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Vitamin D is an extremely important vitamin that has powerful effects on several systems throughout your body (1).

Unlike other vitamins, vitamin D functions like a hormone. Every single cell in your body has a receptor for it.

Your body makes it from cholesterol when your skin is exposed to sunlight.

It’s also found in certain foods such as fatty fish and fortified dairy products, though it’s very difficult to get enough from diet alone.

For most adults, the Recommended Dietary Allowance (RDA) for vitamin D is around 600–800 IU, but many experts recommend getting even more than that.

Vitamin D deficiency is very common. It’s estimated that about 1 billion people worldwide have low blood levels of the vitamin (2).

According to one review, 41.6% of adults in the United States are deficient. This number goes up to 69.2% in Hispanic adults and 82.1% in African American adults (3).

Here are some of the most common risk factors for vitamin D deficiency:

  • having dark skin
  • being an older adult
  • being overweight or having obesity
  • not eating much fish or dairy
  • living far from the equator in areas where there is little sun year-round
  • always using sunscreen when going out; however, using sunscreen is important in helping prevent sun-damaging effects to the skin, including skin cancer
  • staying indoors
  • having chronic kidney disease, liver disease, or hyperparathyroidism
  • having a health condition that affects nutrient absorption, such as Crohn’s disease or celiac disease
  • having gastric bypass surgery
  • using certain medications that impact vitamin D metabolism

People who live near the equator and get frequent sun exposure are less likely to be deficient, as their skin produces enough vitamin D to satisfy their bodies’ needs.

Most people do not realize that they’re deficient, as symptoms are generally subtle. You may not recognize them easily, even if they’re having a significant impact on your quality of life.

Here are 8 signs and symptoms of vitamin D deficiency.

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One of vitamin D’s most important roles is keeping your immune system strong so you’re able to fight off viruses and bacteria that cause illness.

It directly interacts with the cells that are responsible for fighting infection (4).

If you often become sick, especially with colds or the flu, low levels of vitamin D may be a contributing factor.

Several large observational studies have shown a link between a deficiency and respiratory tract infections like colds, bronchitis, and pneumonia (5, 6).

A number of studies have found that taking vitamin D supplements at a dosage of up to 4,000 IU daily may reduce your risk for respiratory tract infections (7, 8, 9).

In one older study involving people with chronic obstructive pulmonary disease (COPD), only those who were severely deficient in vitamin D experienced a significant benefit after taking a high-dose supplement for 1 year (10).

Summary

Vitamin D plays important roles in immune function. One of the most common symptoms of a deficiency is an increased risk of illness or infections.

Feeling tired can have many causes, and vitamin D deficiency may be one of them.

Unfortunately, it’s often overlooked as a potential cause.

Case studies have shown that very low blood levels of vitamin D can cause fatigue that can have a severe negative effect on quality of life (11, 12).

In one older case study, a woman who complained of chronic daytime fatigue and headaches was found to have a vitamin D blood level of only 5.9 ng/ml. This is extremely low, as anything under 20 ng/ml is considered deficient.

When the woman took a vitamin D supplement, her level increased to 39 ng/ml, and her symptoms resolved (12).

One observational study in female nurses also found a strong connection between low vitamin D levels and self-reported fatigue.

What’s more, the researchers found that 89% of the nurses were deficient (13).

Interestingly, several studies have also found that supplementing with vitamin D could reduce the severity of fatigue in people with a deficiency (14, 15).

For more information on how to reduce fatigue, consider reading about the 11 best vitamins and supplements to boost energy.

Summary

Excessive fatigue and tiredness may be a sign of vitamin D deficiency. Taking supplements may help improve energy levels.

Vitamin D helps maintain bone health in a number of ways.

For one, it improves your body’s absorption of calcium.

Bone pain and lower back pain may be signs of inadequate vitamin D levels in the blood.

Large observational studies have found a relationship between a deficiency and chronic lower back pain (16, 17).

One study in 98 people with lower back pain found that increased pain severity was associated with lower levels of vitamin D (18).

Another review of 81 studies also found that people with arthritis, muscle pain, and chronic widespread pain tended to have lower levels of vitamin D compared with people without these conditions (19).

Summary

Low blood levels of vitamin D may be a cause or contributing factor to bone pain and lower back pain.

4. Depression

A depressed mood may also be a sign of vitamin D deficiency.

In review studies, researchers have linked vitamin D deficiency to depression, particularly in older adults (20, 21).

In one analysis, 65% of the observational studies found a relationship between low blood levels and depression.

