Ethnic variation in vitamin D-binding protein (GC): a review of isoelectric focusing studies in human populations

Vitamin D binding protein in psychiatric and neurological disorders: Implications for diagnosis and treatment

Vitamin D binding protein (VDBP) serves as a key transporter protein responsible for binding and delivering vitamin D and its metabolites to target organs. VDBP plays a crucial part in the inflammatory reaction following tissue damage and is engaged in actin degradation. Recent research has shed light on its potential role in various diseases, leading to a growing interest in understanding the implications of VDBP in psychiatric and neurological disorders. The purpose of this review was to provide a summary of the existing understanding regarding the involvement of VDBP in neurological and psychiatric disorders. By examining the intricate interplay between VDBP and these disorders, this review contributes to a deeper understanding of underlying mechanisms and potential therapeutic avenues. Insights gained from the study of VDBP could pave the way for novel strategies in the diagnosis, prognosis, and treatment of psychiatric and neurological disorders.

The potential role of vitamin D binding protein in kidney disease: a comprehensive review

Chronic kidney disease (CKD) is a growing health concern with a complex etiological landscape. Among the numerous factors implicated, vitamin D binding protein (VDBP) has emerged as a focal point of scientific studies because of its critical role in vitamin D metabolism and immune modulation. The relationship between VDBP and CKD reveals a complex web of molecular and biochemical details that have great potential for improving diagnostic understanding and treatment strategies for CKD. This review summarizes the multifaceted roles of VDBP, including its molecular dynamics, interactions with vitamin D, and subsequent implications for kidney function. The main focus of the discussion is how VDBP affects bone mineral homeostasis, highlighted by the dysregulation of calcium and phosphorus metabolism, which is a part of the pathophysiology of CKD. The discussion also touches on the immunomodulatory scope of VDBP and how it may reduce the chronic inflammatory environment that accompanies CKD. The diagnostic potential of VDBP as a biomarker for CKD has been rigorously examined, highlighting its capacity to improve early detection and prognostic assessment. Modification of VDBP activity has the potential to slow the course of CKD and improve patient outcomes. Furthermore, a detailed examination of the genetic polymorphisms of VDBP and their implications for CKD susceptibility and treatment responsiveness provides a perspective for personalized medical methods. Prospects for the future depend on the expansion of studies that try to understand the molecular mechanisms underlying the VDBP-CKD interaction, in addition to clinical trials that evaluate the effectiveness of VDBP-focused treatment approaches.

The Importance, Challenges, and Possible Solutions for Sharing Proteomics Data While Safeguarding Individuals' Privacy

Proteomics data sharing has profound benefits at the individual level as well as at the community level. While data sharing has increased over the years, mostly due to journal and funding agency requirements, the reluctance of researchers with regard to data sharing is evident as many shares only the bare minimum dataset required to publish an article. In many cases, proper metadata is missing, essentially making the dataset useless. This behavior can be explained by a lack of incentives, insufficient awareness, or a lack of clarity surrounding ethical issues. Through adequate training at research institutes, researchers can realize the benefits associated with data sharing and can accelerate the norm of data sharing for the field of proteomics, as has been the standard in genomics for decades. In this article, we have put together various repository options available for proteomics data. We have also added pros and cons of those repositories to facilitate researchers in selecting the repository most suitable for their data submission. It is also important to note that a few types of proteomics data have the potential to re-identify an individual in certain scenarios. In such cases, extra caution should be taken to remove any personal identifiers before sharing on public repositories. Data sets that will be useless without personal identifiers need to be shared in a controlled access repository so that only authorized researchers can access the data and personal identifiers are kept safe.

Association of functional, inherited vitamin D-binding protein variants with melanoma-specific death

BACKGROUND

It is unclear whether genetic variants affecting vitamin D metabolism are associated with melanoma prognosis. Two functional missense variants in the vitamin D-binding protein gene (GC), rs7041 and rs4588, determine 3 common haplotypes, Gc1s, Gc1f, and Gc2, of which Gc1f may be associated with decreased all-cause death among melanoma patients based on results of a prior study, but the association of Gc1f with melanoma-specific death is unclear.

METHODS

We investigated the association of the Gc1s, Gc1f, and Gc2 haplotypes with melanoma-specific and all-cause death among 4490 individuals with incident, invasive primary melanoma in 2 population-based studies using multivariable Cox-proportional hazards regression.

RESULTS

In the pooled analysis of both datasets, the patients with the Gc1f haplotype had a 37% lower risk of melanoma-specific death than the patients without Gc1f (hazard ratio [HR] = 0.63, 95% confidence interval [CI] = 0.47 to 0.83, P = .001), with adjustments for age, sex, study center, first- or higher-order primary melanoma, tumor site, pigmentary phenotypes, and Breslow thickness. Associations were similar in both studies. In pooled analyses stratified by Breslow thickness, the corresponding melanoma-specific death HRs for those patients with the Gc1f haplotype compared with those without Gc1f were 0.89 (95% CI = 0.63 to 1.27) among participants with tumor Breslow thickness equal to or less than 2.0 mm and 0.40 (95% CI = 0.25 to 0.63) among participants with tumor Breslow thickness greater than 2.0 mm (Pinteraction = .003).

CONCLUSIONS

Our findings suggest that individuals with the GC haplotype Gc1f may have a lower risk of dying from melanoma-specifically from thicker, higher-risk melanoma-than individuals without this Gc1f haplotype.

Identifying individuals using proteomics: are we there yet?

Multi-omics approaches including proteomics analyses are becoming an integral component of precision medicine. As clinical proteomics studies gain momentum and their sensitivity increases, research on identifying individuals based on their proteomics data is here examined for risks and ethics-related issues. A great deal of work has already been done on this topic for DNA/RNA sequencing data, but it has yet to be widely studied in other omics fields. The current state-of-the-art for the identification of individuals based solely on proteomics data is explained. Protein sequence variation analysis approaches are covered in more detail, including the available analysis workflows and their limitations. We also outline some previous forensic and omics proteomics studies that are relevant for the identification of individuals. Following that, we discuss the risks of patient reidentification using other proteomics data types such as protein expression abundance and post-translational modification (PTM) profiles. In light of the potential identification of individuals through proteomics data, possible legal and ethical implications are becoming increasingly important in the field.

