Vitamin D is the name for a small family of compounds, and vitamin D3 is one member of it. Vitamin D3 (cholecalciferol) is one of the two main forms; the other is vitamin D2 (ergocalciferol). Both count toward your daily intake, but studies show D3 usually raises blood levels a bit more.
This guide covers where each form comes from, what the research says, how much you need at each age, and what a blood test can and cannot tell you. Figures are current as of October 2026.
What vitamin D is and why the body needs it
Vitamin D helps your gut absorb calcium and phosphorus. Without enough of it, bones can become thin, soft or misshapen. In children, for example, severe deficiency causes rickets. In adults, it can lead to osteomalacia, which means soft, painful bones.
It also plays a part in muscle function and the immune system. However, large trials have not shown that extra vitamin D prevents cancer, heart disease or most other illnesses in people who already get enough.
Neither D2 nor D3 does anything on arrival. First, the liver turns either form into 25-hydroxyvitamin D, also called calcidiol. Then the kidneys convert some of that into the active hormone, calcitriol. That middle compound, 25-hydroxyvitamin D, is what a standard blood test measures.
Vitamin D vs. D3: the two forms side by side
Vitamin D2 comes from plants and fungi. For example, mushrooms make it when they are exposed to ultraviolet light. Vitamin D3, by contrast, comes from animals. Your own skin also makes D3 when sunlight hits it, which is why vitamin D is often called the “sunshine vitamin.”
Both forms appear in supplements and fortified foods. D3 sold as a supplement is usually made from lanolin, the oil in sheep’s wool. That said, plant-based D3 made from lichen also exists, so vegans are not limited to D2.
| Feature | Vitamin D2 (ergocalciferol) | Vitamin D3 (cholecalciferol) |
|---|---|---|
| Natural source | Fungi and yeast exposed to UV light | Animal foods; made in human skin from sunlight |
| Typical food examples | UV-treated mushrooms, some fortified plant milks | Fatty fish, egg yolks, most fortified dairy milk |
| Supplement origin | Made from yeast or fungi | Usually lanolin; lichen-based versions exist |
| Counts toward the RDA | Yes | Yes |
| Effect on blood levels | Raises 25(OH)D, but less on average | Raises 25(OH)D more on average, especially with large, infrequent doses |
| Prescription use in the US | Common in high-dose prescription capsules | Mostly over the counter |
Which form raises blood levels more
Several reviews have compared the two forms head to head. A 2012 meta-analysis of randomized trials in the American Journal of Clinical Nutrition found that D3 raised blood levels more than D2. However, the edge was clear mainly when people took large single doses. With daily dosing, the difference shrank and was not statistically significant.
A larger 2021 systematic review in Nutrients pooled 22 trials with about 1,300 participants. On average, D3 raised total 25-hydroxyvitamin D by roughly 16 nmol/L (about 6 ng/mL) more than D2. Still, the authors noted that at the low doses found in fortified foods, D3 may be only slightly better.
In practice, both forms can fix a shortfall. D3 just tends to get there a little more efficiently. If a doctor prescribes D2, that is a standard and accepted treatment.
How much vitamin D you need by age
The US recommendations come from the National Academies’ 2011 report. The NIH Office of Dietary Supplements fact sheet lists them alongside the upper limits. In short, the RDA covers nearly all healthy people. The upper limit (UL) is the most you can take daily from all sources without likely harm. It is a ceiling, not a target.
| Age | Recommended daily amount | Upper limit per day |
|---|---|---|
| 0-6 months | 10 mcg (400 IU)* | 25 mcg (1,000 IU) |
| 7-12 months | 10 mcg (400 IU)* | 38 mcg (1,500 IU) |
| 1-3 years | 15 mcg (600 IU) | 63 mcg (2,500 IU) |
| 4-8 years | 15 mcg (600 IU) | 75 mcg (3,000 IU) |
| 9-70 years | 15 mcg (600 IU) | 100 mcg (4,000 IU) |
| 71 years and older | 20 mcg (800 IU) | 100 mcg (4,000 IU) |
| Pregnant or breastfeeding | 15 mcg (600 IU) | 100 mcg (4,000 IU) |
*For infants, this is an Adequate Intake rather than an RDA, because there is less data for that age.
Doctors sometimes prescribe doses above the UL for a short time to treat a confirmed deficiency. When that happens, it is done under medical supervision, with follow-up testing.
IU vs. mcg: reading the label
Labels can be confusing, because vitamin D is measured in two units. The conversion is simple: 1 microgram (mcg) equals 40 international units (IU). So 15 mcg is 600 IU, and 100 mcg is 4,000 IU. To go from IU to mcg, divide by 40.
The FDA’s Daily Value for vitamin D is 20 mcg for adults and children 4 and older. Nutrition Facts labels must list vitamin D in micrograms. Many supplement labels show both units, but some show only one. Doing the quick math helps you avoid doubling up by mistake. The same habit is useful when you look at other nutrient breakdowns, such as the nutrition data some restaurants publish for their menus.
Food sources and sunlight
Few foods contain much vitamin D naturally. So fatty fish are the best source. For example, 3 ounces of cooked sockeye salmon provides roughly 570 IU, close to a full day’s RDA for most adults. A large egg has about 45 IU, mostly in the yolk.
