What Is Vitamin D? Functions, Sources, Deficiency Signs, Dosage, and More

Medically Reviewed
You probably already know vitamin D can help you build and maintain strong bones. You may even know that it is produced in your body when the sun’s ultraviolet B (UVB) rays shine upon your skin, as the Harvard T.H. Chan School of Public Health notes.

But what’s really going on when our bodies get enough, too much, or too little of it? And how does?vitamin D?affect our health?

What Does Vitamin D Do?

For starters, know that this so-called sunshine vitamin is different from other nutrients because it also functions as a?hormone?— a switch that occurs when your body absorbs the vitamin. That means it acts as a messenger in your metabolism, potentially affecting everything from weight to organ functioning, per the National Academies.

Yet up to 42 percent of Americans may have inadequate vitamin D, based on a study that defined this as having levels under 20 nanograms per milliliter (ng/mL).

(However, note that officially, the National Institutes of Health, or NIH, defines deficiency has being less than 12 ng/mL, though optimal levels are 30 ng/mL.)

“Not getting enough vitamin D poses health concerns because the vitamin is said to help absorb calcium, which ultimately helps build?strong and healthy bones,” says Kerry Clifford, RD, who is the director of Sustainable Nutrition Affairs for the National Dairy Council in Chicago.

More specifically, vitamin D helps the body to get sufficient?calcium?and phosphorus. These minerals are critical for building and maintaining strong bones. Getting an ample amount of the nutrient is key for warding off conditions like rickets in children, osteomalacia in adults, and?osteoporosis?in the elderly, per the NIH.

“Vitamin D may also be helpful in reducing inflammation and boosting immune function and cell growth,” says Clifford.

She notes that some groups of people are at especially high risk of being vitamin D deficient: “Breastfed infants, older adults, people with limited sun exposure and darker-skinned people are the most at risk for vitamin D deficiency.”

When you look at Black Americans, 82 percent aren’t getting enough; and neither are 69 percent of people who are Hispanic. There is also an association between being vitamin D-deficient and having obesity; not having a college education is also linked with being deficient.

Common Questions & Answers

Where does vitamin D come from?
When your skin is exposed to UVB rays, vitamin D is produced. Certain foods, like salmon, tuna, plant-based beverages, and fortified foods and drinks can also provide vitamin D. A supplement may be necessary if your healthcare team recommends one.
How much vitamin D do you need?
Up to age 70, aim to get 600 IU (15 mcg) of vitamin D per day. Over age 70, the recommended amount is 800 IU (20 mcg). Infants need less: 400 IU (10 mcg) per day. Don't get more than 4,000 IU (100 mcg) from supplements if you're over age 9. Up to age 8, don't exceed 1,000–3,000 IU (25–75 mcg).
What does vitamin D do?
Vitamin D helps grow strong and healthy bones by aiding calcium and phosphorus absorption. In addition to reducing inflammation and helping prevent and treat certain diseases, vitamin D has important roles in cell growth and immunity.
What foods are high in vitamin D?
Though sunshine and supplements are the top sources of vitamin D, certain foods can help you reach the ideal threshold. Foods high in vitamin D include fatty fish, egg yolks, beef liver, and fortified foods like such as milk, plant-based beverages, yogurt and orange juice.
What happens when your vitamin D is low?
Vitamin D deficiency can contribute to bone diseases, including osteoporosis, rickets, and osteomalacia. Hyperparathyroidism, which causes excess calcium accumulation in the blood, is another potential result of low vitamin D.

Where Does Vitamin D Come From?

Getting enough vitamin D is no easy task. For one thing, vitamin D is fat-soluble, which means the body can only absorb it when you eat foods with fat, such as avocado,?chicken, and peanut butter. People who have trouble absorbing fat may be among those predisposed to a?vitamin D deficiency.

Additionally, food sources of vitamin D may be difficult for some people to come by.

The following foods provide vitamin D:

  • Fatty fish, such as salmon, tuna, sardines, and mackerel
  • Fortified milk and other dairy products
  • Fortified plant-based beverages, typically used as milk substitutes
  • Fish liver oils
  • Egg yolk and mushrooms that have been exposed to ultraviolet light

However, people relying on sourcing vitamin D through diet alone typically don’t take in more than 288 IU a day on average. That falls far shorter than the recommended daily amounts, which vary based on your age:

  • Ages 1 to 70: 600 international units (IU) or 15 micrograms (mcg)
  • Ages 71 and older: 800 IU or 20 mcg

Even if you love drinking milk, an 8-ounce glass of the 2 percent variety will only get you 120 IU (2.9 mcg) — about 20 percent of the recommended daily amount.

When Do You Need a Vitamin D Supplement?