The association of vitamin D binding protein levels and genotypes with type 1 diabetes in the black South African population

BACKGROUND

Vitamin D deficiency and the vitamin D pathway have previously been associated with type 1 diabetes (T1D). The majority of vitamin D is transported through the blood bound to the vitamin D binding protein (VDBP). Two polymorphisms in the VDBP gene (rs4588 and rs7041) result in different VDBP variants and have been associated with T1D, however the results are not consistent. The association of VDBP levels and its polymorphisms with T1D have not been investigated in the black South African population. Therefore, this study aimed to determine whether rs4588, rs7041 or serum VDBP levels were associated with T1D in this population.

METHODS

Participants with type 1 diabetes and controls were recruited from the greater Johannesburg area, South Africa. Participants were genotyped for rs4588 and rs7041 using PCR-RFLP and serum VDBP levels were determined by ELISA.

RESULTS

There was no difference in VDBP allelic or genotypic frequencies between participants with T1D and controls (rs4588 C allele frequency 0.92 vs. 0.94; p = 0.390 and rs7041 T allele frequency 0.95 vs. 0.95; p = 0.890). In univariate analysis, the rs4588 CC genotype was associated with increased serum VDBP levels, however, this association was lost with multivariate analysis. The VDBP genotypes were not associated with any other study variables. In logistic regression analysis, higher VBDP levels were associated with T1D (OR: (95% CI): 6.58 (1.45-29.9); p = 0.015), and within a linear regression analysis, T1D disease status was found to be associated with 0.044 mg/ml higher VDBP levels (p = 0.028).

CONCLUSIONS

These data suggest that serum VDBP levels are positively associated with the presence of T1D in the African population. Whether VDBP lies in the causal pathway or its elevation is an effect of T1D is uncertain and requires further investigation.

Distribution of vitamin D-binding protein/group-specific component gene subtypes in Kuwaiti population

BACKGROUND

Vitamin D-binding protein or group-specific component (Gc) is the major plasma carrier protein of Vitamin D. Two single nucleotide polymorphisms, rs7041 (NM_000583.3:c.1296G>T;NP_000574.2:p.Asp432Glu) and rs4588 (c.1307C>A; p.Thr436Lys), in the GC gene result in three major genotypes, that is, GC1F (c.1296T, c.1307C), GC1S (c.1296G, c.1307C), GC2 (c.1296T, c.1307A), and phenotypes such as Gc1F (p.432Asp, p.436Thr), Gc1S (p.432Glu, p.436Thr), and Gc2 (p.432Asp, p.436Lys). Significant variations in the frequencies of GC subtypes (genotypes/phenotypes) are reported in different populations living in different geographical locations, for example, GC1S/Gc1S (c.1296G, c.1307C/p.432Glu, p.436Thr) and GC2/Gc2 (c.1296T, c.1307A/p.432Asp, p.436Lys) are predominant in Caucasians and people living in the northern hemisphere, and GC1F/Gc1F (c.1296T, c.1307C/p.432Asp, p.436Thr) is predominant in Africans. However, frequencies of major GC subtypes are not known in the Kuwaiti population. In this study, we investigated 512 alleles to identify the major GC subtypes in Kuwaiti nationals.

METHODS

Genomic DNA was isolated from blood samples of 128 healthy subjects. DNA regions covering the targeted mutations were amplified by PCR. Amplified DNAs were sequenced by the Sanger method and analyzed for specific mutations to determine the GC genotypes and phenotypes.

RESULTS

The results identified the presence of four GC genotypes/phenotypes namely GC1F/Gc1F (c.1296T, c.1307C/p.432Asp, p.436Thr), GC1S/Gc1S (c.1296G, c.1307C/p.432Glu, p.436Thr), GC2/Gc2 (c.1296T, c.1307A/p.432Asp, p.436Lys), and GC3/Gc3 (c.1296G;c.1307A/p.432Glu, p.436Lys). Among the allelic subtypes (n = 512), GC1S (c.1296G; c.1307C) (n = 270, 52.7%) was predominant, followed by GC1F (c.1296T; c.1307C) (n = 138, 27%), GC2 (c.1296T; c.1307A) (n = 72, 14%), and GC3 (c.1296G; c.1307A) (n = 32, 6.3%). Three common subtypes, that is, GC1F (c.1296T; c.1307C), GC1S (c.1296G; c.1307C), and GC2 (c.1296T; c.1307A) are well documented in the literature, but GC3 (c.1296T; c.1307A) is an uncommon variant found in our study subjects.

CONCLUSION

We found that GC subtype distribution was unique in the Kuwaiti population, with some affinity to Caucasians. Several factors including ancestral origin, migration history, and environmental forces such as solar intensity may be responsible for the unique distribution of GC subtypes in this population.

Maternal Vitamin D Status Correlates to Leukocyte Antigenic Responses in Breastfeeding Infants

It is unknown if vitamin D (vitD) sufficiency in breastfeeding mothers can lead to physiological outcomes for their children that are discernible from infant vitD sufficiency per se. In a 3-month, randomized vitD supplementation study of mothers and their exclusively breastfeeding infants, the effects of maternal vitD sufficiency were determined on infant plasma concentrations of 25-hydroxyvitamin D (i.e., vitD status) and 11 cytokines. An inverse correlation was seen between maternal vitD status and infant plasma TNF concentration (r = −0.27; p < 0.05). Infant whole blood was also subjected to in vitro antigenic stimulation. TNF, IFNγ, IL-4, IL-13, and TGFβ1 responses by infant leukocytes were significantly higher if mothers were vitD sufficient but were not as closely correlated to infants’ own vitD status. Conversely, IL-10 and IL-12 responses after antigenic challenge were more correlated to infant vitD status. These data are consistent with vitD-mediated changes in breast milk composition providing immunological signaling to breastfeeding infants and indicate differential physiological effects of direct-infant versus maternal vitD supplementation. Thus, consistent with many previous studies that focused on the importance of vitD sufficiency during pregnancy, maintenance of maternal sufficiency likely continues to affect the health of breastfed infants.

