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Vitamin D deficiency occurs when the body doesn’t get enough vitamin D from sunlight or diet. Vitamin D deficiency can cause loss of bone density, osteoporosis, and broken bones.

Vitamin D is sometimes called the sunshine vitamin because your body makes it from cholesterol when your skin is exposed to sunlight (1).

This vitamin has recently garnered a lot of attention for its role in immune health, specifically regarding COVID-19. It’s also critical to bone health and many important functions throughout your body (1, 2).

Most adults should get 1,500–2,000 international units (IU) of vitamin D daily. While certain foods, such as fatty fish and fortified dairy products, do contain this vitamin, it’s difficult to get enough through your diet alone (1).

It’s no surprise, then, that vitamin D deficiency is one of the most common nutritional deficiencies worldwide (1).

This article explores vitamin D and why it’s important to get enough of it.

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Vitamin D is a fat-soluble vitamin that plays critical roles in the proper functioning of your body, including bone health and immunity. It may even help prevent cancer and protect against several chronic conditions, including (3):

  • bone loss
  • depression
  • type 2 diabetes
  • heart disease
  • multiple sclerosis

An estimated 1 billion people around the globe have low blood levels of the vitamin (4).

One research review found that almost 42% of U.S. adults have a vitamin D deficiency. This figure goes up to almost 63% in Hispanic adults and 82% in African American adults (5).


Vitamin D is a fat-soluble vitamin that plays a critical role in several aspects of health, including bone health and immunity.

Vitamin D deficiency can be difficult to notice because symptoms may not occur for several months or years. Sometimes, you may have no symptoms at all.

Keeping that in mind, it’s still helpful to know what signs and symptoms to look for.

Frequent illness or infections

One of the most important roles of vitamin D is supporting immune health, which helps you ward off viruses and bacteria that cause illness.

Vitamin D directly interacts with the cells that are responsible for addressing infections (6).

If you often become sick, especially with colds or the flu, low vitamin D levels may be a contributing factor. Several large observational studies have shown a link between a deficiency and respiratory tract infections such as colds, bronchitis, and pneumonia (7, 8).

A number of studies have found that taking up to 4,000 IU of vitamin D daily may reduce the risk of respiratory tract infections (9, 10, 11).

Recently, vitamin D deficiency has been linked to an increased risk of COVID-19, as well as an increased risk of experiencing severe effects from the condition. However, it is important to note that taking vitamin D supplements — at any dosage — will not prevent COVID-19 (2).

Fatigue and tiredness

Feeling tired can stem from a number of causes, one of which may be vitamin D deficiency.

Unlike more visible causes like stress, depression, and insomnia, vitamin D deficiency is often overlooked as a potential cause of fatigue.

One study in 480 older adults linked vitamin D deficiency with fatigue symptoms (12).

Plus, a study in 39 children associated low vitamin D levels with poor sleep quality, shorter sleep duration, and delayed bedtimes (13).

One observational study in female nurses also found a strong connection between low vitamin D levels and self-reported fatigue. What’s more, 89% of the participants were deficient in this vitamin (14).

Interestingly, several studies show that supplementing with this vitamin may reduce the severity of fatigue in people with a deficiency (15, 16).

All the same, more research is needed.

Bone and back pain

Bone and lower back pain may be symptoms of inadequate vitamin D levels (17, 18).

Vitamin D helps maintain bone health by improving your body’s absorption of calcium.

One study in 98 adults with lower back pain linked lower levels of vitamin D to more severe pain. However, a large research review found that this association was inconsistent across other similar studies (19, 20).

A review of 81 studies also found that people with arthritis, muscle pain, and chronic widespread pain tended to have lower levels of vitamin D than people without these conditions (21).

Still, more studies are necessary.


Vitamin D deficiency has been linked to depression, especially in older adults — although some study results are conflicting (22, 23, 24).

The effects of vitamin D supplements have been mixed, but some reviews have found that they helped relieve symptoms of depression (25, 26, 27, 28).

Still, more research is needed to understand the relationship between vitamin D and depression.

Impaired wound healing

Slow wound healing after surgery or injury may be a sign that your vitamin D levels are too low.