“I would consider vitamin D supplementation if [you’re] unable to get most of your daily needs from food,” says Lori Zanini, RD, CDCES, who’s based in Los Angeles. The Endocrine Society recommends 1500–2,000 IU (37.5–50 mcg) per day, from supplements and foods, for adults and 1,000 IU (25 mcg) for children. That’s in order to get blood serum levels of the vitamin at 30 ng/ml (75 nmol/liter) or higher, which is what they recommend for maximum health benefits.

Learn More About How to Pick a Vitamin D Supplement

What’s the Difference Between Vitamin D2 and D3?

There are two main forms of vitamin D — vitamin D2 (ergocalciferol) and vitamin D3 (cholecalciferol) — and the difference between them has to do with their chemical structure and the way they’re made. Vitamin D3 is the form of vitamin D that the body makes from sunshine hitting your skin. You can get it from animal-derived sources. Vitamin D2, on the other hand, is plant based.

“The best recommended form of vitamin D supplementation is vitamin D3 or cholecalciferol,” says Zanini. Of the two types, vitamin D3 is most effective at raising your blood serum levels to where they need to be, research suggests. Yet if you are following a vegan diet, you can opt for D2 supplements.

How Much Do You Know About Vitamin D?


What Are the Potential Health Benefits of Vitamin D?

Not having enough vitamin D in your body can adversely affect your health in a number of ways, though research is mixed on whether supplements can help to repair the damage.

Bone Health

Vitamin D deficiency can lead to bone-related ailments, especially in women as they age. Women who are past?menopause?are at the highest risk of osteoporosis, an age-related disease that results when bone loss exceeds bone creation in the body. It is one of the top causes of bone fractures and breaks in seniors, according to the Mayo Clinic.

Other bone diseases that can result from not having enough vitamin D include rickets, which causes soft and weak bones in children

, and?osteomalacia, which is a similar ailment in adults, as the Mayo Clinic notes.

Yet recent research indicates that supplementing with vitamin D won’t help prevent common bone health problems. Supplements don’t help guard against falls or bone fractures, or have any clinically meaningful effect on bone mineral density, according to large review of over 81 clinical trials published in October 2018 in The Lancet.

Thyroid Health

Getting enough?vitamin?D may help prevent hyperparathyroidism, according to the American Association of Endocrine Surgeons.

The condition involves an excess of the parathyroid hormone in the bloodstream, which can cause calcium levels in your blood to rise and can lead to osteoporosis, joint pain, and other problems, the Cleveland Clinic notes.


Taking vitamin D supplements may help prevent death from cancer, as well as reduce the cancer risk in Black Americans. Findings published in January 2019 in The New England Journal of Medicine — from the VITAL study, a large, randomized clinical trial involving over 25,000 participants — suggested that people who developed cancer had a 25 percent lower death rate when they were taking vitamin D. The researchers also reported a possible reduction in cancer risk for Black Americans, though they said that further study is needed.

Mood Disorders

Vitamin D is an established therapy for seasonal depression, also called?seasonal affective disorder (SAD). Yet research on the effectiveness of supplements for addressing the disorder are mixed, notes the National Institute of Mental Health.

Heart Disease and Diabetes

Prior claims about how vitamin D supplements can help protect against cardiovascular events or type 2 diabetes haven’t held up in the face of more rigorous randomized, controlled clinical trials. Vitamin D supplements did not reduce the risk of heart attack, stroke, or cardiovascular death in VITAL study participants.

Nor did vitamin D supplements affect the risk of developing type 2 diabetes in a randomized, controlled trial published in June 2019 in The?New England Journal of Medicine.

Researchers compared type 2 diabetes rates in people taking 4,000 IU of vitamin D3 per day versus those taking a placebo, and didn’t observe a significant difference in the groups’ risk.

Learn More About How Vitamin D May Affect Your Health?

How Much Vitamin D Do You Need?

Because?the body needs different amounts of vitamin D?at different stages of life, vitamin D dosage is not a one-size-fits-all situation, notes Harvard Health.

Here are the aforementioned age-based recommended daily allowance (RDA) amounts for vitamin D, broken down in even greater detail:


Growing bodies need 400 IU, or 10 mcg, of vitamin D per day. Note that because breast milk contains only 25 IU per liter, an exclusively breastfed infant will likely need a vitamin D supplement to reach the recommended threshold. Vitamin D deficiency in infants and babies is linked with rickets, a rare but serious condition that can lead to growth delays, notes the National Center for Advancing Translational Sciences, which is part of the U.S. Department of Health and Human Services.

Children and Teenagers

Youngsters need 600 IU, or 15 mcg,?of vitamin D per day.


When you reach adulthood, 600 IU per day (15 mcg) is recommended.

Elderly?(Over Age 70)?

These individuals need 800 IU (20 mcg) per day. Talk to your doctor to get a blood test to find out if you may benefit from a vitamin D supplement, because vitamin D deficiency is more prevalent in older adults than in middle-aged and young adults, past research shows.