Serum vitamin D-binding protein (VDBP) concentration and rs7041 genotype may be associated with preterm labor

PURPOSE

Vitamin D deficiency is common during pregnancy and may cause complications such as preterm labor (PTL). This study was aimed to investigate the effect of the vitamin D-binding protein (VDBP) rs7041 genotype, which has a significant effect on vitamin D metabolism and PTL.

METHODS

This cross-sectional study was conducted with 32 pregnant women who had spontaneous PTL and 54 pregnant women who had no specific findings as a control group. Serum total vitamin D 25-hydroxy vitamin D (25(OH)D) levels were measured using the Elecsys Vitamin D Total Kit. VDBP was measured using a VDBP Quantikine ELISA Kit. The levels of bioavailable 25(OH)D were calculated based on the total 25(OH)D and VDBP concentrations. DNA was extracted using the DNeasy Blood and Tissue Kit. Single nucleotide polymorphisms (rs7041) in GC were analyzed using a TaqMan SNP Genotyping Assay Kit. The unpaired t-test, Chi-squared, and ANCOVA tests were performed. Firth's penalized logistic regression was applied. The area under the curve (AUC) was calculated and the cutoff value was determined. All statistical analyses were performed using R version 4.0.3 (R Foundation for Statistical Computing, Vienna, Austria).

RESULTS

Total 25(OH)D levels were not significantly different between the two groups. Bioavailable 25(OH)D was significantly decreased in PTL women (p= .011), and VDBP was significantly increased in PTL women (p= .004) compared to the controls. Bioavailable 25(OH)D was lower in women with GT/TG and TT rs7041 genotypes than in those with GG, with statistical significance in women with the TT allele (p= .048). VDBP was higher in women with GT/TG and TT than those with GG, but there was no statistical significance. In PTL prevalence, bioavailable 25(OH)D and VDBP, the odds ratio increased by 1.463 times in GT/TG (p= .728) and increased by 1.675 times in TT compared to the GG allele (p= .640). In receiver operating characteristic (ROC) analysis for bioavailable 25(OH)D and VDBP, the AUC was 0.665 and 0685, respectively. The optimum cutoff of bioavailable 25(OH)D and VDBP levels for the diagnosis of PTL was calculated as 0.6 ng/mL and 523 µg/mL, respectively.

CONCLUSIONS

Pregnant women with the VDBP rs7041(c.1296 T > G) T allele genotype had reduced serum levels of bioavailable 25(OH)D and were more likely to develop PTL. Therefore, if the T allele is found in the VDBP rs7041 SNP genotyping test before or during pregnancy, more careful prenatal care may be required because of the increased risk of PTL.

Free, bioavailable 25-hydroxyvitamin D levels and their association with diabetic ketoacidosis in children with type 1 diabetes at diagnosis

BACKGROUND

Considering the roles of 25-hydroxyvitamin D (25OHD) in glucose homeostasis and immune modulation, vitamin D deficiency may be related to the development of type 1 diabetes (T1DM) and diabetic ketoacidosis (DKA). We evaluated the total, free, bioavailable 25OHD levels and vitamin D binding protein (VDBP) levels and genotypes between T1DM patients and controls.

METHODS

This retrospective, cross-sectional study included 84 children with T1DM (38 boys and 46 girls, 8.0 ± 3.6 years) and 1:1 age- and sex-matched healthy controls. A multiplex liquid chromatography-tandem mass spectrometry-based assay was used to simultaneously measure vitamin D metabolites.

RESULTS

Patients with T1DM had lower levels of total 25OHD (16.3 ± 5.1 vs. 19.9 ± 6.5 ng/mL, P< 0.001) and VDBP (146.0 ± 27.8 vs. 224.9 ± 36.1 µg/mL, P = 0.001), but higher free 25OHD (8.0 ± 2.5 vs. 6.5 ± 2.3 pg/mL, P< 0.001) than controls. Patients who presented with DKA had lower levels of 25OHD in the total (15.0 ± 4.6 vs. 17.6 ± 5.2 ng/mL, P = 0.020), free (7.5 ± 2.6 vs. 8.4 ± 2.4 pg/mL, P = 0.059), and bioavailable (2.3 ± 0.9 vs. 2.8 ± 0.8 ng/mL, P = 0.014) forms than those without DKA at the T1DM diagnosis. The lower the total, free, and bioavailable 25OHD levels at diagnosis, the lower the pH and HCO3-. The proportions of the VDBP genotypes did not differ between the patients and controls.

CONCLUSION

Patients with T1DM had higher levels of free 25OHD than healthy children, despite lower levels of total 25OHD. However, patients with DKA exhibited lower levels of bioavailable 25OHD than those without DKA at the T1DM diagnosis. The lower the concentrations of free and bioavailable 25OHD, the more severe the acidosis at the initial T1DM presentation.

Effect of vitamin D on cognitive decline: results from two ancillary studies of the VITAL randomized trial

Low vitamin D levels have been associated with cognitive decline; however, few randomized trials have been conducted. In a trial, we evaluated vitamin D3 supplementation on cognitive decline. We included participants aged 60+ years (mean[SD] = 70.9[5.8] years) free of cardiovascular disease and cancer in two substudies in the VITAL 2 × 2 randomized trial of vitamin D3 (2000 IU/day of cholecalciferol) and fish oil supplements: 3424 had cognitive assessments by phone (eight neuropsychologic tests; 2.8 years follow-up) and 794 had in-person assessments (nine tests; 2.0 years follow-up). The primary, pre-specified outcome was decline over two assessments in global composite score (average z-scores of all tests); substudy-specific results were meta-analyzed. The pooled mean difference in annual rate of decline (MD) for vitamin D3 versus placebo was 0.01 (95% CI - 0.01, 0.02; p = 0.39). We observed no interaction with baseline 25-hydroxyvitamin-D levels (p-interaction = 0.84) and a significant interaction with self-reported race (p-interaction = 0.01). Among Black participants (19%), those assigned vitamin D3 versus placebo had better cognitive maintenance (MD = 0.04, 95% CI 0.01, 0.08, similar to that observed for Black participants 1.2 years apart in age). Thus, vitamin D3 (2000 IU/day cholecalciferol) supplementation was not associated with cognitive decline over 2-3 years among community-dwelling older participants but may provide modest cognitive benefits in older Black adults, although these results need confirmation.Trial registration ClinicalTrials.gov; VITAL (NCT01169259), VITAL-DEP (NCT01696435) and VITAL-Cog (NCT01669915); the date the registration for the parent trial (NCT01169259) was submitted to the registry: 7/26/2010 and the date of first patient enrollment in either of the ancillary studies for cognitive function in a subset of eligible VITAL participants: 9/14/2011.

