Vitamin D and vitamin D3 are not exactly the same thing. “Vitamin D” is an umbrella term that covers two forms: vitamin D2 (ergocalciferol) and vitamin D3 (cholecalciferol). So when you see “vitamin D” on a label or in a health recommendation, it could refer to either form. Vitamin D3 is one specific type, and it happens to be the more effective one at raising your blood levels.
Two Forms of Vitamin D
Vitamin D2 comes from plants and fungi. Vitamin D3 comes from animal sources and is also the form your skin produces when exposed to sunlight. Both share the same basic molecular structure, but their side chains differ: D2 has an extra double bond and methyl group that D3 lacks. This small chemical difference changes how your body processes each one.
Your body handles both forms through the same pathway. After you ingest or produce either form, it travels to the liver, where it’s converted into a circulating form called 25-hydroxyvitamin D. This is what doctors measure in a blood test. From there, the kidneys convert it into the fully active hormone that regulates calcium absorption, bone health, and immune function. Your body tightly controls this final activation step using feedback loops involving parathyroid hormone and other signals.
D3 Raises Blood Levels More Effectively
A systematic review and meta-analysis published in Advances in Nutrition compared daily supplementation with D2 versus D3 across 12 studies. The result: D2 raised total blood levels of vitamin D about 40% less than D3 did at the same dose. That’s a significant gap. It means if you take 1,000 IU of D2 and someone else takes 1,000 IU of D3, the D3 user will end up with meaningfully higher vitamin D status.
This difference is why most doctors and dietitians now recommend D3 specifically when suggesting a supplement. It’s also why many over-the-counter vitamin D products have shifted to D3 as the default form. If a supplement label just says “vitamin D” without specifying, check the ingredients list for “cholecalciferol” (D3) or “ergocalciferol” (D2).
Where Each Form Comes From
Your skin makes D3 naturally when UVB rays from sunlight hit a precursor molecule called 7-dehydrocholesterol. This triggers a chemical reaction that produces previtamin D3, which slowly converts into active D3 through body heat. The amount you produce depends on time of day, season, latitude, skin tone, and how much skin is exposed.
Food sources of D3 are mostly animal-based: fatty fish like salmon and mackerel, egg yolks, and liver. Most D3 supplements historically came from lanolin, a waxy substance extracted from sheep’s wool. For people who avoid animal products, lichen is one of the very few non-animal sources of D3. Lichen produces D3 through a process remarkably similar to human skin, using the same precursor molecule and UVB exposure.
D2, on the other hand, comes from fungi and some plants. Mushrooms are the standout source. When exposed to UV light, common white button mushrooms can produce impressive amounts of D2. In one study, 100 grams of sliced mushrooms exposed to midday summer sunlight produced 17.5 micrograms of D2 after just 15 minutes and 32.5 micrograms after an hour. UV-treated mushrooms are now sold in many U.S. grocery stores, typically containing 100% of the estimated average daily requirement per serving. D2 is also the form used in most fortified plant milks and some prescription vitamin D supplements.
How Much You Need
The National Institutes of Health sets the recommended daily allowance for vitamin D (in either form) at 600 IU for most people ages 1 through 70. Adults over 70 need 800 IU. Infants up to 12 months have an adequate intake of 400 IU. These recommendations apply equally during pregnancy and lactation.
The tolerable upper limit is 4,000 IU per day for adults. Going well above this over time can cause a dangerous buildup of calcium in the blood, leading to nausea, vomiting, weakness, frequent urination, bone pain, and kidney stones. Toxicity is essentially impossible from sun exposure or food alone. It only happens through excessive supplementation.
Which One Should You Take
For most people, D3 is the better choice for supplementation because it’s roughly 40% more effective at raising and maintaining your blood levels. If you follow a vegan diet, you have two practical options: UV-exposed mushrooms for D2, or a lichen-derived D3 supplement. D2 from mushrooms still works, it just takes a higher dose to achieve the same result as D3.
If you’re unsure about your vitamin D status, a simple blood test measuring 25-hydroxyvitamin D will tell you where you stand. This test captures both D2 and D3 together, giving your total vitamin D level. When a label, doctor, or guideline says “vitamin D” without specifying a number, they’re referring to total vitamin D from any source. When they say “vitamin D3,” they mean the specific, more potent form your body also makes from sunlight.