In fact, results from a test-tube study suggest that vitamin D increases the production of compounds that are crucial for forming new skin as part of the wound-healing process (29).

One review of four studies found that vitamin D deficiency compromised certain aspects of healing in people who had dental surgery (30).

Vitamin D’s role in controlling inflammation and addressing infections may also be important for proper healing.

One older study in 221 people, 112 of whom had diabetes-related foot infections, found that those with severe vitamin D deficiency were more likely to have higher levels of inflammatory markers that can jeopardize healing (31).

In a 12-week study involving 60 people with diabetes-related foot ulcers, those who took a vitamin D supplement experienced significant improvements in wound healing compared with the placebo group (32).

However, further research is needed (33).

Bone loss

Vitamin D plays a crucial role in calcium absorption and bone metabolism.

This is important because taking vitamin D and calcium at the same time helps your body maximize absorption (34).

Low bone mineral density is an indication that your bones have lost calcium and other minerals. This places older adults, especially women, at an increased risk of fractures (35).

In a large observational study in more than 1,100 middle-aged menopausal or postmenopausal women, researchers found a strong link between low vitamin D levels and low bone mineral density (36).

However, research on vitamin D supplementation therapy in independent older adults has yielded mixed results. While some studies show some benefits, such as reduced muscle pain, others have not found that it wards off fractures related to bone loss (37, 38, 39).

One study found that women deficient in vitamin D experienced no improvement in bone mineral density when they took high dose supplements, even if their blood levels improved (40).

Nonetheless, adequate vitamin D intake may be a good strategy to protect your bone mass and reduce fracture risk (39).

Hair loss

Many foods and nutrients may affect hair health.

While stress is a common cause of hair loss, severe hair loss may be the result of a disease or nutrient deficiency (41, 42, 43).

Hair loss in women is linked to low vitamin D levels, though research is lacking (44).

In particular, studies tie low vitamin D levels to alopecia areata, an autoimmune disease characterized by severe hair loss (45, 46, 47).

One study in people with this condition associated lower vitamin D levels with more severe hair loss. In another study in 48 people with this condition, applying a synthetic form of vitamin D topically for 12 weeks significantly increased hair regrowth (48, 49).

Another research review found that vitamin D levels may have an inverse relationship with non-scarring hair loss. This means the higher the vitamin D levels, the less hair loss detected in the study, and vice versa (50).

Muscle pain

The causes of muscle pain are often difficult to pinpoint. However, evidence suggests that vitamin D deficiency is a potential cause (51, 52).

In an older study, 71% of people with chronic pain were found to have a deficiency in the vitamin (52).

The vitamin D receptor is present in nerve cells called nociceptors, which sense pain. This vitamin may also be involved in your body’s pain signaling pathways, which may play a role in chronic pain (53).

A few studies note that high dose vitamin D supplements may reduce various types of pain in people with a vitamin D deficiency (54, 55).

One study in 120 children with vitamin D deficiency who had growing pains found that a single dose of this vitamin reduced pain scores by an average of 57% (55).

Weight gain

Obesity is one risk factor for vitamin D deficiency (3).

One study in adults found a possible link between low vitamin D status and both belly fat and increased weight, although these effects were more pronounced in men (56).

While vitamin D deficiency may be seen in cases of obesity, further studies are needed to determine whether supplementing with this vitamin helps prevent weight gain (57).


Vitamin D deficiency is linked to anxiety disorders.

One review found that levels of calcidiol, a form of vitamin D, were lower in people with anxiety, as well as in those with depression (58).

A separate study in pregnant women found that having adequate vitamin D levels may help reduce anxiety symptoms, improve sleep quality, and even help prevent postpartum depression (59).

Still, more research is necessary.


Vitamin D deficiency may manifest in a variety of ways, including fatigue, back pain, hair loss, poor wound healing, and symptoms of depression.

Vitamin D deficiency is typically defined as having blood levels below 20 ng/mL, while levels from 21–29 ng/mL are considered insufficient (60).

