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Vitamin D deficiency: What to know

The best way for someone to prevent vitamin D deficiency is to ensure they’re getting enough vitamin D in their diet and through sun exposure.

Published: September 12, 2025 | 8 minutes to read

Written by: Carrie Weisman

A person with a vitamin D deficiency sits in the sun.

There are about a billion people living with vitamin D deficiency, worldwide. That’s a lot of people. In other words, yes, the diagnosis is incredibly common. It also includes 35 percent of American adults. While vitamin D deficiency typically occurs without symptoms, the condition can also have a significant impact on overall health. Check out the article below to learn what vitamin D does, what might lead to a deficiency and how to help reverse it.

In this article:

What is vitamin D?

Vitamin D is often referred to as the "sunshine vitamin" because it can be produced by spending time in the sun. The main functions of vitamin D are to help the body absorb calcium from food, which is necessary for building and maintaining strong bones; reduce the loss of calcium from the kidneys; and regulate the balance of calcium and phosphorus in the body. In addition to the regulation of calcium in the body, vitamin D has many functions outside of bone health, including immune system function.

What is a vitamin D deficiency?

A deficiency occurs when the body doesn't get enough of a certain vitamin over a long period of time, so people with a diagnosis of vitamin D deficiency don’t have enough vitamin D in their bodies. Low levels of vitamin D can cause a range of health issues, including:

  • Hypocalcemia (occurs when the calcium levels in the blood are too low)
  • Increased risk of fractures
  • Osteomalacia (weakened bones)
  • Osteoporosis (occurs when bones become weak and brittle)

In rare cases, children with severe vitamin D deficiencies may develop rickets. The condition causes growing bones to become soft and weak over time.

What causes vitamin D deficiency?

Vitamin D deficiencies can occur for different reasons. But the main causes include:

Vitamin D is produced in the skin when exposed to sunlight. People who spend most of their time indoors, live in high latitudes or wear clothes that cover most of their skin don’t always receive as much vitamin D as they need.

Vitamin D is found naturally in some food sources, like fatty fish, fortified dairy products and some mushrooms, but most of our vitamin D intake comes from fortified foods, such as fortified dairy products or cereals. People who leave these items out of their diet (without getting their vitamin D from somewhere else) may have an increased chance of developing a deficiency.

Melanin reduces the skin's ability to produce vitamin D from sunlight. That’s why people with darker skin tones tend to develop vitamin D deficiencies more often than others.

As people age, they have fewer skin cells capable of producing vitamin D.

Fat cells can store vitamin D, making it unavailable for the rest of the body to use. That’s why a body mass index (BMI) over 30 is sometimes associated with lower vitamin D levels.

Certain illnesses can impair vitamin D absorption or metabolism. That list includes celiac disease, Crohn's disease and kidney or liver diseases. In addition, gastric bypass surgery is a major cause of vitamin D deficiency.

Some medications, including anticonvulsants and steroids, can affect vitamin D levels. That means there’s less vitamin D available for the body to use.

Symptoms of low vitamin D

Symptoms of low vitamin D vary from person to person. And some people show no signs of a deficiency at all. But when they do, common issues typically include:

  • Bone pain or tenderness
  • Bowed legs
  • Dental problems
  • Depression
  • Fatigue
  • Joint deformities
  • Mood changes
  • Muscle weakness
  • Muscle cramps or spasms

Who does vitamin D deficiency affect?

Vitamin D deficiencies can happen to anyone and at any age. However, some groups are more prone to it than others. They include:

  • Breastfed infants
  • Older adults
  • People with certain medical conditions, like chronic kidney or liver disease
  • People with darker skin tones
  • People with limited sun exposure
  • Those who are obese or have undergone gastric bypass surgery

How can vitamin D deficiency be diagnosed?

Vitamin D deficiency doesn’t always cause symptoms. And that can make diagnosing the condition a challenge. But health care providers can perform blood tests to monitor levels. It's not a routine procedure, but health care providers may check someone’s vitamin D level in certain circumstances.

How can people help raise their vitamin D levels?

Young adult smiles on the beach while applying sunscreen to their nose.

People can help raise their vitamin D levels through a combination of dietary adjustments, lifestyle changes and supplements, but it's always best to consult with a health care provider first. Some possible options to help raise vitamin D levels may include:

Food

Consuming vitamin D-rich foods can help increase vitamin D levels. Foods that are naturally high in vitamin D include fatty fish, like salmon or mackerel. There are some fortified food options, too. That means extra nutrients have been added to them during processing. That list includes breakfast cereals, dairy milk, plant milks, orange juice and dairy yogurt.

Sun exposure

The best way for someone to prevent vitamin D deficiency is to ensure they are getting enough vitamin D in their diet and through sun exposure. But be careful about being in the sun for too long without sunscreen: Excessive sun exposure puts people at an increased risk for skin cancer.

Dietary supplements

Taking vitamin D supplements can help increase vitamin D levels in the body, too. This can be particularly useful for individuals with limited sun exposure or certain health conditions that affect vitamin D absorption. Speak with a health care provider before starting or stopping any supplement.

Is it possible to get too much vitamin D?

It is possible to get too much vitamin D, and that’s not good for the body, either. Excess vitamin D can lead to vitamin D toxicity, which is characterized by high levels of calcium in the blood. This often results in symptoms such as nausea, vomiting and weakness. It can also cause more severe problems, including damage to the kidneys and other organs.

Key takeaways

Vitamin D is important for maintaining healthy bones. Certain groups, such as people with darker skin tones, those who don't get enough sunlight, older individuals and those with obesity, have an increased chance of developing a deficiency. Symptoms may vary, but the condition can be confirmed with a blood test. Dietary adjustments, lifestyle changes and supplements can help bring levels back up to normal. For personalized guidance, it's always best to consult a health care professional.

Frequently asked questions (FAQs)

Low levels of vitamin D can cause weakened bones, increased risk of fractures and fatigue.

Vitamin D can be found naturally in foods like fish-liver oils, fatty fish, egg yolks and liver. In the U.S., vitamin D is also often added to milk and other foods.

A lack of vitamin D has been linked to various health conditions, including osteoporosis, osteomalacia and an increased risk of fractures. Vitamin D deficiency might also be associated with a higher risk of autoimmune diseases like multiple sclerosis, along with depression.

This content is for informational purposes only and is not intended to replace professional medical advice, diagnosis or treatment. Consult your health care provider if you have any questions about medications, vitamins or supplements you may be considering or changes to your wellness or health care routines.

FOR VITAMINS AND SUPPLEMENTS: Consult your health care provider before taking any vitamins or supplements. These statements have not been evaluated by the Food and Drug Administration. These products are not intended to diagnose, treat, cure or prevent any disease.

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