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Vitamin D is essential for a range of bodily functions. Dietary sources provide some vitamin D, but most comes from exposure to sunlight. After the body takes in vitamin D, it needs to convert it to its active form.

Deficiencies can arise if a person does not take in enough vitamin D or their skin has an impaired ability to synthesize it from the sun. A person can also become deficient if the body is not able to absorb the vitamin or convert it to its active form in the liver and kidneys.

Low vitamin D levels can lead to a range of problems, especially relating to the bones and muscles.

Some people have an annual blood test to check for a vitamin D deficiency. The result will show serum vitamin D levels in nanomoles/liter (nmol/l). Healthy levels of serum vitamin D are between 50 nmol/l and 125 nmol/l.

In this article, we look at how to detect the signs of vitamin D deficiency and how to treat it.

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A person may experience vitamin D deficiency if they do not get exposure to enough sunlight.

Vitamin D is a fat-soluble vitamin that the body produces when the skin gets exposed to sunlight.

It is present in a small number of foods, including fortified products.

When vitamin D enters the body, it is not in an active form.

To use it, the body needs to convert it to an active form called 25-hydroxyvitamin D [25(OH)D] or calcidiol.

How much is a deficiency?

The results of a serum vitamin D blood test may show the following:

  • Too high and possibly harmful: 125 nmol/l or more
  • Sufficient: 50–125 nmol/l
  • At risk of inadequacy: 30–49 nmol/l
  • At risk of deficiency: 30 nmol/l or less

Functions of vitamin D

Vitamin D has many important functions, including:

Low vitamin D and diabetes

Some experts have suggested that vitamin D may help prevent type 2 diabetes.

However, in a 2019 study, 2,423 people at risk of type 2 diabetes took either a vitamin D supplement of 4,000 international units (IU) a day or a placebo.

Vitamin D levels increased in those who took the supplement. However, taking the supplement did not appear to reduce the risk of diabetes developing.

Learn more about the health benefits of vitamin D.

Vitamin D deficiency can happen when a person:

  • does not consume enough vitamin D
  • is unable to absorb or metabolize the vitamin D
  • does not spend enough time in ultraviolet B (UVB) sunlight

Risk factors

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A person's diet may contribute to vitamin D deficiency.

Various factors can increase the risk of a deficiency.

Diet: People who do not eat enough vitamin D-rich foods, including fortified dairy products and cereals, may have low levels of vitamin D.

Lifestyle factors: Some people spend little time outdoors due to work, ill health, a lack of outdoor space in their neighborhood, or other factors. These people have less opportunity to expose their skin to sunlight. Those who wear clothes that cover all of their body, whether to protect it from the sun or for cultural or religious reasons, may also have a higher risk of a deficiency.

The Office of Dietary Supplements (ODS) recommend that people who use a lot of sunscreen or wear clothing that covers the body should include sources of vitamin D in their diet.

Geographical factors: People living in certain parts of the word — Northern Canada and Alaska, for example — may have less access to the sun's UVB rays, especially in winter. People who live in a hot climate may also be at risk, as they often try to avoid the heat and strong sunlight by staying indoors.

Pollution: Particles in the air can block UVB rays and prevent them from reaching the skin. People who live in highly polluted areas may also be more likely to avoid spending time outside.

Absorption problems: Crohn's disease, celiac disease, and other conditions can affect how the intestines absorb nutrients, including vitamin D.

Medications: Some drugs reduce the body's ability to absorb or synthesize vitamin D. These include steroids and some drugs for lowering cholesterol, among others.

Smoking: Levels of deficiency appear to be higher among smokers. Some experts have suggested that smoking may affect the gene that activates the production of vitamin D-3 in the body.

Obesity: Research has found lower vitamin D levels in people with obesity, or a body mass index (BMI) of 30 or more. This link may be stem from the ways in which body fat affects vitamin D absorption. Some people with obesity may spend less time outdoors due to mobility issues. Those who have undergone bariatric surgery may also have absorption problems. Conversely, people whose BMI of 25–29.9 classified them as overweight appeared to have a lower risk of a deficiency than those who were not overweight. The authors suggest that dietary factors are responsible for this difference.

Skin type: People with darker skin need more sunlight exposure to produce vitamin D than those with lighter skin. People with pale skin or a history of skin cancer may avoid sun exposure to protect their skin from damage.

Age: The ability to convert vitamin D to calcitriol may decline with age due to decreased kidney function. As a result, calcium absorption will fall.

Kidney and liver health: People with liver disease and kidney disease tend to have lower vitamin D levels. These diseases can affect the body's ability to synthesize vitamin D or turn it into its active form.