Comparative analysis of the association between various serum vitamin D biomarkers and sarcopenia

Background

Vitamin D status is associated with muscle strength and maintenance of muscle fibers. However, which serum vitamin D biomarker better reflects sarcopenia remains unclear. The aim of this study was to investigate associations between various serum vitamin D biomarkers (total 25‐hydroxy vitamin D [25(OH)D], bioavailable 25(OH)D, 24,25‐dihydroxyvitamin D [24,25(OH)₂D], and vitamin D metabolite ratio [VMR]) and sarcopenia.

Methods

The data for 83 hip fracture patients were finally included in the analysis. Sarcopenia was defined according to the Asia Working Group for Sarcopenia (AWGS) criteria. Measurements of 24,25(OH)₂D and 25(OH)D were made using solid‐phase extraction (SPE) and subsequent liquid chromatography‐tandem mass spectrometry (LC‐MS/MS). Vitamin D binding protein (VDBP) concentration was measured using an enzyme‐linked immunosorbent assay. The VMR was calculated by dividing serum 24,25(OH)₂D by serum 25(OH)D and then multiplying by 100. Based on total 25(OH)D, VDBP, and albumin concentrations, bioavailable 25(OH)D concentrations were calculated using the equations from the other previous studies.

Results

Bioavailable 25(OH)D levels were significantly (p = 0.030) decreased in the sarcopenia group compared with the non‐sarcopenia group. Results of ROC analysis for the diagnosis of sarcopenia using serum level of bioavailable of 25(OH)D revealed that the cutoff point for bioavailable 25(OH)D was 1.70 ng/ml (AUC = 0.649, p < 0.001). In the group with a bioavailable 25(OH)D less than 1.70 ng/ml, the incidence of sarcopenia increased by 3.3 times (odds ratio: 3.33, p = 0.013).

Conclusion

We demonstrated that bioavailable 25(OH)D was associated with sarcopenia among the various serum vitamin D biomarkers. Bioavailable vitamin D might be helpful for assessing the risk of sarcopenia.

Association of GC Variants with Bone Mineral Density and Serum VDBP Concentrations in Mexican Population

Vitamin D-binding protein (VDBP) is encoded by the GC gene and is an active participant in the control of bone metabolism. However, the effect of its major variants on VDBP concentration and bone mineral density (BMD) remains unclear. Our aim was to analyze the effect of major GC variants on serum VDBP concentration and BMD. We recruited individuals from the Health Workers Cohort Study, which includes employees of the Mexican Institute of Social Security (IMSS). A total of 1853 adults were included. The single nucleotide polymorphisms (SNPs) rs7041 and rs4588 were genotyped to identify the three best characterized haplotypes of GC. Serum VBDP, 25(OH)D and BMD were also measured. Among women, the G allele of rs7041 was associated with higher VDBP and BMD compared to homozygous TT. The A allele of rs4588 was associated with lower VDBP and BMD compared to CC homozygous. In men, GC variants were only associated with VDBP levels. We did not observe an association between free/bioavailable 25(OH)D and BMD in men and women. Our results support an association of VDBP in bone health. The G and C alleles, from rs7041 and rs4588, respectively, are associated with high concentrations of VDBP and BMD in this sample of Mexican postmenopausal women.

Effect of oral contraceptives on total and bioavailable 25-hydroxyvitamin D

Studies show an increase in circulating levels of 25-hydroxyvitamin D [25(OH)D] in women using combined oral contraceptives (COCs). 25(OH)D is a quantitatively important metabolite and widely used clinical marker of vitamin D status and is regulated by vitamin D binding protein (VDBP). However, studies have not identified the type of formulations used by the women, and there are no data on the effect of progestins on 25(OH)D levels. Our study objective was to compare the effects of two COC formulations [ethinyl estradiol (EE)/norethindrone acetate (NETA) vs. EE/levonorgestrel (LNG)] as well as LNG alone on total and bioavailable (free plus albumin-bound) 25(OH)D levels in serum samples collected at baseline, mid treatment, and end of treatment. Total 25(OH)D and VDBP were measured by immunoassay, and bioavailable 25(OH)D was calculated. The results show that with the EE/NETA formulation, total and bioavailable 25(OH)D and VDBP levels increased non-significantly by 7.4 %, 14.9 %, and 10 %, respectively, from baseline to end of treatment. In contrast, the corresponding changes with EE/LNG showed an increase of 4.4 % in total 25(OH)D but a significant decrease of 18.2 % in bioavailable 25(OH)D and increase of 19.1 % in VDBP. When LNG was administered alone, no significant changes were observed in total and bioavailable 25(OH)D or VDBP levels during the course of treatment. Our findings show considerably different effects on total and bioavailable 25(OH)D levels, as well as VDBP levels, with different oral contraceptive formulations. LNG may have a suppressive effect on VDBP, similar to its well-known androgenic effect on SHBG. Further studies are needed to determine the effect of hormonal contraceptive formulations on vitamin D status and its potential impact on women's health.

Clinical utility of cerebrospinal fluid vitamin D-binding protein as a novel biomarker for the diagnosis of viral and bacterial CNS infections

Background

Rapid and accurate diagnosis of central nervous system (CNS) infections is important, and laboratory tests help diagnose CNS infections. Even when the patient has symptoms, laboratory tests often do not reveal any specific findings. The potential of vitamin D-binding protein (VDBP) to be used as a biomarker for viral and bacterial CNS infections was studied.

