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Vitamin D

Written by Ada’s Medical Knowledge Team

Updated on

Vitamin D is a critical vitamin for good health and many different body functions. Bones, muscles, nerves, and the immune system all rely on vitamin D to work properly. 1

Many studies suggest that the micronutrient vitamin D might play an even bigger role in our health. But, for now, let’s focus on its confirmed health benefits and how we can make sure to get enough vitamin D.

What is vitamin D?

Vitamin D is a fat-soluble vitamin that helps us absorb calcium and phosphate from food through the intestines. 2 Both of these minerals are crucial for building healthy bones.

Recent studies suggest that while vitamin D is a nutrient, it also acts like a hormone in some cases and plays a role in many different processes in our body. Vitamin D contributes to proper function of the cardiovascular system, the immune system, and metabolic pathways. 3

Our body can get the vitamin D it needs in two ways. It can directly absorb vitamin D from food or create it when our skin is exposed to sunlight. This is why vitamin D is often referred to as the “sunshine vitamin”.

There are 2 main forms of vitamin D:

  • Vitamin D2: Ergocalciferol
  • Vitamin D3: Cholecalciferol

Vitamin D2 is formed in plants and yeast. It's also the form that’s usually used in supplements. On the other hand, Vitamin D3 is the most active form of Vitamin D and can be formed by the skin through direct exposure to sunlight. D3 can also be obtained from fortified foods and other food sources.

What is vitamin D good for?

We know that vitamin D is critical for our body and observational studies have seen strong evidence that higher levels of vitamin D can play a preventive role in certain diseases. However, clinical trials often fail to prove the connection. Let’s explore how studies have claimed that vitamin D benefits us and where it might be limited.

  • Bone health: As vitamin D helps the bones absorb essential minerals, it’s critical to preventing bone conditions like osteomalacia and osteoporosis. With osteomalacia, bones soften as a result of vitamin D deficiency. 4 Osteoporosis is a condition that weakens the bones and often causes fractures in the elderly. However, vitamin D supplements do not prevent fractures and have rarely shown a clinically-relevant effect on bone mineral density. 5
  • Heart disease: Vitamin D can help to control high blood pressure as it keeps arteries flexible. However, vitamin D supplements do not lower the risk for strokes or heart attacks. 6
  • Respiratory infections: Studies suggest that vitamin D supplementation can decrease the risk of flu and colds. 7
  • Depression: Vitamin D deficiency is sometimes present in people with depression. However, vitamin D supplements are not an adequate treatment for depression.
  • COVID-19: Since the outbreak of COVID-19 in late 2019, many studies have examined whether a person’s vitamin D levels might have an influence on their likelihood of contracting the virus. Several studies found that a vitamin D deficiency was associated with an increased risk of infection and more severe symptoms. 8

What foods have vitamin D?

Only a few foods naturally contain vitamin D. This means it’s hard to get enough vitamin D every day from your diet. Some foods are fortified with vitamin D, which means vitamin D is added during processing so these can help to increase your vitamin D intake. Here are some examples of vitamin D rich foods:

  • Fortified foods like milk, orange juice, breakfast cereal, and others
  • Salmon
  • Tuna
  • Mackerel
  • Trout
  • Fish liver oil
  • Beef liver (also contains high amounts of cholesterol)
  • Egg yolk
  • Cheese
  • Mushrooms

Who should take vitamin D?

Our body only gets small amounts of vitamin D from food but can form enough vitamin D with sunlight exposure during summer. From October to March in many places, light from the sun is not strong enough for our bodies to make vitamin D. So you may consider taking a vitamin D supplement during these months. 9

Vitamin D supplements are available in different forms, such as pills, vitamin D drops, or vitamin D gummies. Make sure to take your supplement with food as vitamin D needs some fat to be absorbed properly by the body.

Certain additional factors may require you to take vitamin D. Find out more about the risk factors of vitamin D deficiency.

What are side effects of vitamin D?

When exposed to sunlight, your body won’t overdose on vitamin D. However, if you take too much vitamin D from supplements, you may experience side effects. A daily vitamin D dosage of less than 100 micrograms or 4,000 IU (international units) is considered safe. 9

Side effects of very high vitamin D levels include:

  • nausea
  • muscle weakness
  • loss of appetite
  • dehydration

In extreme cases, vitamin D can lead to kidney failure and irregular heartbeat.

When is a vitamin D test useful?

People with bone disorders, muscle weakness, liver or kidney disease, or other conditions that could be attributed to vitamin D deficiency should get tested for vitamin D levels in their blood. The most common symptoms of vitamin D deficiency include bone pain, fatigue, muscle pain, and frequent infections. People living in countries where there’s very little sunshine may also think about getting a vitamin D test.

Visit Ada's article on how to test for vitamin deficiency to find out more.

Vitamin D FAQ

Q: What does Vitamin D do?

Vitamin D is an important contributor to our health. It helps our bodies absorb calcium and phosphorus, which are essential micronutrients for proper bone development.

Q: How much Vitamin D do I need?

For adults and teens, the recommended daily intake of vitamin D is 600 IU (international units). If you’d prefer to think about it on a weekly basis, aim for an intake of Vitamin D of 5000 IU.

Q: Why is Vitamin D important?

Our body needs vitamin D to maintain healthy bones, muscles, nerves, and immune function. A lack of vitamin D may lead to fatigue, osteoporosis, and frequent infections.

Q: When do I need to seek medical help?

If you experience vitamin D deficiency, symptoms like bone disorders, fatigue, or muscle weakness and pain, you should seek medical help. These symptoms may indicate vitamin D deficiency.

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  1. NIH (2021). Vitamin D. Accessed April 29, 2022.

  2. MSD Manuals (2020). Vitamin D Deficiency and Dependency. Accessed May 8, 2022.

  3. Ellison, D.L., Moran, H.R., Nursing Clinics of North America (2021), doi: 10.1016/j.cnur.2020.10.004.

  4. NHS (2021). Rickets and osteomalacia. Accessed May 2, 2022.

  5. Bollad, M.J., et. al, The Lancet Diabetes & Endocrinology (2018), doi: 10.1016/S2213-8587(18)30265-1.

  6. Manson, J. E., et. al, NEJM (2019), doi: 10.1056/NEJMoa1809944.

  7. Martineau, A.R., et. al, BMJ (2017), doi: 10.1136/bmj.i6583.

  8. Demir M., et. al, J Med Virol. (2021), doi: 10.1002/jmv.26832.

  9. NHS (2020). Vitamin D. Accessed April 29, 2022.