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The prevalence of vitamin D deficiency in intensive care units ranges typically between 40 and 70%. There are many reasons for being or becoming deficient in the ICU. Hepatic, parathyroid and renal dysfunction additionally increases the risk for developing vitamin D deficiency. Moreover, therapeutic interventions like fluid resuscitation, dialysis, surgery, extracorporeal membrane oxygenation, cardiopulmonary bypass and plasma exchange may significantly reduce vitamin D levels. Many observational studies have consistently shown an association between low vitamin D levels and poor clinical outcomes in critically ill adults and children, including excess mortality and morbidity such as acute kidney injury, acute respiratory failure, duration of mechanical ventilation and sepsis. It is biologically plausible that vitamin D deficiency is an important and modifiable contributor to poor prognosis during and after critical illness. Although vitamin D supplementation is inexpensive, simple and has an excellent safety profile, testing for and treating vitamin D deficiency is currently not routinely performed. Overall, less than 800 patients have been included in RCTs worldwide, but the available data suggest that high-dose vitamin D supplementation could be beneficial. Two large RCTs in Europe and the United States, together aiming to recruit >5000 patients, have started in 2017, and will greatly improve our knowledge in this field. This review aims to summarize current knowledge in this interdisciplinary topic and give an outlook on its highly dynamic future.

Vitamin D deficiency is common in pregnant women and may contribute to adverse events in pregnancy such as preeclampsia (PET). To date, studies of vitamin D and PET have focused primarily on serum concentrations vitamin D, 25-hydroxyvitamin D3 (25(OH)D3) later in pregnancy. The aim here was to determine whether a more comprehensive analysis of vitamin D metabolites earlier in pregnancy could provide predictors of PET. Using samples from the SCOPE pregnancy cohort, multiple vitamin D metabolites were quantified by liquid chromatography–tandem mass spectrometry in paired serum and urine prior to the onset of PET symptoms. Samples from 50 women at pregnancy week 15 were analysed, with 25 (50%) developing PET by the end of the pregnancy and 25 continuing with uncomplicated pregnancy. Paired serum and urine from non-pregnant women (n = 9) of reproductive age were also used as a control. Serum concentrations of 25(OH)D3, 25(OH)D2, 1,25(OH)₂D3, 24,25(OH)₂D3 and 3-epi-25(OH)D3 were measured and showed no significant difference between women with uncomplicated pregnancies and those developing PET. As previously reported, serum 1,25(OH)₂D3 was higher in all pregnant women (in the second trimester), but serum 25(OH)D2 was also higher compared to non-pregnant women. In urine, 25(OH)D3 and 24,25(OH)₂D3 were quantifiable, with both metabolites demonstrating significantly lower (P < 0.05) concentrations of both of these metabolites in those destined to develop PET. These data indicate that analysis of urinary metabolites provides an additional insight into vitamin D and the kidney, with lower urinary 25(OH)D3 and 24,25(OH)₂D3 excretion being an early indicator of a predisposition towards developing PET.

Fibroblast growth factor‐23 (FGF23) controls the homeostasis of both phosphate and vitamin D. Bone-derived FGF23 can suppress the transcription of 1α‐hydroxylase (1α(OH)ase) to reduce renal activation of vitamin D (1,25(OH)₂D₃). FGF23 can also activate the transcription of 24‐hydroxylase to enhance the renal degradation process of vitamin D. There is a counter-regulation for FGF23 and vitamin D; 1,25(OH)₂D₃ induces the skeletal synthesis and the release of FGF23, while FGF23 can suppress the production of 1,25(OH)₂D₃ by inhibiting 1α(OH)ase synthesis. Genetically ablating FGF23 activities in mice resulted in higher levels of renal 1α(OH)ase, which is also reflected in an increased level of serum 1,25(OH)₂D₃, while genetically ablating 1α(OH)ase activities in mice reduced the serum levels of FGF23. Similar feedback control of FGF23 and vitamin D is also detected in various human diseases. Further studies are required to understand the subcellular molecular regulation of FGF23 and vitamin D in health and disease.

The CC genotype of the vitamin D receptor (VDR) polymorphism TaqI rs731236 has previously been associated with a higher risk of developing myopathy compared to TT carriers. However, the mechanistic role of this polymorphism in skeletal muscle is not well defined. The effects of vitamin D on patients genotyped for the VDR polymorphism TaqI rs731236, comparing CC and TT carriers were evaluated. Primary human myoblasts isolated from 4 CC carriers were compared with myoblasts isolated from four TT carriers and treated with vitamin D in vitro. A dose-dependent inhibitory effect on myoblast proliferation and differentiation was observed concurrent with modifications of key myogenic regulatory factors. RNA sequencing revealed a vitamin D dose–response gene signature enriched with a higher number of VDR-responsive elements (VDREs) per gene. Interestingly, the greater the expression of muscle differentiation markers in myoblasts, the more pronounced was the vitamin D-mediated response to suppress genes associated with myogenic fusion and myotube formation. This novel finding provides a mechanistic explanation to the inconsistency regarding previous reports of the role of vitamin D in myoblast differentiation. No effects in myoblast proliferation, differentiation or gene expression were related to CC vs TT carriers. Our findings suggest that the VDR polymorphism TaqI rs731236 comparing CC vs TT carriers did not influence the effects of vitamin D on primary human myoblasts and that vitamin D inhibits myoblast proliferation and differentiation through key regulators of cell cycle progression. Future studies need to employ strategies to identify the primary responses of vitamin D that drive the cellular response towards quiescence.