Methods

A total of 302 subjects with suspected CNS infection who underwent lumbar puncture were included. Clinical and laboratory data were collected retrospectively. VDBP levels were measured in the cerebrospinal fluid (CSF) samples. Genotyping for the GC gene encoding VDBP was also performed. VDBP levels were analyzed and compared by CNS infection, pathogen, CSF opening pressure, and GC genotype.

Results

A CNS infection group (n = 90) and a non-CNS infection group (n = 212) were studied. In terms of its receiver operating characteristic, CSF VDBP showed an area under the curve of 0.726 for the diagnosis of CNS infection. CSF VDBP levels were significantly different between the CNS infection and non-infection groups. The CNS infection group with enterovirus showed a statistically lower distribution of CSF VDBP levels than the other virus groups. The group with CSF opening pressure > 25 cmH₂O showed higher CSF VDBP levels than the other groups. There was no significant difference in GC gene allele distribution between the CNS infection and non-infection groups.

Conclusions

CSF VDBP levels were increased in patients with CNS infection. The CSF VDBP showed potential as a new biomarker for viral and bacterial CNS infections.

Serum 24,25-dihydroxyvitamin D level in general Korean population and its relationship with other vitamin D biomarkers

Background

Vitamin D status is presently assessed by measuring total serum concentration of 25-hydroxyvitamin D [25(OH)D]. However, 25(OH)D concentration alone might not accurately reflect vitamin D status owing to its weak relationship with various clinical indices and inconsistency across races. Recently, 24,25-dihydroxyvitamin D [24,25(OH)₂D] and vitamin D metabolite ratio [VMR; ratio of 24,25(OH)₂D to 25(OH)D] have emerged as vitamin D biomarkers. The present study aimed to determine the values of 24,25(OH)₂D and VMR in healthy Koreans and compare them with other vitamin D biomarkers, including 25(OH)D and bioavailable 25(OH)D.

Methods

Serum samples and medical information were collected from 200 individuals (100 females and 100 males) who underwent general health checks without self-reported symptoms. We measured 24,25(OH)₂D concentration using liquid chromatography–tandem mass spectrometry, and concentrations of 25(OH)D and vitamin D binding protein using immunoassays. VMR and bioavailable 25(OH)D concentration were calculated using the above data. Serum parathyroid hormone level, and bone mineral density (BMD) data were collected as clinical outcomes, and the effects of the vitamin D markers on them were tested using multiple linear regression models.

Results

The mean values of 25(OH)D, 24,25(OH)₂D, VMR, and bioavailable 25(OH)D were 24.3 ± 8.5 ng/mL, 1.9 ± 1.1 ng/mL, 7.6 ± 2.5, and 3.2 ± 1.2 ng/mL, respectively. The concentration of 25(OH)D closely correlated with 24,25(OH)₂D (R = 0.868, P < 0.001) and bioavailable 25(OH)D (R = 0.862, P < 0.001). No significant effects of 24,25(OH)₂D, VMR, and bioavailable 25(OH)D were observed on the prediction of PTH and BMD in the multiple linear regression models.

Conclusion

Our study presents the distribution of 24,25(OH)₂D concentration and VMR in Korean population for the first time. Overall, our data reaffirm that 25(OH)D is the primary marker for determining vitamin D status in the general population.

Plasma Proteomes Can Be Reidentifiable and Potentially Contain Personally Sensitive and Incidental Findings

The goal of clinical proteomics is to identify, quantify, and characterize proteins in body fluids or tissue to assist diagnosis, prognosis, and treatment of patients. In this way, it is similar to more mature omics technologies, such as genomics, that are increasingly applied in biomedicine. We argue that, similar to those fields, proteomics also faces ethical issues related to the kinds of information that is inherently obtained through sample measurement, although their acquisition was not the primary purpose. Specifically, we demonstrate the potential to identify individuals both by their characteristic, individual-specific protein levels and by variant peptides reporting on coding single nucleotide polymorphisms. Furthermore, it is in the nature of blood plasma proteomics profiling that it broadly reports on the health status of an individual-beyond the disease under investigation. Finally, we show that private and potentially sensitive information, such as ethnicity and pregnancy status, can increasingly be derived from proteomics data. Although this is potentially valuable not only to the individual, but also for biomedical research, it raises ethical questions similar to the incidental findings obtained through other omics technologies. We here introduce the necessity of-and argue for the desirability for-ethical and human-rights-related issues to be discussed within the proteomics community. Those thoughts are more fully developed in our accompanying manuscript. Appreciation and discussion of ethical aspects of proteomic research will allow for deeper, better-informed, more diverse, and, most importantly, wiser guidelines for clinical proteomics.

Differences in 25-Hydroxyvitamin D Clearance by eGFR and Race: A Pharmacokinetic Study

BACKGROUND

Conversion of 25-hydroxyvitamin D (25[OH]D) to the active form of vitamin D occurs primarily in the kidney. Observational studies suggest 25(OH)D clearance from the circulation differs by kidney function and race. However, these potential variations have not been tested using gold-standard methods.

METHODS

We administered intravenous, deuterated 25(OH)D3 (d-25[OH]D3) in a pharmacokinetic study of 87 adults, including 43 with normal eGFR (≥60 ml/min per 1.73 m2), 24 with nondialysis CKD (eGFR <60 ml/min per 1.73 m2), and 20 with ESKD treated with hemodialysis. We measured concentrations of d-25(OH)D3 and deuterated 24,25-dihydroxyvitamin D3 at 5 minutes and 4 hours after administration, and at 1, 4, 7, 14, 21, 28, 42, and 56 days postadministration. We calculated 25(OH)D clearance using noncompartmental analysis of d-25(OH)D3 concentrations over time. We remeasured 25(OH)D clearance in a subset of 18 participants after extended oral vitamin-D3 supplementation.

