Vitamin D

Last Updated: April 4, 2024

Vitamin D is a fat-soluble vitamin that our skin synthesizes when exposed to the sun. It benefits us in many ways, from bone health to mood.

Vitamin D is most often used for

What is vitamin D?

Vitamin D is a fat-soluble nutrient. It is one of the 24 micronutrients critical for human survival. The sun is the major natural source, through eliciting vitamin D production in the skin, but vitamin D is also found naturally in oily fish and eggs and is often added to milk and milk alternatives.

What are vitamin D’s main benefits?

Supplemental vitamin D is associated with a range of benefits, including improved immune health, bone health, and well-being. Supplementation may also reduce the risk of cancer mortality, diabetes, and multiple sclerosis. Vitamin D is often taken for depression and similar mental health issues, but the evidence is mixed as to whether supplementation improves these outcomes.

The effects of vitamin D likely depend on a person’s circulating levels of 25-hydroxyvitamin D (25(OH)D; a form of vitamin D that is measured in blood samples to determine vitamin D status), and many of the benefits of supplementation will only be seen when a deficiency is reversed.

What are vitamin D’s main drawbacks?

Very high vitamin D levels in the blood exceeding 375 nmol/L or 150 ng/mL are toxic, potentially causing side effects such as loss of appetite, nausea, vomiting, muscle weakness, confusion, dehydration, excessive thirst, and kidney stones.[7] Extremely high blood levels of vitamin D can even be fatal.[8]

Since vitamin D production in the skin is self-limiting, sun exposure is unlikely to produce vitamin D levels that cause harm (although it may be possible for people with constant, high-level sun exposure, e.g., lifeguards, to synthesize harmfully high levels of Vitamin D).[9] However, vitamin D levels in the blood can readily reach toxic levels with chronic, high-level supplementation.

Although ultra-high-level supplementation is universally considered to be toxic, there’s some debate on what constitutes the upper limit for safe, “moderate-level” supplementation. Taking around 4,000 IU (100 μg) of vitamin D per day for extended periods (≥6 months) seems to increase the risk of hypercalcemia (high blood calcium) and, among older adults, the likelihood of experiencing a fall.[10]

A few trials on older adults have found that vitamin D increased the risk of falls,[11][12] and one study observed a decrease in bone mineral density among women taking high doses of vitamin D.[13]

How does vitamin D work?

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Vitamin D exerts its effects by binding to and activating the vitamin D receptor (VDR). Upon binding vitamin D, the VDR functions as a transcription factor, regulating the activity of over 1,000 different genes.[14] This “genomic” action of vitamin D tends to be slower-acting, requiring the synthesis of new mRNA and proteins to take effect.

Vitamin D also works through “non-genomic” mechanisms, causing rapid activation of various signaling pathways within the cell. Although VDRs have been identified on cell membranes, it isn’t currently known whether the more rapid, non-genomic action of vitamin D occurs through membrane-associated vitamin D receptors or a different cellular receptor.[15]

Although the most commonly associated effects of vitamin D on the body are associated with bone metabolism, the almost ubiquitous presence of VDRs in cells and tissues throughout the body indicates that vitamin D can affect a wide range of physiological processes.[16] The additional effects of vitamin D on the body include, but aren’t limited to, the following:[17]

  • Anticancer effects (promotion of cell death or suppressing proliferation in cancer cells)
  • Brain development[18]
  • Cell differentiation[19][20]
  • Anti-inflammatory effects[21]
  • Antioxidant effects[22]
  • Antibacterial effects (through vitamin D’s effects on immune cells, and its promotion of the expression of genes encoding antimicrobial peptides)[23]
  • Blood vessel protection[24]
  • Nervous system health
  • Calcium regulation
What else is Vitamin D known as?
Note that Vitamin D is also known as:
  • Cholecalciferol (Vitamin D3)
  • Ergocalciferol (Vitamin D2)
Vitamin D should not be confused with:
  • 25-Dihydroxyvitamin D (Hormonally active yet not directly supplemented form)
  • Calcitriol
Dosage information

IU vs. μg

Historically, it’s been more common to see vitamin D dosed in international units (IU), rather than micrograms (μg). In 2016, however, the US Food and Drug Administration (FDA) announced that, by 2021, supplement labels had to display vitamin D content in μg.[1] IU is still commonly seen alongside μg, so it’s good to become familiar with both units: 40 IU is equivalent to 1 μg.

How many grams of vitamin D should I take per day?

The ideal daily dose of vitamin D should correspond to its recommended daily allowance (RDA), which is currently 400–800 IU (10–20 μg) a day, but this may be too low for many adults. For moderate supplementation, a standard daily dose of 1,000–2,000 IU (25–50 μg) of vitamin D3 is sufficient to meet the needs of most people. Higher daily doses are in the range of 20–80 IU (0.5–2 μg) per kilogram of body weight.

Vitamin D should be taken daily with meals or a source of fat.[2][3]

Regarding the daily recommended dose for babies and children, please refer to the question “Should vitamin D supplements be given to infants and children?”.

What is the ideal daily dose of vitamin D for pregnant women?

The recommended daily dose of vitamin D for pregnant women varies across different countries. In the US, pregnant women are recommended to take 600 IU (15 μg) of vitamin D daily, unless they have a vitamin D deficiency, which will require higher doses.[4] In the UK, the recommended dosage is slightly lower, 400 IU (10 μg) daily.[5].

What is the ideal dose of vitamin D for menopausal women?

The recommended daily dose of vitamin D for women during menopause has not yet been established. However, research has shown that daily doses of 2000–4800 IU (50–120 μg) taken regularly were effective at correcting vitamin D deficiencies and maintaining vitamin D levels.[6]

What is the maximum dose of vitamin D?

The Upper Tolerable Intake Level), which is the maximum daily dose of a nutrient that is unlikely to cause side effects, is 4,000 IU (100 μg) per day in the United States and Canada.[4] This is considered a high dose of vitamin D; however, doses above this level are sometimes used for short periods of time (e.g., to reverse a severe vitamin D deficiency quickly), with safety likely determined by the dose and duration of supplementation. High-dose vitamin D supplementation should be done under medical supervision in order to monitor blood levels and watch for signs of vitamin D toxicity (e.g., hypercalcemia).

What types of vitamin D are there?

Vitamin D3 supplementation (cholecalciferol) is recommended over D2 supplementation (ergocalciferol), because D3 tends to raise blood levels more effectively.

Vitamin D is usually available in a wide range of doses and forms (e.g., tablets, capsules, soft gels, drops, powder). Which form to choose and how much to take will depend on the vitamin D content in each supplement and the individual’s specific requirements. For instance, for an adult wanting to take 2000 IU (50 μg) a day of vitamin D, the correct daily dose of a vitamin D drops supplement containing 1000 IU (25 μg) per drop would be 2 drops a day.

Examine Database: Vitamin D