That is why fortified foods fill much of the gap for Americans. Almost all US cow’s milk is fortified with about 120 IU per cup. Many plant milks, breakfast cereals and some orange juices are fortified too, but amounts vary, so check the label. Mushrooms treated with UV light can contain a few hundred IU per serving, as D2.
Sunlight is the other route. When UVB rays hit bare skin, the skin makes D3. How much depends on the season, time of day, latitude, cloud cover, skin tone and sunscreen. In Los Angeles, the sun is strong for much of the year. Farther north, however, winter sun is often too weak. Because UV exposure also raises skin cancer risk, health agencies do not set a “safe” amount of sun for vitamin D.
Who is at risk of low vitamin D
Some groups have a harder time getting enough:
- Breastfed infants. Breast milk is low in vitamin D, so a daily infant supplement of 400 IU is commonly advised. Ask your pediatrician.
- Older adults. Skin makes less D3 with age, and many people spend more time indoors.
- People with limited sun exposure. This includes people who are homebound or who cover most of their skin.
- People with darker skin. More melanin means the skin makes less vitamin D from the same sunlight.
- People with conditions that limit fat absorption. Examples include Crohn’s disease, celiac disease, cystic fibrosis and some liver diseases.
- People with obesity or after gastric bypass surgery. Body fat holds onto vitamin D, and surgery can reduce absorption.
The blood test and what the numbers mean
The standard test measures 25-hydroxyvitamin D in the blood. Depending on the lab, results come in either nanomoles per liter (nmol/L) or nanograms per milliliter (ng/mL). To convert, 1 ng/mL equals about 2.5 nmol/L.
The National Academies committee used these ranges, which the NIH fact sheet also lists:
- Below 30 nmol/L (12 ng/mL): at risk of deficiency.
- 30-50 nmol/L (12-20 ng/mL): possibly too low for some people’s bone health.
- 50 nmol/L (20 ng/mL) or higher: adequate for most people.
- Above 125 nmol/L (50 ng/mL): linked to possible harm.
Not every expert group agrees, though. The Endocrine Society’s 2011 guideline aimed for at least 30 ng/mL. Then its 2024 guideline dropped specific target levels. It also advised against routine testing in healthy adults, since the evidence does not show clear benefits tied to a given number. As a result, two labs or two doctors may label the same result differently. Testing still makes sense for people with bone disease, malabsorption or other medical reasons.
Risks of too much and drug interactions
Vitamin D toxicity almost never comes from food or sunlight. Instead, it comes from taking too many high-dose supplements, often for months. In that case, the main problem is too much calcium in the blood. That can cause nausea, vomiting, muscle weakness, confusion, frequent urination and kidney stones. In severe cases, it can damage the kidneys or cause an irregular heartbeat.
Vitamin D supplements can also interact with medicines. According to the NIH, these include:
- Orlistat, a weight-loss drug that can reduce vitamin D absorption.
- Statins, some of which may interact with vitamin D in either direction.
- Steroids such as prednisone, which can lower vitamin D levels and calcium absorption.
- Thiazide diuretics, which together with vitamin D can push blood calcium too high, especially in older adults.
For more practical guides on nutrition, exercise and everyday health, see our health and sports guides.
This is general information, not medical advice. Talk to your doctor before you start a vitamin D supplement, especially if you take other medicines.
Frequently asked questions
Is vitamin D3 the same as vitamin D?
Vitamin D3 is one form of vitamin D. The label “vitamin D” can mean D2, D3 or both. Both count toward your daily needs, but D3 tends to raise blood levels a bit more.
What is the difference between vitamin D and vitamin D3?
Vitamin D is the umbrella term. D3 comes from animal sources and your skin, while D2 comes from fungi and yeast. Reviews favor D3 for raising blood levels, but with daily doses the gap is small.
How many IU are in 50 mcg of vitamin D?
Multiply mcg by 40, so 50 mcg equals 2,000 IU. That is still only half the 4,000 IU adult upper limit.
What is a normal vitamin D level?
The NIH says 20 ng/mL (50 nmol/L) or higher is adequate for most people. Some labs and doctors use 30 ng/mL instead. Levels above 50 ng/mL (125 nmol/L) may cause harm.
Can you get enough vitamin D from the sun alone?
Some people can, especially in sunny places like Southern California. However, it depends on skin tone, age, season and sunscreen use. So many people also rely on fortified foods.
Have a question about this topic? Ask in the comments below. Our editors read every comment and answer questions here, and we add useful answers to the guide.
Sources
- ods.od.nih.gov/factsheets/VitaminD-HealthProfessional
- ods.od.nih.gov/factsheets/VitaminD-Consumer
- nap.nationalacademies.org/catalog/13050/dietary-reference-intakes-for-calcium-and-…
- pmc.ncbi.nlm.nih.gov/articles/PMC3349454
- pmc.ncbi.nlm.nih.gov/articles/PMC8538717
- academic.oup.com/jcem/article/109/8/1907/7685305
- fda.gov/food/nutrition-facts-label/daily-value-nutrition-and-sup…
- medlineplus.gov/lab-tests/vitamin-d-test
- dietaryguidelines.gov/resources/2020-2025-dietary-guidelines-online-materials/…