RESULTS

The mean age of the study cohort was 64 years; 41% were female, and 30% were Black. Mean 25(OH)D clearances were 360 ml/d, 313 ml/d, and 263 ml/d in participants with normal eGFR, CKD, and kidney failure, respectively (P=0.02). After adjustment for age, sex, race, and estimated blood volume, lower eGFR was associated with reduced 25(OH)D clearance (β=-17 ml/d per 10 ml/min per 1.73 m2 lower eGFR; 95% CI, -21 to -12). Black race was associated with higher 25(OH)D clearance in participants with normal eGFR, but not in those with CKD or kidney failure (P for interaction=0.05). Clearance of 25(OH)D before versus after vitamin-D3 supplementation did not differ.

CONCLUSIONS

Using direct pharmacokinetic measurements, we show that 25(OH)D clearance is reduced in CKD and may differ by race.

CLINICAL TRIAL REGISTRY NAME AND REGISTRATION NUMBER

Clearance of 25-hydroxyvitamin D in Chronic Kidney Disease (CLEAR), NCT02937350; Clearance of 25-hydroxyvitamin D3 During Vitamin D3 Supplementation (CLEAR-PLUS), NCT03576716.

Genetic Variants Shaping Inter-individual Differences in Response to Dietary Intakes-A Narrative Review of the Case of Vitamins

Recent advances in the field of nutrigenetics have provided evidence on how genetic variations can impact the individuals' response to dietary intakes. An objective and reliable assessment of dietary exposures should rely on combinations of methodologies including frequency questionnaires, short-term recalls or records, together with biological samples to evaluate markers of intake or status and to identify genetic susceptibilities. In an attempt to present current knowledge on how genetic fingerprints contribute to an individual's nutritional status, we present a review of current literature describing associations between genetic variants and levels of well-established biomarkers of vitamin status in free-living and generally healthy individuals. Based on the outcomes of candidate gene, genome-wide-association studies and meta-analyses thereof, we have identified several single nucleotide polymorphisms (SNPs) involved in the vitamins' metabolic pathways. Polymorphisms in genes encoding proteins involved in vitamin metabolism and transport are reported to have an impact on vitamin D status; while genetic variants of vitamin D receptor were most frequently associated with health outcomes. Genetic variations that can influence vitamin E status include SNPs involved in its uptake and transport, such as in SCAR-B1 gene, and in lipoprotein metabolism. Variants of the genes encoding the sodium-dependent vitamin C transport proteins are greatly associated with the body's status on vitamin C. Regarding the vitamins of the B-complex, special reference is made to the widely studied variant in the MTHFR gene. Methodological attributes of genetic studies that may limit the comparability and interpretability of the findings are also discussed. Our understanding of how genes affect our responses to nutritional triggers will enhance our capacity to evaluate dietary exposure and design personalized nutrition programs to sustain health and prevent disease.

Adaptation and co-adaptation of skin pigmentation and vitamin D genes in native Americans

We carried out an exhaustive review regarding human skin color variation and how much it may be related to vitamin D metabolism and other photosensitive molecules. We discuss evolutionary contexts that modulate this variability and hypotheses postulated to explain them; for example, a small amount of melanin in the skin facilitates vitamin D production, making it advantageous to have fair skin in an environment with little radiation incidence. In contrast, more melanin protects folate from degradation in an environment with a high incidence of radiation. Some Native American populations have a skin color at odds with what would be expected for the amount of radiation in the environment in which they live, a finding challenging the so-called "vitamin D-folate hypothesis." Since food is also a source of vitamin D, dietary habits should also be considered. Here we argue that a gene network approach provides tools to explain this phenomenon since it indicates potential alleles co-evolving in a compensatory way. We identified alleles of the vitamin D metabolism and pigmentation pathways segregated together, but in different proportions, in agriculturalists and hunter-gatherers. Finally, we highlight how an evolutionary approach can be useful to understand current topics of medical interest.

Association of prediagnostic vitamin D status with mortality among colorectal cancer patients differs by common, inherited vitamin D-binding protein isoforms

Lower prediagnostic circulating 25-hydroxyvitamin D (25[OH]D)-considered the best marker of total vitamin D exposure-is associated with higher mortality risk among colorectal cancer (CRC) patients. However, it is unknown whether this association differs by the vitamin D-binding protein (GC) isoform Gc2 (encoded by GC rs4588*C>A, Thr436Lys), which may substantially affect vitamin D metabolism and modify associations of 25(OH)D with colorectal neoplasm risk. Prediagnostic 25(OH)D-mortality associations according to Gc2 isoform were estimated using multivariable Cox proportional hazards regression among 1281 CRC cases (635 deaths, 483 from CRC) from two large prospective cohorts conducted in the United States (Cancer Prevention Study-II) and Europe (European Prospective Investigation into Cancer and Nutrition). 25(OH)D measurements were calibrated to a single assay, season standardized, and categorized using Institute of Medicine recommendations (deficient [<30], insufficient [30 - <50], sufficient [≥50 nmol/L]). In the pooled analysis, multivariable-adjusted hazard ratios (HRs) for CRC-specific mortality associated with deficient relative to sufficient 25(OH)D concentrations were 2.24 (95% CI 1.44-3.49) among cases with the Gc2 isoform, and 0.94 (95% CI 0.68-1.22) among cases without Gc2 (Pinteraction = .0002). The corresponding HRs for all-cause mortality were 1.80 (95% CI 1.24-2.60) among those with Gc2, and 1.12 (95% CI 0.84-1.51) among those without Gc2 (Pinteraction = .004). Our findings suggest that the association of prediagnostic vitamin D status with mortality among CRC patients may differ by functional GC isoforms, and patients who inherit the Gc2 isoform (GC rs4588*A) may particularly benefit from higher circulating 25(OH)D for improved CRC prognosis.

Vitamin D Binding Protein (VDBP) and Its Gene Polymorphisms-The Risk of Malignant Tumors and Other Diseases

Vitamin D is an important component of the endocrine system that controls calcium homeostasis and bone mineralization. Because of the very short half-life of free serum vitamin D it is stabilized and transported to target tissues by being bound to the vitamin D binding protein (VDBP). The most common polymorphisms: rs4588 and rs7041 in the vitamin D binding protein gene may correlate with differences in vitamin D status in the serum. This review presents data that relate to the presence of genetic variants in the VDBP gene in correlation with certain diseases, mostly concerning cancers (breast, prostate, pancreatic, lung, colorectal, basal cell carcinoma cancer and cutaneous melanoma) or other related diseases (thyroid autoimmunity disorders, obesity, diabetes mellitus, bone metabolism, rheumatoid arthritis, ankylosing spondylitis, asthma, chronic obstructive pulmonary disease, tuberculosis and coronary artery diseases).

