Vitamin D is completely different than most other vitamins.

In fact, it’s a steroid hormone produced from cholesterol when your skin is exposed to the sun.

For this reason, vitamin D is often referred to as “the sunshine vitamin.”

However, sun exposure rarely provides adequate vitamin D, making it necessary to obtain it from supplements or your diet.

Yet, only a handful of foods contain significant amounts of this crucial vitamin, and deficiency is very common (1, 2, 3).

In fact, around 41.6% of the U.S. population is deficient (4).

This article explains everything you need to know about vitamin D.

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Vitamin D is a fat-soluble vitamin, meaning that it dissolves in fats and oils and can be stored in your body for a long time.

Two main dietary forms exist (5):

  • Vitamin D3 (cholecalciferol). Found in some animal foods, like fatty fish and egg yolks.
  • Vitamin D2 (ergocalciferol). Found in some plants, mushrooms, and yeasts.

Of the two, D3 (cholecalciferol) seems to be almost twice as effective at increasing blood levels of vitamin D as D2 (ergocalciferol) (6, 7).

SUMMARY Vitamin D is a fat-soluble vitamin that your body can store for long periods of time. Of the two main forms — D2 and D3 — the latter is more effective at raising vitamin D levels in your blood.

Vitamin D needs to undergo two conversion steps to become active (8, 9).

First, it is converted to calcidiol, or 25(OH)D, in your liver. This is the storage form of the vitamin.

Second, it is converted to calcitriol, or 1,25(OH)2D, mostly in your kidneys. This is the active, steroid-hormone form of vitamin D.

Calcitriol interacts with the vitamin D receptor (VDR), which is found in almost every single cell in your body (10, 11).

When the active form of vitamin D binds to this receptor, it turns genes on or off, leading to changes in your cells. This is similar to how most other steroid hormones work (12, 13).

Vitamin D affects various cells related to bone health. For example, it promotes the absorption of calcium and phosphorus from your gut (14).

But scientists have recently discovered that it also plays roles in other areas of health, such as immune function and protection against cancer (15).

SUMMARY Vitamin D is turned into calcidiol, the storage form of the vitamin, which is then converted into calcitriol, the active steroid form. Calcitriol binds to the vitamin D receptor inside your cells, turning genes on or off.

Vitamin D can be produced from cholesterol in your skin when it’s exposed to ultraviolet B (UVB) rays from the sun (16).

If you live in an area with abundant sunshine, you can probably get all the vitamin D you need by sunbathing a few times per week.

Keep in mind that you need to expose a large part of your body. If you're only exposing your face and hands, you will produce much less vitamin D.

Also, if you stay behind glass or use sunscreen, you will produce less vitamin D — or none at all (17).

However, you should make sure to use sunscreen when staying in the sun for extended periods. Sunshine is healthy, but sunburns can cause premature skin aging and raise your risk of skin cancer (18, 19).

If you're staying in the sun for a long time, consider going without sunscreen for the first 10–30 minutes — depending on your sensitivity to sunlight — then applying it before you start to burn.

As vitamin D gets stored in your body for weeks or months at a time, you may only need occasional sunshine to keep your blood levels adequate.

That said, if you live in an area without adequate sunlight, getting vitamin D from foods or supplements is absolutely essential — especially during winter.

SUMMARY Sunshine is an effective way to get vitamin D, but sunscreen blocks its production. While sunbathing safely can help you obtain adequate levels, many people don't have access to sunshine for most of the year.

Here is the vitamin D3 content of a few of the best food sources (20):

FoodAmount% RDI
Cod liver oil, 1 tablespoon (15 ml)1,360 IU / 34 mcg227%
Salmon, cooked, 3 ounces (85 grams)447 IU / 11 mcg75%
Tuna, canned in water, 3 ounces (85 grams)154 IU / 4 mcg26%
Beef liver, cooked, 3 ounces (85 grams)42 IU / 1 mcg7%
1 large whole egg (D is found in yolk)41 IU / 1 mcg7%
1 sardine, canned in oil, drained23 IU / 0.6 mcg4%

Although fatty fish like salmon, mackerel, swordfish, trout, tuna, and sardines are decent sources, you would have to eat them almost every day to get enough.

The only excellent dietary source of vitamin D is fish liver oil — such as cod liver oil — which contains upwards of two times the Reference Daily Intake (RDI) in a single tablespoon (15 ml).

Keep in mind that dairy products and cereals are often fortified with vitamin D (21).

Some rare mushrooms also harbor vitamin D, and egg yolks contain small amounts.

SUMMARY Cod liver oil is the single best source of vitamin D3. Fatty fish is also a good source, but you have to eat it frequently to get enough.

Vitamin D deficiency is one of the most common nutrient deficiencies.

Some people are at a greater risk than others. In the United States, 41.6% of the total population is deficient, although minorities fare worse — 82.1% and 69.2% of blacks and Hispanics are deficient, respectively (4).