While there’s no single cause for deficiency, your overall risk may be higher as a result of certain underlying conditions or lifestyle factors. Some of the most common risk factors for vitamin D deficiency are (5):

  • having dark skin
  • being older
  • having overweight or obesity
  • not eating much fish or dairy
  • living far from the equator or in regions with little sunlight year-round (61)
  • staying or working indoors (62)
  • working overnight shifts (62)
  • having chronic kidney disease, liver disease, or hyperparathyroidism (63, 64)
  • having a health condition that affects nutrient absorption, such as Crohn’s disease or celiac disease (65, 66, 67)
  • having gastric bypass surgery (68)
  • using certain medications that affect vitamin D metabolism, such as statins and steroids (69)

People who live near the equator and get frequent sun exposure are less likely to have a deficiency, because their skin produces enough vitamin D (61, 62).

While people who often wear sunscreen outdoors are also at an increased risk of deficiency, using sunscreen is important to reduce skin damage and cancer risk due to sun exposure (70, 71).

Consult a healthcare professional about your vitamin D status if you’re at increased risk of deficiency.


While vitamin D deficiency has no single cause, numerous environmental, lifestyle, genetic, medical, and dietary factors may be at play.

Vitamin D deficiency is usually treated with supplements. If a healthcare professional finds that you have a deficiency, they may recommend the following options.


Oral supplements are the go-to treatment for vitamin D deficiency. You can easily buy these over the counter but should ask a doctor for dosage recommendations.

Magnesium helps activate vitamin D, so you may want to take this mineral too (72).

For a severe deficiency, a doctor may recommend prescription vitamin D, which comes in much stronger doses of up to 50,000 IU. Your doctor may also consider vitamin D injections (1, 73).

Food sources

Eating more vitamin D-rich foods may also boost your levels. Options include (3):

  • fatty fish
  • egg yolks
  • fortified cereals
  • fortified milk and juices
  • yogurt
  • beef liver

Because sunlight is a natural source of vitamin D, your doctor may also recommend going outdoors more often. However, given the negative effects of excess ultraviolet exposure, it’s important to take precautions by limiting your total time in the sun and applying sunscreen.


Vitamin D deficiency is usually treated with supplements, food sources, and mindful sunlight exposure.

Simple blood work can help determine whether you have a vitamin D deficiency. This vitamin exists in your blood in two forms (74):

  • 25-hydroxy vitamin D (25-OH D), or calcidiol
  • 1,25-dihydroxy vitamin D, or calcitriol

The most common test for vitamin D deficiency is the 25-hydroxy vitamin D test. This is because 25-hydroxy, or calcidiol, has higher concentrations and stays in your blood longer, which makes it easier to detect (75, 76).

You may also be able to take an at-home test in which you collect a small drop of blood yourself through a simple finger prick. These tests are accessible and convenient, but you may still want a healthcare professional to help you understand the results.


You can determine your vitamin D status through blood work at a doctor’s office or healthcare clinic, as well as via at-home tests.

It can be difficult to tell whether you have a vitamin D deficiency, as the symptoms may be subtle. Furthermore, it’s possible to have a vitamin D deficiency without experiencing any symptoms.

As a general rule, consider asking a doctor to check for vitamin D deficiency if you notice any possible symptoms and especially if you have any risk factors.

Your doctor may also be able to rule out other causes for some of the symptoms you’re experiencing.


If you’re experiencing symptoms of vitamin D deficiency or are at increased risk of this condition, ask a healthcare professional about getting tested.

Vitamin D deficiency is surprisingly common, but the symptoms are often subtle and nonspecific, so it may be hard to know whether you have a deficiency or some other health condition.

If you think you may have a deficiency, ask a healthcare professional for a blood test.

Vitamin D deficiency is usually treated with supplements, but you may need a doctor’s advice to get the right dosage. Increasing your sun exposure and eating more vitamin D-rich foods, such as fatty fish and fortified dairy products, can also help.

Addressing a vitamin D deficiency is worthwhile and can have lasting benefits for your health.


Just one thing

Try this today: Interested in which foods pack vitamin D? Check out this evidence-based article on 7 delicious vitamin D-rich foods.

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