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Calcidiol – 25-Hydroxyvitamin D Explained

Calcidiol is the main storage form of vitamin D in the blood and serves as the primary marker for assessing vitamin D status. It is produced in the liver from vitamin D3.

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Things worth knowing about "Calcidiol"

Calcidiol is the main storage form of vitamin D in the blood and serves as the primary marker for assessing vitamin D status. It is produced in the liver from vitamin D3.

What is Calcidiol?

Calcidiol (chemical name: 25-hydroxyvitamin D or 25-hydroxycholecalciferol) is an intermediate metabolite in the vitamin D pathway. It is produced in the liver from vitamin D3 (cholecalciferol) and represents the main circulating form of vitamin D in the bloodstream. Calcidiol is the compound measured in blood tests to assess a person's vitamin D status.

Formation and Metabolism

Vitamin D enters the body either through the skin via UV-B radiation from sunlight or through dietary intake. In the liver, it is converted to calcidiol by the enzyme 25-hydroxylase. Calcidiol itself has limited biological activity. It is only in the kidneys that it is further converted by the enzyme 1alpha-hydroxylase into the biologically active form calcitriol (1,25-dihydroxyvitamin D), which exerts the actual physiological effects throughout the body.

Biological Functions

As a precursor to calcitriol, calcidiol is involved in many important bodily functions:

Calcium and phosphate balance: Promotes intestinal absorption of calcium and phosphate and supports their incorporation into bone tissue.
Bone health: Supports bone mineralization and helps prevent osteoporosis and rickets.
Immune system: Regulates immune responses and supports the body's natural defense mechanisms.
Muscle health: Contributes to normal muscle function and reduces the risk of falls in older adults.
Cell growth and differentiation: Plays a role in regulating cell proliferation processes.

Clinical Relevance and Measurement

The calcidiol level in the blood (reported in nmol/L or ng/mL) is considered the most reliable marker for assessing a person's vitamin D supply. The level reflects both dietary intake and the body's own production through sunlight exposure.

Reference Values (according to the European Food Safety Authority, EFSA)

Deficiency: below 30 nmol/L (below 12 ng/mL)
Suboptimal levels: 30–50 nmol/L (12–20 ng/mL)
Sufficient levels: 50–125 nmol/L (20–50 ng/mL)
Potential excess: above 125 nmol/L (above 50 ng/mL)

Causes of Low Calcidiol Levels

A low calcidiol level can result from various factors:

Insufficient sun exposure (e.g., indoor work, winter months, high latitudes)
Darker skin tone (reduced UV-B absorption)
Advanced age (reduced skin synthesis capacity)
Fat malabsorption disorders (e.g., Crohn's disease, celiac disease)
Liver or kidney disease (impaired conversion)
Certain medications (e.g., antiepileptics, corticosteroids)
Obesity (sequestration of vitamin D in adipose tissue)

Symptoms of Vitamin D Deficiency

A persistently low calcidiol level can cause the following symptoms:

Muscle weakness and muscle pain
Bone pain and increased risk of fractures
Osteoporosis in adults or rickets in children
Increased susceptibility to infections
General fatigue and exhaustion

Treatment and Supplementation

In cases of confirmed calcidiol deficiency, targeted supplementation with vitamin D3 (cholecalciferol) is recommended, which is then converted to calcidiol in the liver. In specific clinical situations (e.g., liver disease), calcidiol itself can be administered directly as a supplement, bypassing the hepatic conversion step.

Recommended daily doses vary depending on age, baseline levels, and individual needs. The European Food Safety Authority (EFSA) recommends a daily intake of 800 IU (20 mcg) of vitamin D for adults when adequate sun-induced synthesis cannot be assured. Supplementation should always be medically supervised, as excessive intake can lead to hypercalcemia and other adverse effects.

References

Holick MF. Vitamin D Deficiency. New England Journal of Medicine. 2007;357(3):266–281.
European Food Safety Authority (EFSA): Dietary Reference Values for Vitamin D. EFSA Journal. 2016;14(10):4547.
World Health Organization (WHO): Vitamin D supplementation guidance. www.who.int (2022).

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