Additionally, older adults are at a much greater risk of being deficient (22).

Those who have certain diseases are also very likely to be deficient. One study showed that 96% of people who had experienced heart attacks were low in vitamin D (23).

Overall, vitamin D deficiency is a silent epidemic. The symptoms are usually subtle and may take years or decades to surface.

The most well-known symptom of vitamin D deficiency is rickets, a bone disease common in children in developing countries.

Rickets has been mostly eliminated from Western countries because of the fortification of some foods with vitamin D (24).

Deficiency is also linked to osteoporosis, reduced mineral density, and increased risk of falls and fractures in older adults (25).

What’s more, studies indicate that people with low vitamin D levels have a much greater risk of heart disease, diabetes (types 1 and 2), cancer, dementia, and autoimmune diseases like multiple sclerosis (26).

Finally, vitamin D deficiency is linked to a reduced life expectancy (27, 28, 29).

That said, it’s unclear whether deficiency contributes to these diseases or whether people with low levels are just more likely to get them.

SUMMARY Vitamin D deficiency is associated with a variety of health issues, as well as reduced life expectancy.

Here are some potential benefits of vitamin D:

  • Reduced risk of osteoporosis, falls, and fractures. Higher doses of vitamin D can help prevent osteoporosis, falls, and fractures in older adults (30).
  • Better strength. Vitamin D can increase physical strength in both upper and lower limbs (31).
  • Cancer prevention. Vitamin D may help prevent cancer. One study noted that 1,100 IU per day — alongside calcium — reduced cancer risk by 60% (32, 33).
  • Depression management. Studies show that vitamin D may ease symptoms in people with clinical depression (34).
  • Reduced risk of type 1 diabetes. One study in infants linked 2,000 IU of vitamin D per day to a 78% reduced risk of type 1 diabetes (35).
  • Improved mortality. Some studies suggest that vitamin D reduces people's risk of dying during the study periods, indicating that it may help you live longer (36, 37).

However, many of these results are preliminary. According to a recent review, more evidence is necessary to confirm many of these benefits (38).

SUMMARY Research suggests that vitamin D may have numerous benefits related to cancer, bone health, mental health, and autoimmune diseases. However, more studies are needed.

The only way to know if you are deficient — and thus need to supplement — is by having your blood levels measured.

Your healthcare provider will measure the storage form of vitamin D, which is known as calcifediol. Anything under 12 ng/ml is considered deficient, and anything above 20 ng/ml is considered adequate.

The RDI for vitamin D is as follows (39):

  • 400 IU (10 mcg): infants, 0–12 months
  • 600 IU (15 mcg): children and adults, 1–70 years old
  • 800 IU (20 mcg): older adults and pregnant or breastfeeding women

Although adequacy is measured at 20 ng/ml, many health experts believe that people should aim for blood levels higher than 30 ng/ml for optimal health and disease prevention (40).

Additionally, many believe that the recommended intake is far too low and that people need much more to reach optimal blood levels (41).

According to the U.S. National Academy of Medicine, the safe upper limit is 4,000 IU (100 mcg) per day (42).

Vitamin D3 supplements appear to be more effective at raising vitamin D levels than D2 supplements. D3 capsules are available in most supermarkets and health food stores, as well as online.

SUMMARY The RDI for vitamin D is 400 IU (10 mcg) for infants, 600 IU (15 mcg) for children and adults, and 800 IU (20 mcg) for older adults and pregnant or breastfeeding women.

It's important to keep in mind that nutrients usually don't work in isolation.

Many of them depend on one another, and increased intake of one nutrient may increase your need for another.

Some researchers claim that fat-soluble vitamins work together and that it’s crucial to optimize your vitamin A and K intake while supplementing with vitamin D3 (43, 44).

This is especially important for vitamin K2, another fat-soluble vitamin that most people don't get enough of (45).

Magnesium — another important mineral often lacking in the modern diet — may also be important for vitamin D function (46, 47).

SUMMARY Evidence suggests that vitamin D works with magnesium and vitamins A and K to promote health.

It is a myth that it is easy to overdose on vitamin D.

Vitamin D toxicity is very rare and only happens if you take very high doses for extended periods (48).

The main symptoms of toxicity include confusion, lack of concentration, drowsiness, depression, vomiting, abdominal pain, constipation, and high blood pressure (49).

SUMMARY Vitamin D toxicity is very rare. The symptoms include confusion, drowsiness, depression, constipation, and high blood pressure.

Vitamin D is a fat-soluble vitamin important for bone health.

For those low in this nutrient, increasing intake may also reduce depression and improve strength.

Your skin produces vitamin D when exposed to sunlight. Foods like fatty fish, fish oil, and liver also contain vitamin D — as well as certain fortified foods and supplements.

Deficiency is fairly common due to limited sunlight exposure and a small selection of rich dietary sources.

If you don’t spend much time in the sun and rarely eat fatty fish, consider supplementing.

Getting enough vitamin D can go a long way to boosting your health.