Pregnant or Breastfeeding Women

The NIH recommends pregnant and lactating women ages 14 to 50 get 600 IU (15 mcg) of vitamin D daily. Prior research suggested that as much as 4,000 IU daily may be beneficial for women who are pregnant and breastfeeding

. But as of October 2019, the World Health Organization (WHO) does not recommend supplements for pregnant women, noting that more research is needed about possible effects on preterm births. Moms-to-be are advised to focus on getting the nutrient from a balanced diet and sunlight exposure. Pregnant women who have been diagnosed with vitamin D deficiency should take no more than 200 IU (5 mcg) in supplements, says the WHO.

Talk to your doctor to see what dosage is best for you and your growing baby.

Learn More About the Factors That May Affect How Much Vitamin D You Need

Are You Vitamin D Deficient? Signs and Risks

Numerous factors can affect how much vitamin D you’re able to take in, including:

  • The amount of clothing you’re wearing
  • Whether you have sunscreen on
  • The season of the year

All of these can affect how much vitamin D your body is producing from sunlight exposure.

Risk Factors for Vitamin D Deficiency

Certain individuals are known to be at an elevated risk for low vitamin D, including being:

The symptoms of vitamin D deficiency are often hard to detect because they’re so subtle.

Potential Signs of Vitamin D Deficiency

But if you’re vitamin D deficient, you may experience:

  • Bone pain

  • Muscle weakness

  • Bone fractures

If you have a risk factor for vitamin D deficiency or live in an area with poor sun exposure during the winter, request a blood test with your doctor to check your vitamin D levels.

Learn More About the Signs, Symptoms, and Risks of Vitamin D Deficiency

What Are Some of the Top Sources of Vitamin D?

There are a handful of top ways to ensure you’re getting adequate vitamin D:


When you step into the sun, your body synthesizes vitamin D3 in your skin, absorbing it through cholesterol and changing it into a hormone, where it performs its various functions in the body. About 10 to 15 minutes of direct sunlight (through a window or screen doesn’t count!) should do the trick if you’re light-skinned; more time may be needed for those with darker skin, per U.S. News & World Report.

Any longer and you’ll need to apply sunscreen, which may interfere with vitamin D being synthesized in the skin but can reduce your risk for skin cancer, notes the Skin Cancer Foundation.


Vitamin D2 and D3 supplements are readily available at the pharmacy or health food store, and come in both liquid and capsule form, or may be mixed into products like multivitamins and health shake powders.

Because the U.S. Food and Drug Administration (FDA) doesn’t regulate supplements, be sure to check with your doctor to pick a responsible brand and dosage that works for you.

Also, keep in mind supplements have the potential to cause drug interactions. If you have an underlying health condition, having a conversation with your doctor about how much and what kind of vitamin D supplement is best for you is even more critical.

Vitamin D–Rich Foods

Fueling up with foods high in vitamin D combined with getting enough sunlight and taking a supplement can help you reach sufficient levels, notes the Mayo Clinic.

Learn More About Vitamin D Sources

Which Foods Are High in Vitamin D?

Because food sources of vitamin D are limited, even a healthy diet may be lacking in the nutrient. Peruse the list below, and keep an eye out for foods are fortified with vitamin D.

Foods That Contain Vitamin D

Vitamin D occurs naturally in the following foods:

  • Salmon
  • Tuna
  • Sardines
  • Eggs
  • Cod liver oil
  • Beef liver
  • Mackerel
  • Oysters
  • Mushrooms

Foods That May Be Fortified With Vitamin D or Calcium

During manufacturing, these eats may be fortified with the sunshine vitamin and calcium:

  • Milk
  • Packaged cheese, such as Swiss
  • Egg yolks
  • Yogurt
  • Kefir
  • Orange juice
  • Cereal
  • Margarine
  • Protein shakes
  • Snack bars
  • Packaged fruit chews or snacks
  • Oatmeal
  • Soy milk
  • Tofu

Foods That Contain Calcium, a Nutrient Partner to Vitamin D

As the National Osteoporosis Foundation points out, here are some foods with calcium:

Learn More About Foods High in Vitamin D

Vitamin D Toxicity: Can You Have Too Much Vitamin D?

It’s rare to get too much vitamin D, but it isn’t out of the question. And in the cases where vitamin D toxicity does?occur, the health consequences can be serious.

The Mayo Clinic notes vitamin D toxicity (hypervitaminosis) occurs when people overdose on vitamin D supplements.

It doesn’t happen from eating too much food with vitamin D or getting too much sunlight. As previously described, the?NIH?recommends that people ages 9 and up do not exceed 4,000 IU per day in supplements, and that those ages 8 and under take no more than 1,000–3,000 IU, depending on their age.

Vitamin D toxicity is marked by a?buildup?of calcium in the body (hypercalcemia). Signs that you have had too much include weakness, loss of appetite, and kidney problems.

Additional reporting by?Jamie Ludwig?and?Melinda?Carstensen.

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