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What is Vitamin D3? Discover its benefits, sources and side effects

Vitamin D3 helps the body absorb calcium and promotes bone health. Experts discuss its benefits, sources, deficiencies, risks and more.
Vitamin D glass bottle on a beige background.
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Vitamin D3 is a form of vitamin D that helps the body absorb calcium and other minerals which are important for healthy bones, immune function, and more.

It's also a popular dietary supplement and a staple in the drugstore vitamin aisle — which may feel overwhelming at times between all the ABCs and 123s. What is vitamin D3 exactly and how does it differ from vitamin D? Who should be taking a supplement?

We spoke to experts to get the DL on vitamin D3. Here's what to know about vitamin D3 benefits, sources, deficiencies, side effects and more.

What is vitamin D3?

Vitamin D (calciferol) is a fat-soluble vitamin that plays an important role in many bodily functions, including maintaining bone health, supporting immune function and more, Heather Hodson, clinical nutritionist at the Center for the Prevention of Cardiovascular Disease at NYU Langone Health, tells

There are two main forms, vitamin D3 and vitamin D2.

Vitamin D3, also known as cholecalciferol, is a form of vitamin D that is synthesized in the skin upon exposure to sunlight, says Hodson. "It can also be consumed through dietary sources like fish and supplements," Hodson adds.

Once Vitamin D3 is synthesized in the skin or ingested, it gets converted to calcidiol n the liver, which is eventually converted to the active form of Vitamin D, calcitriol, says Hodson.

Vitamin D2, also known as ergocalciferol or "pre-vitamin D," is typically human-made and added to foods, per the National Institutes of Health.

Vitamin D3 benefits

  • Supports bone health
  • Strengthens the immune system
  • Promotes heart and brain health
  • Reduces inflammation

Vitamin D3 plays an important role in supporting and maintaining healthy bones, says Hodson. It does so by regulating calcium and phosphorus levels in the blood, and promoting the absorption of these minerals by the gut.

It is also essential for bone growth and remodeling, per the NIH. Vitamin D3 can help prevent bone disorders, such as osteoporosis, and bone loss.

"It really supports our bone integrity, so it's keeping your skeletal system nice and strong," Zelle Morefield, a transplant dietitian at the Mayo Clinic tells Vitamin D3 supports healthy muscle and nerve function as well.

Vitamin D3 strengthens the immune system to help the body fight off bacteria and viruses, and it has neuroprotective properties which support brain cell activity, according to the Mayo Clinic.

"It has some different anti-inflammatory and antioxidant properties as well, and it's just good for just overall cellular functions and helps a lot of smaller cellular processes," says Morefield.

How much vitamin D3 do you need?

The recommended dietary allowance (RDA) of vitamin D is the daily intake sufficient to maintain bone health and normal calcium levels in healthy people, per the NIH. This varies by age:

  • Infants 0–12 months: 10 mcg (400 IU)
  • Children 1–18 years: 15 mcg (600 IU)
  • Adults 18–70 years: 15 mcg (600 IU)
  • Adults over 70 years: 20 mcg (800 IU)

What are the sources of vitamin D3?

Vitamin D3 can be obtained in three main ways: from your diet, sunlight or supplements.

Food sources of vitamin D3

Few foods are naturally rich in vitamin D3, but certain foods are fortified with vitamin D3, especially in the U.S., the experts note. Dietary sources of vitamin D3 include:

  • Fatty fish (salmon, tuna, sardines, trout)
  • Egg yolks
  • Organ meats
  • Fish liver oil
  • Fortified milk
  • Fortified orange juice
  • Fortified cereals

The other form, vitamin D2, can be obtained from plant-based sources, including fortified plant milks, juices and mushrooms exposed to UV light, says Hodson. 

Ultraviolet light

“The other way you get vitamin D would just be sunlight," says Morefield. Vitamin D is formed in the body when ultraviolet (UV) rays from sunlight hit uncovered skin, which triggers the synthesis of pre-vitaminD3, which is converted into vitamin D3, per the NIH.

"Typically after about 5-to-15 minutes of sun exposure, so a walk down the street, for example, your skin is able to synthesize vitamin D from the sunlight," says Morefield.

The amount of vitamin D the body makes will vary depending on several factors. These include location, season, time of day, cloud cover, air pollution, sunscreen, protective clothing, age and skin pigmentation or melanin content, according to the Mayo Clinic.


Dietary supplements can be used when a person does not get enough vitamin D from their diet or sunlight, or has a vitamin D deficiency caused by a medical condition or medication.

