Vitamin D is an extremely important vitamin that greatly influences various systems in your body ( 1 ).

Unlike other vitamins, vitamin D works like a hormone, and 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 is also found in certain foods such as fatty fish and dairy products, although it is very difficult to get enough from diet alone.

The recommended daily intake (FEI) is usually between 400 and 800 IU, but many experts believe you should get even more.

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

According to a 2011 study, 41.6% of adults in the United States are deficient. This number increases to 69.2% among Hispanics and 82.1% among African Americans ( 3 ).

Here are 7 common risk factors for vitamin D deficiency:

• Have dark skin
• To be older
• being overweight or obese.
• Don't eat a lot of fish or dairy products.
• Far from the equator, where there is little sun all year round.
• Always use sunscreen when you go out.
• Stay inside.

People who live near the equator and are more frequently exposed to sunlight are less likely to be deficient because their skin produces enough vitamin D to meet their body's needs.

Most people don't know they are deficient because the symptoms are generally subtle. You may not recognize them easily, even if they significantly affect your quality of life.

Here are 8 signs and symptoms of vitamin D deficiency.

One of vitamin D's most important functions is maintaining a strong immune system, allowing you to fight off viruses and bacteria that cause disease.

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

If you get sick frequently, especially with colds or flu, low vitamin D levels may contribute.

Several large observational studies have shown an association between deficiency and respiratory infections such as colds, bronchitis, and pneumonia ( 5 , 6 ).

Several studies have shown that taking vitamin D supplements at a dosage of up to 4,000 IU daily can reduce the risk of respiratory infections ( 7 , 8 , 9 ).

In a study of people with the chronic lung disease COPD, only people who were severely vitamin D deficient had significant benefit after taking a high-dose supplement for a year ( 10 ).

Fatigue can have many causes, and vitamin D deficiency can be one of them.

Unfortunately, it is often overlooked as a possible cause.

Case studies have shown that very low blood levels can cause fatigue, which severely impacts quality of life ( 11 , 12 ).

In one case, a woman who complained of chronic daytime sleepiness and headaches was found to have a blood vitamin D level of just 5.9 ng/mL. This is extremely low as anything below 20 ng/ml is considered deficient.

When the woman took a vitamin D supplement, her levels rose to 39 ng/mL and her symptoms disappeared ( 12 ).

However, even blood levels that are not extremely low can have a negative impact on your energy.

A large observational study examined the connection between vitamin D and fatigue in young women.

The study found that women with blood counts below 20 ng/ml or 21–29 ng/ml complained of fatigue more often than women with blood counts above 30 ng/ml ( 13 ).

Another observational study of female nurses found a strong association between low vitamin D levels and fatigue.

Additionally, researchers found that 89% of nurses were deficient ( 14 ).

For more information on reducing fatigue, check out the 11 Best Vitamins and Supplements.

Vitamin D helps maintain bone health in several ways.

On the one hand, it improves calcium absorption in the body.

Bone pain and lower back pain can be signs of insufficient vitamin D levels in the blood.

Large observational studies have shown an association between deficiency and chronic back pain ( 15 , 16 , 17 ).

One study examined the connection between vitamin D levels and back pain in over 9,000 older women.

Researchers found that individuals with a deficiency had more back pain, including severe back pain that limited their daily activities ( 17 ).

In a controlled study, people with vitamin D deficiency were almost twice as likely to have bone pain in the legs, ribs, or joints compared to those with blood levels within the normal range ( 18 ).

A depressed mood can also be an indication of a vitamin D deficiency.

In review studies, researchers have linked vitamin D deficiency to depression, particularly in older adults ( 19 , 20 ).

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

On the other hand, most controlled studies, which have more scientific weight than observational studies, showed no association between the two studies ( 19 ).

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

Additionally, they observed that some of the studies may not have lasted long enough to detect the effects of taking supplements on mood.

Some controlled studies have shown that giving vitamin D to people who are deficient helps improve depression, including seasonal affective disorder that occurs in the colder months ( 21 , 22 ).

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

Results from a test tube study suggest that the vitamin increases the production of compounds that are crucial for the formation of new skin as part of the wound healing process ( 23 ).

A study of people undergoing dental surgery found that certain aspects of healing were affected by vitamin D deficiency ( 24 ).

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

One analysis looked at patients with diabetic foot infections.

Individuals with severe vitamin D deficiency have been found to have higher rates of pro-inflammatory markers, which can jeopardize healing ( 25 ).

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

However, one study found that in patients with vitamin D deficiency who had leg ulcers treated with the vitamin, the size of the ulcers was reduced by an average of 28% ( 26 ).

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, you may also be deficient in vitamin D.

Low bone density is an indication that your bones have lost calcium and other minerals. This leads to an increased risk of fractures in older adults, especially women.

In a large observational study of over 1,100 middle-aged, menopausal or postmenopausal women, researchers found a strong association between low vitamin D levels and low bone density ( 27 ).

However, a controlled study found that women with vitamin D deficiency showed no improvement in bone mineral density when they took high-dose supplements, even if their blood levels improved ( 28 ).

Regardless of these findings, adequate vitamin D intake and maintaining blood levels within the optimal range may represent a good strategy to protect bone mass and reduce fracture risk.

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

However, if hair loss is severe, it may be due to illness or a nutrient deficiency.

Hair loss in women is associated with low vitamin D levels, although little research has been done in this regard ( 29 ).

Alopecia areata is an autoimmune disease characterized by severe hair loss in the head and other parts of the body. It is associated with rickets, a condition that causes soft bones in children due to vitamin D deficiency ( 30 ).

Low levels of vitamin D are linked to alopecia areata and may be a risk factor for developing the disease ( 31 , 32 , 33 ).

A study of patients with alopecia areata showed that lower blood levels of vitamin D tended to be associated with more hair loss ( 33 ).

In a case study, topical application of a synthetic form of the vitamin was found to successfully treat hair loss in a young boy with a vitamin D receptor defect ( 34 ).

Many other foods and nutrients can affect the health of your hair. If you are suffering from hair loss, you may be interested in the 14 Best Foods for Hair Growth.

The causes of muscle pain are often difficult to pinpoint.

There is evidence that vitamin D deficiency may be a possible cause of muscle pain in children and adults ( 35 , 36 , 37 ).

In one study, 71% of people with chronic pain were found to be deficient ( 37 ).

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

A study in rats showed that deficiency resulted in pain and tenderness due to stimulation of nociceptors in muscles ( 38 ).

Some studies have shown that taking high-dose vitamin D supplements can reduce various types of pain in people with a deficiency ( 39 , 40 ).

A study of 120 children with vitamin D deficiency who experienced growing pains found that a single dose of the vitamin reduced pain by an average of 57% ( 40 ).

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

This is because symptoms are often subtle and nonspecific, meaning it's difficult to know whether they're caused by low vitamin D levels or something else.

If you think you may have a deficiency, it is important to speak to your doctor and have your blood levels checked.

Fortunately, vitamin D deficiency is usually easy to correct.

You can either increase your sun exposure or eat more vitamin D-rich foods, such as fatty fish or fortified dairy products. You can also find a variety of vitamin D supplements on Amazon.

Correcting your deficiency is simple, easy and can have great benefits for your health.