phosphorus. This is a different situation from the low-dose D2 you might find in a daily multivitamin or fortified food.
Safety and Upper Limits
For adults aged 19 and older, the tolerable upper intake level for vitamin D (including both D2 and D3) is 4,000 IU per day. This is the highest daily amount considered unlikely to cause harm in the general population. The limits are lower for children: 1,000 IU for infants under 6 months, 1,500 IU for infants 6 to 12 months, 2,500 IU for children 1 to 3, and 3,000 IU for children 4 to 8. From age 9 onward, including during pregnancy and breastfeeding, the upper limit is 4,000 IU.
The primary risk of taking too much vitamin D is hypercalcemia, a buildup of calcium in the blood. This occurs when serum calcium rises above 10.5 mg/dL. Symptoms can include nausea, frequent urination, weakness, and in severe cases, kidney damage. Toxicity from food sources alone is essentially impossible. It occurs from supplement overuse, particularly with the high-dose prescription capsules.
Storage and Stability
Vitamin D2 is somewhat less stable than D3 under certain conditions. In laboratory testing, ergocalciferol broke down rapidly at room temperature (25°C) and even faster at elevated temperatures (40°C) when stored in dry air. D3 held up better under dry conditions but also degraded quickly at high heat. If you’re storing D2 supplements, keep them in a cool, dry place away from direct light. Mushrooms with UV-generated D2 retain adequate levels through their shelf life if consumed before the best-before date, but the D2 content does decline over time.