What Is Vitamin D and What Does It Do?

Vitamin D belongs to the group of fat-soluble vitamins, alongside vitamins A, E and K. This means it dissolves in fat and can be stored in the body.

Unlike water-soluble vitamins, fat-soluble vitamins are absorbed with dietary fat and are handled differently by the body. Our comparison of fat-soluble vs. water-soluble vitamins explains these differences.

Vitamin D from food, supplements or sunlight isn’t immediately active. The liver first converts it into 25-hydroxyvitamin D, and the kidneys then help produce its active form, called calcitriol.^[5]

This active form helps regulate calcium and phosphorus levels, supporting healthy bones. Vitamin D also contributes to normal muscle and immune function, although taking more than you need doesn’t automatically improve these processes.^[1]

Vitamin D is one of the 13 essential vitamins, but it’s unusual because the body can also produce it under the right conditions.

How Does Vitamin D Work With Calcium?

Calcium helps build and maintain bones, but your body needs vitamin D to absorb it efficiently from food.^[3]

When vitamin D levels are too low, calcium absorption can decrease. The body then makes adjustments to maintain calcium levels in the blood, sometimes drawing minerals from bone tissue.^[3]

Over time, this can affect bone strength and development. Our guide to vitamin D and calcium explains how these nutrients work together and why both matter for bone health.

Do You Need Vitamin D3 With K2?

Vitamin D3 and K2 are often sold together in supplements marketed for bone health. Vitamin D helps with calcium absorption, while vitamin K is involved in activating certain proteins needed for normal bone metabolism.^[2]

However, there isn’t enough evidence to recommend K2 supplements routinely to everyone taking D3.^[2]

If you take warfarin or similar blood-thinning medications, vitamin K supplements require particular caution because they can interfere with treatment.^[2]

Does Vitamin D Support the Immune System?

Vitamin D plays a role in normal immune function, and researchers have studied whether supplements help prevent respiratory infections.^[4]

Results have been mixed, so maintaining adequate vitamin D is important, but high doses shouldn’t be considered a proven way to prevent infections.^[1]

What Happens When Vitamin D Is Low?

Low vitamin D doesn’t always cause obvious symptoms. Some people become aware of a problem only when testing is performed for a medical reason.

More significant or prolonged deficiency can lead to bone pain, muscle weakness and impaired bone mineralization. In children, severe deficiency can cause rickets, while adults may develop osteomalacia, a condition in which bones become poorly mineralized.^[1]

Fatigue can also occur, but it has many possible causes and cannot confirm vitamin D deficiency on its own.

Risk may be higher in older adults, people with limited sunlight exposure and those with conditions affecting fat absorption. Certain liver or kidney diseases can also interfere with how the body processes vitamin D.^[5]

When testing is appropriate, a blood test measuring 25-hydroxyvitamin D, written as 25(OH)D, is commonly used to assess vitamin D status.^[1]

How Much Vitamin D Do You Need Per Day?

Daily vitamin D recommendations depend mainly on age. They are expressed in micrograms (mcg) or International Units (IU).

These are U.S. dietary reference values, which assume minimal sun exposure. They describe usual nutritional requirements, not doses used to treat a diagnosed deficiency.^[1]

Where Can You Get Vitamin D?

Your body can obtain vitamin D from sunlight, food, fortified products and supplements. Each source has its own advantages and limitations.

Vitamin D From Sunlight

When UVB rays reach the skin, they trigger reactions that lead to vitamin D3 production.^[1]

The amount produced varies with season, latitude, age, skin pigmentation, clothing and time spent outdoors. There is no universal number of minutes in the sun that guarantees enough vitamin D for everyone.^[5]

However, this doesn’t mean that spending longer in direct sunlight is always better. Excessive ultraviolet exposure increases the risk of skin cancer, and tanning or sunburn should never be treated as a way to obtain vitamin D. Tanning beds are not a safe alternative.^[1]

Which Foods Contain Vitamin D?

Relatively few foods naturally contain substantial amounts of vitamin D. Fatty fish are among the richest sources, while some mushrooms and fortified products can also contribute.^[1]

Amounts are approximate and may vary by product, preparation and fortification.^[1]

If you’re wondering whether everyday meals provide enough essential nutrients, read Can You Get All Your Vitamins From Food?

Because vitamin D is fat-soluble, consuming it with a meal containing some fat may improve its absorption. Our guide to vitamin absorption explains how digestion and food combinations can influence this process.

Vitamin D2 vs. D3: What’s the Difference?

Vitamin D comes in two main nutritional forms. Both can raise vitamin D levels, but they differ in their sources and effects.

Research generally shows that D3 increases and maintains blood levels of 25(OH)D more effectively than D2. However, D2 can still contribute to vitamin D intake, and this difference alone doesn’t establish whether an individual needs supplementation.^[1]

If you’re comparing vitamin D supplements, check which form the product contains and how much vitamin D is listed per serving. Our guide to reading a Supplement Facts label explains how to interpret these details.

Frequently Asked Questions

Can You Have Too Much Vitamin D?

Yes. Excessive vitamin D from supplements can cause calcium to rise too high in the blood, potentially damaging the kidneys and other tissues. Toxicity is generally associated with excessive supplementation rather than ordinary food intake or normal sunlight exposure.^[1]

Can You Get Vitamin D Through a Window?

Not effectively. Ordinary window glass blocks most UVB radiation, which the skin needs to produce vitamin D3.^[5]

Is Vitamin D Deficiency More Common in Winter?

It can be, especially in places where winter sunlight provides less UVB radiation. However, vitamin D status also depends on diet, supplements, individual health and other factors.^[4]

Final Thoughts

Getting enough vitamin D matters, but more isn’t always better. Your needs depend on age, diet, health and other factors, so the right approach isn’t necessarily the same for everyone.

If you’re concerned about your vitamin D status, discuss your individual risk factors with a healthcare professional rather than guessing which supplement or dose you need.

References

1. NIH Office of Dietary Supplements.
   
   Vitamin D — Health Professional Fact Sheet
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2. NIH Office of Dietary Supplements.
   
   Vitamin K — Health Professional Fact Sheet
   .
3. NIH Office of Dietary Supplements.
   
   Calcium — Health Professional Fact Sheet
   .
4. Harvard T.H. Chan School of Public Health.
   
   Vitamin D
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5. Linus Pauling Institute, Oregon State University.
   
   Vitamin D
   .