On the other hand, most of the controlled trials, which carry more scientific weight than observational studies, did not show a link between the two (20).

However, the researchers who analyzed the studies noted that the dosages of vitamin D in controlled studies were often very low.

In addition, they observed that some of the studies may not have lasted long enough to see the effect of taking supplements on mood.

Some studies have shown that giving vitamin D to people who are deficient helps improve depression, including seasonal depression, which typically occurs during the colder months (22, 23).

Summary

Depression is associated with low vitamin D levels, and some studies have found that supplementing improves mood.

Slow healing of wounds 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 the vitamin increases the production of compounds that are crucial for forming new skin as part of the wound-healing process (24).

One review of four studies found that vitamin D deficiency compromised certain aspects of healing in people who had dental surgery (25).

It’s also been suggested that vitamin D’s role in controlling inflammation and fighting infection is important for proper healing.

One analysis looked at patients with diabetic foot infections.

It found that those with severe vitamin D deficiency were more likely to have higher levels of inflammatory markers that can jeopardize healing (26).

Unfortunately, there is very little research about the effects of vitamin D supplements on wound healing in people with a deficiency at this point.

However, one study involving 60 people with diabetic foot ulcers found that taking a vitamin D supplement for 12 weeks significantly improved wound healing compared with a control group (27).

Summary

Inadequate vitamin D levels may lead to poor wound healing following surgery, injury, or infection.

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

Many older people who are diagnosed with bone loss believe they need to take more calcium. However, they may be deficient in vitamin D as well.

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 for fractures.

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 (28).

However, a controlled study found that women who were deficient in vitamin D experienced no improvement in bone mineral density when they took high-dose supplements, even if their blood levels improved (29).

Regardless of these findings, adequate vitamin D intake and maintaining blood levels within the optimal range may be a good strategy for protecting bone mass and reducing fracture risk.

Summary

A diagnosis of low bone-mineral density may be a sign of vitamin D deficiency. Getting enough of this vitamin is important for preserving bone mass as you get older.

7. Hair loss

Hair loss is often attributed to stress, which is certainly a common cause.

However, when hair loss is severe, it may be the result of a disease or nutrient deficiency.

Hair loss in women has been linked to low vitamin D levels, though there is very little research on this to date (30).

In particular, research suggests that low vitamin D levels are linked to alopecia areata and may be a risk factor for developing the disease (31, 32).

Alopecia areata is an autoimmune disease characterized by severe hair loss from the head and other parts of the body. It’s associated with rickets, which is a disease that causes soft bones in children due to vitamin D deficiency (33).

One study in people with alopecia areata showed that lower vitamin D blood levels tended to be associated with more severe hair loss (32).

In another study, applying a synthetic form of the vitamin topically for 12 weeks was found to significantly increase hair regrowth in 48 people with alopecia areata (34).

Many other foods and nutrients may affect the health of your hair. If you experience hair loss, you may be interested in the 14 best foods for hair growth.

Summary

Hair loss may be a sign of vitamin D deficiency in female-pattern hair loss or the autoimmune condition alopecia areata.

The causes of muscle pain are often difficult to pinpoint.

There is some evidence that vitamin D deficiency may be a potential cause of muscle pain in children and adults (35, 36).

In one study, 71% of people with chronic pain were found to be deficient (36).

The vitamin D receptor is present in nerve cells called nociceptors, which sense pain.

According to one review, vitamin D may be involved in the body’s pain signaling pathways, which could play a role in chronic pain (37).

A few studies have found that taking high-dose vitamin D supplements may reduce various types of pain in people who are deficient (38, 39).

One study in 120 children with vitamin D deficiency who had growing pains found that a single dose of the vitamin reduced pain scores by an average of 57% (39).

Summary

There is a link between chronic pain and low blood levels of vitamin D, which may be due to the interaction between the vitamin and pain-sensing nerve cells.

Vitamin D deficiency is incredibly common, and most people are unaware of it.

That’s because the symptoms are often subtle and nonspecific, meaning that it’s hard to know if they’re caused by low vitamin D levels or other factors.

If you think you may have a deficiency, it’s important that you speak with your doctor and get your blood levels measured.

Fortunately, a vitamin D deficiency is usually easy to fix.

You can either increase your sun exposure or add more vitamin-D-rich foods to your diet, such as fatty fish or fortified dairy products.

In some cases, your doctor may also recommend taking a vitamin D supplement. You can find a variety of vitamin D supplements on Amazon. Always talk with your doctor first if you’re considering adding supplements to your treatment regimen.

Fixing your deficiency is simple, easy, and can have big benefits for your health.

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