Calcifediol

Biology

Calcifediol is the precursor for calcitriol, the active form of vitamin D. Variations in the expression and activity of CYP2R1, such as low levels in obesity, affect circulating calcifediol. Similarly, vitamin D2, ergocalciferol, can also be 25-hydroxylated to form 25-hydroxyergocalciferol, (, 25(OH)D2); both forms are measured together in blood as 25(OH)D.

At a typical intake of cholecalciferol (up to 2000 IU/day), conversion to calcifediol is rapid. When large doses are given (100,000 IU), it takes 7 days to reach peak calcifediol concentrations. Calcifediol binds in the blood to vitamin D-binding protein (also known as gc-globulin) and is the main circulating vitamin D metabolite. Calcifediol has an elimination half-life of around 15 to 30 days.

Calcifediol is further hydroxylated at the 1-alpha-position in the kidneys to form 1,25-(OH)2D3, calcitriol.

Calcifediol is also converted into 24,25-dihydroxycholecalciferol by 24-hydroxylation. This enzymatic reaction is performed by CYP24A1 which is expressed in many vitamin D target tissues including kidney, and is induced by calcitriol. This will inactivate calcitriol to calcitroic acid, but 24,25-(OH)2D3 may have some biological actions itself.

Blood test for vitamin D deficiency

Main article: Vitamin D#Recommended serum levels

In medical practice, a blood test for 25-hydroxy-vitamin D, 25(OH)D, is used to determine an individual's vitamin D status. The name 25(OH)D refers to any combination of calcifediol (25-hydroxy-cholecalciferol), derived from vitamin D3, and (25-hydroxy-ergocalciferol),

This measurement is considered the best indicator of overall vitamin D status. US labs generally report 25(OH)D levels as ng/mL. Other countries use nmol/L. Multiply ng/mL by 2.5 to convert to nmol/L.

This test can be used to diagnose vitamin D deficiency, and is performed in people with high risk for vitamin D deficiency, when the results of the test can be used to support beginning replacement therapy with vitamin D supplements., which cites

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Patients with osteoporosis, chronic kidney disease, malabsorption, obesity, and some other infections may be at greater risk for being vitamin D-deficient and so are more likely to have this test. Although vitamin D deficiency is common in some populations including those living at higher latitudes or with limited sun exposure, the 25(OH)D test is not usually requested for the entire population. Physicians may advise low risk patients to take over-the-counter vitamin D supplements in place of having screening.

It is the most sensitive measure, though experts have called for improved standardization and reproducibility across different laboratories. while other studies have defined levels below 80 nmol/L (32 ng/mL) as indicative of vitamin D deficiency.

Increasing calcifediol levels up to levels of 80 nmol/L (32 ng/mL) are associated with increasing the fraction of calcium that is absorbed from the gut.

Supplementation

Calcifediol supplements have been used in some studies to improve vitamin D status. Indications for their use include vitamin D deficiency or insufficiency, refractory rickets (vitamin D resistant rickets), familial hypophosphatemia, hypoparathyroidism, hypocalcemia and renal osteodystrophy and, with calcium, in primary or corticosteroid-induced osteoporosis.

Calcifediol may have advantages over cholecalciferol for the correction of vitamin D deficiency states. A review of the results of nine randomized control trials which compared oral doses of both, found that calcifediol was 3.2-fold more potent than cholecalciferol. Calcifediol is better absorbed from the intestine and has greater affinity for the vitamin-D-binding protein, both of which increase its bioavailability. Orally administered calcifediol has a much shorter half-life with faster elimination. These properties may be beneficial in people with intestinal malabsorption, obesity, or treated with certain other medications.

In 2016, the FDA approved a formulation of calcifediol (Rayaldee) 60 microgram daily as a prescription medication to treat secondary hyperparathyroidism in patients with chronic kidney disease.

Interactive pathway map

History

Research in the laboratory of Hector DeLuca identified 25(OH)D in 1968 and showed that the liver was necessary for its formation. The enzyme responsible for this synthesis, cholecalciferol 25-hydroxylase, was isolated in the same laboratory by Michael F. Holick in 1972.

Research

Studies are ongoing comparing the effects of calcifediol with other forms of vitamin D including cholecalciferol in prevention and treatment of osteoporosis.

Other Organisms

Teleost Fish

In teleost fish, calcifediol is predominantly converted to calcitriol in the liver, rather than in the kidneys which means circulating levels of calcifediol can be undetectable and alternative measures of vitamin D status are required. There has been some success in using dietary calcifediol supplementation in salmonids to improve growth and food conversion ratio.

References

References

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