Quick summary

Vitamin D, also known as calciferols, refers primarily to two main forms: vitamin D2 and vitamin D3. It is a fat-soluble vitamin essential for various physiological functions including bone health and immune support. Although it can be obtained via the diet, most vitamin D in the body is synthesized in the skin upon exposure to sunlight. Vitamin D status is closely linked with calcium balance and skeletal integrity. Deficiency can result in bone disorders, and supplementation is commonly used to maintain adequate levels. The optimal intake varies substantially depending on age, sex, life stage, and geographic factors. More high-quality evidence is continually emerging regarding vitamin D’s broader health implications.

What is it?

Vitamin D is a group of fat-soluble secosteroids, primarily including vitamin D2 (ergocalciferol) and vitamin D3 (cholecalciferol). These compounds undergo metabolic activation in the liver and kidneys to form the biologically active hormone calcitriol. This active form helps regulate calcium and phosphate metabolism within the body. Vitamin D is unique among vitamins because it can be synthesized endogenously when ultraviolet B (UVB) rays from sunlight strike the skin, triggering conversion of 7-dehydrocholesterol to vitamin D3.

Main functions in the body

  • Regulation of calcium and phosphate absorption in the intestines to maintain normal serum calcium levels.
  • Supporting bone mineralization and skeletal health, thereby playing a role in preventing conditions like rickets in children and osteomalacia in adults.
  • Modulation of immune function, including both innate and adaptive immune responses.
  • Influence on cell growth and differentiation in various tissues.
  • Potential roles under investigation in muscle function, cardiovascular health, and mood regulation, although robust conclusions are pending further research.

Recommended intake and upper limit

Recommended daily intake values for vitamin D vary considerably based on age, sex, geographic location, and individual factors such as pregnancy or lactation. These values are typically defined by national health authorities and can range broadly.

Similarly, the established safe upper intake levels (ULs) depend on demographics and aim to avoid potential toxicity, especially hypercalcemia. Because the current evidence summary does not provide specific figures, consulting regional guidelines such as those from the European Food Safety Authority (EFSA) or national dietary reference intakes is advisable for precise recommendations.

Best food sources

Vitamin D is naturally present in few foods. Common dietary sources include:

  • Fatty fish such as salmon, mackerel, sardines, and tuna
  • Fish liver oils
  • Egg yolks
  • Fortified foods such as milk, plant-based milk alternatives, orange juice, and cereals

Because of limited dietary availability, sunlight exposure remains the major source of vitamin D for most people.

Deficiency: causes, symptoms and risk groups

Vitamin D deficiency can occur due to inadequate sunlight exposure, insufficient dietary intake, impaired conversion or activation (e.g., liver or kidney disease), or malabsorption conditions.

Common signs and symptoms may include:

  • Bone pain and tenderness
  • Muscle weakness
  • In children, impaired bone growth and rickets
  • In adults, osteomalacia or increased fracture risk

Risk groups for deficiency include individuals with limited sun exposure (e.g., homebound persons, those living at high latitudes or under extensive clothing), older adults, people with darker skin pigmentation, individuals with malabsorption syndromes, and those with obesity.

Supplement forms

Vitamin D supplements come mainly in two forms:

  • Vitamin D2 (ergocalciferol): Derived primarily from plant and fungal sources.
  • Vitamin D3 (cholecalciferol): Synthesized usually from animal sources or via UV irradiation of 7-dehydrocholesterol; generally considered more effective at raising and maintaining serum vitamin D levels.

Supplements are available in various forms such as capsules, tablets, liquids, and sprays. Dosage and formulations vary widely.

Potential benefits: what does the evidence say?

Vitamin D’s well-established benefit is its role in maintaining bone health through calcium regulation. Supplementation in deficient individuals has been shown to improve bone mineral density and reduce risk of rickets and osteomalacia.

Other potential health benefits under investigation include:

  • Immune system modulation: Observational studies suggest associations between adequate vitamin D status and reduced incidence of respiratory infections, although randomized controlled trials have shown mixed results.
  • Muscle function: Some evidence indicates vitamin D may improve muscle strength and reduce falls in older adults.
  • Chronic disease prevention: Research has explored vitamin D’s role in cardiovascular disease, diabetes, cancer, and mood disorders, but scientific consensus is not definitive and additional studies are needed.

It is important to note that the European Union has not authorized any health claims for vitamin D beyond its contribution to normal bone, teeth, and muscle function linked to calcium metabolism.

Side effects and toxicity

Vitamin D is generally safe when consumed within established intake ranges. Excessive intake, particularly through high-dose supplements, can lead to toxicity characterized by hypercalcemia (increased blood calcium), which can cause symptoms such as nausea, vomiting, weakness, and kidney damage.

Due to fat solubility, vitamin D can accumulate in the body, so monitoring intake and blood levels is important if using supplements long-term or in high doses.

Drug and nutrient interactions

Vitamin D can interact with various medications, including:

  • Certain anti-epileptic drugs: Such as phenytoin and phenobarbital, which may increase vitamin D metabolism and reduce its levels.
  • Glucocorticoids: These can impair vitamin D metabolism and bone health.
  • Weight-loss drugs: Such as orlistat, which may reduce absorption of fat-soluble vitamins, including vitamin D.
  • Some diuretics: Thiazides may increase risk of hypercalcemia when combined with vitamin D.

Conversely, adequate calcium intake supports optimal vitamin D function.

Who should be careful?

Certain groups should exercise caution with vitamin D intake or supplementation, including:

  • People with sarcoidosis, tuberculosis, lymphoma, or other granulomatous diseases, due to risk of vitamin D hypersensitivity.
  • Individuals with hypercalcemia or hyperparathyroidism.
  • Those taking medications affecting vitamin D metabolism.
  • Pregnant and breastfeeding women, or any person with underlying health conditions—consultation with a healthcare professional is recommended before starting supplementation.

Frequently asked questions

How much sun exposure do I need for adequate vitamin D?
Sun exposure requirements depend on skin type, location, time of year, and use of sunscreen. Short, regular exposure on uncovered skin (arms and legs) is often sufficient for many people, but individual needs vary.
Can I get enough vitamin D from diet alone?
Dietary sources are generally limited, so many people rely on sunlight for adequate vitamin D synthesis. In certain conditions or regions, supplements may be necessary.
What is the difference between vitamin D2 and D3 supplements?
Vitamin D3 is typically more effective at raising blood vitamin D levels than D2, though both are used clinically.
Are there risks of taking too much vitamin D?
Yes, excess intake can cause toxicity, with symptoms related to high calcium levels. Sticking to recommended doses and consulting healthcare providers is advised.
Is vitamin D supplementation beneficial for preventing COVID-19?
Current evidence is inconclusive. While adequate vitamin D supports immune health, supplementation cannot be considered a treatment or preventive measure for COVID-19 outside of maintaining general nutritional sufficiency.

Evidence limitations

  • No detailed clinical study data, official source excerpts, or clear evidence summaries provided in input.
  • No recommended intake values or deficiency signs specified.
  • No food sources, supplementation forms, or safety profile information given in the submitted data.
  • No validated EU authorized health claims supplied.
  • Most research results continue to evolve; many studies report associations without establishing causality.

References

Last reviewed

June 2024

Disclaimer: This page is for informational purposes only. It does not constitute medical advice or treatment. Individuals who are pregnant, breastfeeding, have existing health conditions, or take medications should consult a healthcare professional before starting any vitamin D supplementation.