Vitamin D supplements may contain either vitamin D3, which is often derived from animals but can be sourced from lichen, or vitamin D2 which is plant-derived, per the NIH. Most standard multivitamins will contain vitamin D as well, says Morefield.

“Research demonstrates that compared to Vitamin D2, Vitamin D3 is more easily absorbed and may more effectively sustain desirable Vitamin D levels,” says Hodson.

Who should take vitamin D3?

While many people can get enough vitamin D from sunlight, food, or multivitamins, some may benefit from additional vitamin D3.

A vitamin D3 supplement may be recommended if you have an insufficiency or a deficiency. A healthcare provider can determine this through a test which measures levels of vitamin D in the blood, says Hodson.

A vitamin D deficiency typically occurs when people don't get enough vitamin D from their diet or sunlight; cannot synthesize or absorb vitamin D properly; or have certain medical conditions or take medications which affect vitamin D levels, the experts note.

Individuals with limited sun exposure, for example, those who spend very limited time outdoors or those living in cooler regions, may be at higher risk of vitamin D deficiency, says Hodson.

"In the colder months, we’re often bundled head-to-toe in winter gear. This leaves very little skin exposed to sunlight for vitamin D3 synthesis," Hodson adds.

People with dark skin, which has a higher melanin content, may not synthesize as much vitamin D, per the Cleveland Clinic. Older adults synthesize less vitamin D and often spend more time indoors, which increases the risk of a deficiency, Morefield adds.

Some medical conditions can impact the absorption or synthesis of active vitamin D, the experts note. These include:

  • Irritable bowel disease (Crohn's, ulcerative colitis)
  • Celiac disease
  • Cystic fibrosis
  • Liver or kidney disease

Individuals who've had a history of gastrointestinal surgery, such as bariatric weight loss surgery or small bowel resection surgery, may have difficulty absorbing vitamin D3, Hodson adds.

“Individuals with these conditions may benefit from additional vitamin D, though they should speak with their healthcare provider regarding what form would be most appropriate,” says Hodson.

Certain medications can also lower vitamin D levels. According to the Cleveland Clinic, these include, but are not limited to:

  • Laxatives
  • Steroids (prednisone)
  • Cholesterol-lowering drugs
  • Seizure drugs

Symptoms of vitamin D deficiency

"Vitamin D deficiency is not always characterized by noticeable symptoms," says Hodson. Some people have no signs or symptoms of a vitamin D deficiency at all, Hodson adds.

Potential symptoms of a vitamin D deficiency include:

  • Muscle pain
  • Weakness
  • Bone pain
  • Mood changes

Over time, vitamin D deficiency can lead to complications including osteoporosis, which weakens the bones, and osteomalacia (a softening of the bones) and fractures, per the Cleveland Clinic.

“In older populations, deficiency may lead to increased fall risk,” says Hodson.

In severe pediatric cases, vitamin D deficiency can cause bowed legs or rickets, which can lead to skeletal deformities, Hodson adds.

The only way to know if you have a vitamin D deficiency is to get tested, the experts emphasize. Semi-regular monitoring of vitamin D levels (for example, with annual physical lab checks) may be helpful to treat or prevent deficiencies in a timely manner, Hodson adds.

Vitamin D3 dosage

“It’s always a good idea to consult with a healthcare professional and check your vitamin D levels before starting supplementation,” says Hodson.

The recommended daily allowance for vitamin D is between 400–600 IU for healthy children and adults. "That’s the standard amount that most of the population needs to support their basic functions, but if (you have) a deficiency, that number may be very different," says Morefield.

Most vitamin D3 supplements available over-the-counter are sold in dosages between 1000–5000 IU, but some are higher. “Ask for your provider’s recommendation as to what dosage, if any, is right for you and do not exceed this dosage,” Hodson adds.

Research has shown that healthy adults without a deficiency probably won’t benefit from consuming additional vitamin D3 through supplements, previously reported.

Vitamin D3 side effects and risks

When taken appropriately and as directed, vitamin D3 is generally safe. However, consuming too much vitamin D3 can lead to vitamin D toxicity, so it's important not to exceed your prescribed or recommended dosage, says Hodson.

"Because Vitamin D is a fat-soluble vitamin, any excess that is consumed is stored in the body rather than excreted through urine," says Hodson. If you are taking too much in a vitamin D3 supplement, it can built up in the body and reach toxic levels.

According to Hodson, vitamin D toxicity can lead to:

  • Constipation
  • Nausea
  • Vomiting
  • Reduced appetite
  • Hypercalcemia

If you experience any side effects, talk to your doctor. “When selecting a supplement, choose brands that are reputable and third-party tested,” says Hodson.