Serum vitamin D binding protein level, but not serum total, bioavailable, free vitamin D, is higher in 30-days survivors than in nonsurvivors with sepsis

The prognostic value of 3 types (total, bioavailable, and free) of 25-hydroxy vitamin D [25(OH)D] and vitamin D binding protein (VDBP) in patients with sepsis is unknown. The aim of this study was to evaluate the association of levels of those 3 types of 25(OH) D and VDBP with 30-day mortality in patients with sepsis. From March to December 2018, patients diagnosed with sepsis and admitted to the medical intensive care unit were enrolled, prospectively. We measured total 25(OH)D and VDBP levels, performed GC genotyping for the polymorphisms rs4588 and rs7041, and calculated bioavailable and free 25(OH)D levels. Total, bioavailable, and free 25(OH)D levels did not differ in 30-days nonsurvivors and survivors. Serum VDBP level was significantly higher in survivors than nonsurvivors (138.6 ug/mL vs 108.2 ug/mL, P = .023) and was associated with 30-day mortality in univariate but not multivariate analysis. VDBP polymorphisms and allele frequencies were not statistically different between the groups. Serum VDBP level was significantly higher in survivors than nonsurvivors over 30-days mortality in septic patients. However, 3 types (total, bioavailable, and free) of 25(OH)D levels did not differ between the survivors and nonsurvivors group.

Association of Circulating Vitamin D With Colorectal Cancer Depends on Vitamin D-Binding Protein Isoforms: A Pooled, Nested, Case-Control Study

BACKGROUND

Higher circulating 25-hydroxyvitamin-D [25(OH)D] concentrations are consistently inversely associated with colorectal cancer (CRC) risk in observational studies. However, it is unknown whether this association depends on the functional GC-rs4588*A (Thr436Lys) variant encoding the vitamin D-binding protein-2 (DBP2) isoform, which may affect vitamin D status and bioavailability.

METHODS

We analyzed data from 1710 incident CRC cases and 1649 incidence-density-matched controls nested within three prospective cohorts of mostly Caucasians. Study-specific incidence rate ratios (RRs) for associations of prediagnostic, season-standardized 25(OH)D concentrations according to DBP2 isoform with CRC were estimated using multivariable unconditional logistic regression and were pooled using fixed-effects models. All statistical significance tests were two-sided.

RESULTS

The odds of having 25(OH)D concentrations less than 50 nmol/L (considered insufficient by the Institute of Medicine) were 43% higher for each DBP2-encoding variant (rs4588*A) inherited (per DBP2 odds ratio [OR] = 1.43, 95% confidence interval [CI] = 1.27 to 1.62, P trend = 1.2 × 10-8). The association of 25(OH)D concentrations with CRC risk differed by DBP2: 25(OH)D concentrations considered sufficient (≥ 50 nmol/L), relative to deficient (< 30 nmol/L), were associated with a 53% lower CRC risk among individuals with the DBP2 isoform (RR = 0.47, 95% CI = 0.33 to 0.67), but with a non-statistically significant 12% lower risk among individuals without it (RR = 0.88, 95% CI = 0.61 to 1.27) (P heterogeneity = .01).

CONCLUSIONS

Our results suggest that the 25(OH)D-CRC association may differ by DBP isoform, and those with a DBP2-encoding genotype linked to vitamin D insufficiency may particularly benefit from adequate 25(OH)D for CRC prevention.

Evolution, Prehistory and Vitamin D

Aspects of human evolutionary biology and prehistory are discussed in relation to vitamin D. The evolution of hairlessness, combined with the need for efficient eccrine sweat production for cooling, provided evolutionary pressure to protect the skin from ultraviolet damage by developing cutaneous pigmentation. There was a subsequent loss of pigmentation as humans journeyed to northern latitudes. Their increasing mastery of technology outstripped evolution's finite pace as further dispersal occurred around the globe. A timeline for the development of clothing to provide warmth, and the consequent shielding from ultraviolet light, which diminished vitamin D synthesis, can be inferred by an examination of mutations in the human louse.

Genetic, environmental and biomarker considerations delineating the regulatory effects of vitamin D on central nervous system function

Studies show that vitamin D (vit-D) (25(OH)D), the bioactive metabolite (1,25(OH)2D3) and vit-D receptors (vit-D receptor; protein disulphide isomerase, family A member 3) are expressed throughout the brain, particularly in regions pivotal to learning and memory. This has led to the paradigm that avoiding vit-D deficiency is important to preserve cognitive function. However, presently, it is not clear if the common clinical measure of serum 25(OH)D serves as a robust surrogate marker for central nervous system (CNS) homeostasis or function. Indeed, recent studies report CNS biosynthesis of endogenous 25(OH)D, the CNS expression of the CYP group of enzymes which catalyse conversion to 1,25(OH)2D3 and thereafter, deactivation. Moreover, in the periphery, there is significant ethnic/genetic heterogeneity in vit-D conversion to 1,25(OH)2D3 and there is a paucity of studies which have actually investigated vit-D kinetics across the cerebrovasculature. Compared with peripheral organs, the CNS also has differential expression of receptors that trigger cellular response to 1,25(OH)2D3 metabolites. To holistically consider the putative association of peripheral (blood) abundance of 25(OH)D on cognitive function, herein, we have reviewed population and genetic studies, pre-clinical and clinical intervention studies and moreover have considered potential confounders of vit-D analysis.