Pregnancy: The need for vitamin D may increase during pregnancy, but experts remain unsure whether supplements are a good idea. The authors of a 2019 Cochrane review concluded that taking supplements during pregnancy may reduce the risk of preeclampsia, gestational diabetes, low birth weight, and severe bleeding after delivery. However, it may also increase the risk of preterm birth, which is birth before 37 weeks. The authors called for further research.

Breastfeeding infants: Human milk is low in vitamin D, which means that breastfeeding infants are at risk of a deficiency. The Centers for Disease Control and Prevention (CDC) recommend giving a vitamin D supplement to all breastfeeding infants from the first few days of life until they consume 1 l or more of formula milk each day. Supplementation becomes unnecessary at this point because formula milk contains added vitamin D.

Groups with a higher risk

A major study in the United States from 2001–2006 found that 24% of the population were at risk of low vitamin D levels, while 8% were at risk of a deficiency. On the other hand, 1% of the population had levels of vitamin D that were so high that they could be harmful.

In 2019, experts published a report on data from 2011–2012. They looked at how vitamin D deficiencies affect specific populations in the U.S. The findings suggested that African American adults have the highest risk of a deficiency, followed by Hispanic people.

A vitamin D deficiency may produce no symptoms, or symptoms may take several years to appear. However, it may increase the risk of long term health problems.

In time, low levels of vitamin D can lead to:

Osteoporosis: The bones become thin or brittle. The first sign may be a bone breaking easily as a result of minor trauma. It often affects older people.

Osteomalacia: This can affect children. The bones become soft, resulting in bone deformities, short stature, dental problems, fragile bones, and pain when walking.

Researchers are looking into whether other symptoms or conditions, such as depression, bone pain, and weakness, may result from low vitamin D levels.

Can low vitamin D levels lead to hair loss? Find out here.

An individual's specific need for vitamin D will depend on various factors, including their age, UVB exposure, diet, and health status.

If a blood test shows that a person has or is at risk of a vitamin D deficiency, the doctor is likely to advise them to increase their intake.

The ODS recommend the following intake each day:

  • 0–12 months: 400 IU (10 micrograms [mcg])
  • 1–70 years: 600 IU (15 mcg)
  • 71 years and over: 800 IU (20 mcg)

However, it is not possible to measure how much vitamin D a person obtains from sunlight.

Individuals should talk to their doctor about their vitamin D needs and how to increase their intake.

Supplements

Some people may need to take supplements, but it is best to talk to a doctor before doing so, as some can have adverse effects. The doctor will also provide advice on a suitable dosage.

Vitamin D supplements are available for purchase online.

Foods sources of vitamin D

Good dietary sources of vitamin D include:

  • oily fish, such as mackerel or salmon
  • beef liver
  • cheese
  • mushrooms
  • egg yolks
  • fortified foods, including some breakfast cereals, orange juice, milk, soy drinks, and margarine

What are the best dietary sources of vitamin D? Learn more here.

Exposure to natural sunlight

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Short periods spent outdoors can boost vitamin D levels.

Sunlight is important for boosting vitamin D, but inappropriate sun exposure increases the risk of skin cancer.

For safe exposure to sunlight, a person should spend a short time outdoors each day without sunscreen and with their forearms, hands, or lower legs exposed to the sun.

When to do this and for how long will depend on the time of year, geographical location, and other factors.

People should ask their doctor about safe ways to increase sunlight exposure, especially if they have fair skin or conditions that affect the skin, such as psoriasis.

How can you get more vitamin D from the sun? Get some tips here.

The best ways to prevent a vitamin D deficiency are to eat foods that are rich in this nutrient and to spend some time outside each day.

Some tips for avoiding a deficiency include:

Maintaining a healthy body weight: Cycling or walking can provide both exercise and exposure to sunlight.

Treating medical conditions: People with health conditions that affect the absorption of nutrients may find that treating the underlying condition helps boost their levels of certain nutrients, including vitamin D.

Being proactive about preventive health: People with a family history of osteoporosis or vitamin D deficiency may wish to consider speaking to their doctor about screening.

Most people can obtain the vitamin D they need from the diet and exposure to sunlight.

Anyone who is worried about their intake of vitamin D or experiences any of the symptoms of a deficiency should speak to a doctor.

Q:

Can you have too much vitamin D?

A:

Absolutely! Excessively high levels are usually due to a person taking too many supplements, and they lead to the body having too much calcium in the blood. High calcium levels often cause nausea, vomiting, and frequent urination, and they may even result in kidney problems. People should take supplements according to the directions and avoid taking too many. There is very little risk of absorbing too much vitamin D through sun exposure.

Debra Sullivan, PhD, MSN, RN, CNE, COI Answers represent the opinions of our medical experts. All content is strictly informational and should not be considered medical advice.