Vitamin D

Last Updated: February 6, 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.[1] Extremely high blood levels of vitamin D can even be fatal.[2]

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)[3]. 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 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.[4]

A few trials on older adults have found that vitamin D increased the risk of falls,[5][6] and one study observed a decrease in bone mineral density among women taking high doses of vitamin D.[7]

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.[8] 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.[9]

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.[10] The additional effects of vitamin D on the body include, but aren’t limited to, the following:[11]

  • Anticancer effects (promotion of cell death or suppressing proliferation in cancer cells)
  • Brain development[12]
  • Cell differentiation[13][14]
  • Anti-inflammatory effects[15]
  • Antioxidant effects[16]
  • Antibacterial effects (through vitamin D’s effects on immune cells, and its promotion of the expression of genes encoding antimicrobial peptides)[17]
  • Blood vessel protection[18]
  • 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

The recommended daily allowance for Vitamin D is currently set at 400–800 IU/day, but this may be too low for many adults. For moderate supplementation, a 1,000–2,000 IU dose of vitamin D3 is sufficient to meet the needs of most of the population. Higher daily doses are in the range of 20–80 IU per kilogram of body weight.

The Upper Tolerable Intake Level in the United States and Canada is 4,000 IU per day (IU/day). It’s been suggested that the true Upper Tolerable Intake Level may actually be as high as 10,000 IU/day, but there are limited data on health outcomes using doses near this amount.

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

Vitamin D should be taken daily, with meals or a source of fat.

Examine Database: Vitamin D