Recommendations on the measurement and the clinical use of vitamin D metabolites and vitamin D binding protein - A position paper from the IFCC Committee on bone metabolism

Abstract

Vitamin D, an important hormone with a central role in calcium and phosphate homeostasis, is required for bone and muscle development as well as preservation of musculoskeletal function. The most abundant vitamin D metabolite is 25-hydroxyvitamin D [25(OH)D], which is currently considered the best marker to evaluate overall vitamin D status. 25(OH)D is therefore the most commonly measured metabolite in clinical practice. However, several other metabolites, although not broadly measured, are useful in certain clinical situations. Vitamin D and all its metabolites are circulating in blood bound to vitamin D binding protein, (VDBP). This highly polymorphic protein is not only the major transport protein which, along with albumin, binds over 99% of the circulating vitamin D metabolites, but also participates in the transport of the 25(OH)D into the cell via a megalin/cubilin complex. The accurate measurement of 25(OH)D has proved a difficult task. Although a reference method and standardization program are available for 25(OH)D, the other vitamin D metabolites still lack this. Interpretation of results, creation of clinical supplementation, and generation of therapeutic guidelines require not only accurate measurements of vitamin D metabolites, but also the accurate measurements of several other "molecules" related with bone metabolism. IFCC understood this priority and a committee has been established with the task to support and continue the standardization processes of vitamin D metabolites along with other bone-related biomarkers. In this review, we present the position of this IFCC Committee on Bone Metabolism on the latest developments concerning the measurement and standardization of vitamin D metabolites and its binding protein, as well as clinical indications for their measurement and interpretation of the results.

Keywords: 1α,25-dihydroxyvitamin D; 24,25-dihydroxyvitamin D; 25-hydroxyvitamin D; Immunoassays; Liquid chromatography; Mass spectrometry; Standardization; Vitamin D; Vitamin D Standardization Program; Vitamin D binding protein.

Fig. 1.

Two forms of vitamin D: ergocalciferol (left) and cholecalciferol (right). The chemical structures are taken from PubChem (https://pubchem.ncbi.nlm.nih.gov).

Fig. 2.

Production of vitamin D2 from ergosterol. Ultraviolet (UV) radiation in the 290–315 nm wavelength range cleaves the B ring of ergosterol, yielding ergocalciferol. The irradiation of milk and yeast is a commercial means of producing D2 from ergosterol. Dihydrotachysterol (DHT) is a synthetic analog of vitamin D2. The chemical structures are taken from PubChem (https://pubchem.ncbi.nlm.nih.gov).

Fig. 3.

When the skin is exposed to UV radiation in the 290–315 nm wavelength range, 7-dehydrocholesterol absorbs this energy, which causes chemical bonds within the molecule to break and rearrange, resulting in the formation of pre-vitamin D. In the skin, pre-vitamin D undergoes rapid, thermally-induced, isomerization to produce vitamin D. Once formed, pre-vitamin D and vitamin D continue to absorb UV. Prolonged exposure to UV radiation results in the breakdown of these molecules into biologically inactive photoproducts. For this reason, during prolonged irradiation, a steady state is reached when only 10–15% of 7-dehydrocholesterol is simultaneously converted to pre-vitamin D3. This ensures that no toxic levels of vitamin D are synthesized under excessive sun exposure conditions. (The chemical structures are taken from PubChem: https://pubchem.ncbi.nlm.nih.gov).

Fig. 4.

The two steps of vitamin D activation. (The chemical structures are taken from PubChem: https://pubchem.ncbi.nlm.nih.gov.

Fig. 5.

Renal and extra renal calcitriol production serves an endocrine, autocrine, and paracrine function (original figure.

Fig. 6.

Schematic representation of catabolic pathways of vitamin D: CYP24A1 catalyzes the C24-oxidation pathway that leads to 1α,25(OH)₂D degradation that ultimately limits the amount of calcitriol in target tissues by accelerating its catabolism. This pathway comprises of 5 enzymatic steps that lead to the production of calcitroic acid that is excreted in bile. When the initial substrate is 25(OH)D the end product is again calcitroic acid.[332,333] CYP24A1 can also catalyze the C23-oxidation pathway which creates the biologically active 1α,25(OH)₂D-26,23 lactone from 1α,25(OH)₂D, and 25(OH)D-26,23 lactone from 25(OH)D.[7,33,333] The biological activity of the C23-oxidation pathway is not clear however there have been claims that the 1α,25(OH)₂D derived end product, 1α,25(OH)₂D-26,23 lactone, may act as a VDR antagonist.[334,335] (The chemical structures are taken from PubChem: https://pubchem.ncbi.nlm.nih.gov).

Fig. 7.

Schematic representation of the epimerization pathways of vitamin D (The chemical structures are taken from PubChem: https://pubchem.ncbi.nlm.nih.gov).