Vitamin D binding protein is required to utilize skin-generated vitamin D

Vitamin D is produced in the skin following exposure to sunlight. Ultraviolet (UV) B (UVB, 280-310 nm) results in isomerization of 7-dehydrocholesterol to previtamin D that spontaneously isomerizes to vitamin D. This pool of skin-derived vitamin D is the major source of vitamin D for animals. However, the mechanisms by which it becomes available remain undefined. It has been assumed that cutaneous vitamin D is transported into the circulation by vitamin D binding protein (DBP), but experimental evidence is lacking. To determine whether cutaneous vitamin D is transported by DBP, we utilized DBP^-/- mice that were made vitamin D-deficient. These animals lack measurable 25(OH)D in blood and are hypocalcemic. As controls, DBP^+/+ animals were vitamin D depleted and made equally hypocalcemic. UV irradiation of DBP^+/+ animals restored serum calcium and serum 25(OH)D while the same treatment of DBP^-/- animals failed to show either a serum calcium or 25(OH)D response despite having normal vitamin D production in skin. Intravenous injection of small amounts of recombinant DBP to the vitamin D-deficient DBP^-/- mice restored the response to UV light. These results demonstrate a requirement for DBP to utilize cutaneously produced vitamin D.

Steroid Hormone Vitamin D: Implications for Cardiovascular Disease

Understanding of vitamin D physiology is important because about half of the population is being diagnosed with deficiency and treated with supplements. Clinical guidelines were developed based on observational studies showing an association between low serum levels and increased cardiovascular risk. However, new randomized controlled trials have failed to confirm any cardiovascular benefit from supplementation in the general population. A major concern is that excess vitamin D is known to cause calcific vasculopathy and valvulopathy in animal models. For decades, administration of vitamin D has been used in rodents as a reliable experimental model of vascular calcification. Technically, vitamin D is a misnomer. It is not a true vitamin because it can be synthesized endogenously through ultraviolet exposure of the skin. It is a steroid hormone that comes in 3 forms that are sequential metabolites produced by hydroxylases. As a fat-soluble hormone, the vitamin D-hormone metabolites must have special mechanisms for delivery in the aqueous bloodstream. Importantly, endogenously synthesized forms are carried by a binding protein, whereas dietary forms are carried within lipoprotein particles. This may result in distinct biodistributions for sunlight-derived versus supplement-derived vitamin D hormones. Because the cardiovascular effects of vitamin D hormones are not straightforward, both toxic and beneficial effects may result from current recommendations.

Cerebrospinal fluid vitamin D-binding protein as a new biomarker for the diagnosis of meningitis

BACKGROUND

Meningitis is an inflammatory process involving meninges. It is difficult to diagnose because of the absence of a diagnostic biomarker. We first report here the possibility of cerebrospinal fluid (CSF) vitamin D-binding protein (VDBP) as a new biomarker for the diagnosis of meningitis.

METHODS

This prospective study enrolled a total of 102 subjects (58 patients with non-neurologic disease, 17 patients with meningitis, and 27 patients with other neurologic diseases) from 2017 to 2018. CSF and blood samples were collected in pairs. Total 25(OH)D in CSF and serum and VDBP levels in serum were measured. GC genotyping was also performed to determine polymorphisms of rs4588 and rs7041. CSF total 25(OH)D and VDBP levels were compared with serum total 25(OH)D and VDBP levels according to disease (meningitis vs. non-meningitis). Receiver operating characteristic (ROC) analysis for the diagnosis of meningitis using CSF VDBP level was performed.

RESULTS

Mean CSF VDBP and serum VDBP levels of all patients were 1.48 ± 1.32 and 181.28 ± 56.90 μg/mL, respectively. CSF VDBP level in the meningitis disease group (3.20 ± 1.49 μg/mL) was significantly (P < 0.001) higher than that in other disease groups. According to ROC curve analysis, the appropriate cut-off value for CSF VDBP was 1.96 μg/mL, showing sensitivity of 82.4% and specificity of 85.9%. AUC of CSF VDBP was 0.879 (95% CI: 0.789-0.962).

CONCLUSIONS

CSF VDBP level showed very good diagnostic performance. It could be used as a potential biomarker for the diagnosis of meningitis.

Vitamin D, bone alkaline phosphatase and parathyroid hormone in healthy subjects and haemodialysed patients from West Africa: impact of reference ranges and parathyroid hormone generation assays on the KDIGO guidelines

BACKGROUND

The Kidney Disease: Improving Global Outcomes (KDIGO) guidelines recommend targets based on multiples of the upper limit of normal (ULN) of parathyroid hormone (PTH) concentration. However, the ULN has not always been correctly established by manufacturers. While it is known that the ULN is supposed to be higher in African Americans than in Caucasians, it is largely unknown in Africans.

METHODS

We established the ULN of PTH concentration in a population of 240 healthy Ivorians using second- and third-generation PTH assays before and after supplementation with 100 000 IU of cholecalferol. We measured the levels of PTH, bone alkaline phosphatase, 25-hydroxyvitamin D and 1,25-dihydroxyvitamin D in 100 haemodialysed Ivorian patients.

RESULTS

The prevalence of vitamin D deficiency in Ivory Coast is low. The ULN obtained using the third-generation PTH assay was similar to that obtained in Caucasians but was higher when PTH was measured using the second-generation PTH assay. According to the KDIGO guidelines, ∼20% of the haemodialysed patients were below twice the ULN and 30% were above nine times the ULN. Approximately 25% of the patients were even >12 times the ULN. We observed a discrepancy in the results between the two PTH assays (14%) that was relatively more important than what we observed from previous studies in Caucasians using the same strategy.

CONCLUSIONS

We found a low prevalence of vitamin D deficiency in a tropical country like Ivory Coast. We also established the PTH reference range, which could prove useful for the follow-up of haemodialysed patients, particularly for the large number of patients suffering from secondary hyperparathyroidism who are at high risk of adverse bone events.

Total 25-hydroxy vitamin D level in cerebrospinal fluid correlates with serum total, bioavailable, and free 25-hydroxy vitamin D levels in Korean population

Epidemiological investigations have suggested that serum 25-hydroxyvitamin D [25(OH)D] level has significantly inverse associations with various neurological and neurodegenerative diseases. However, little is known about 25(OH)D level in human cerebrospinal fluid (CSF). Thus, the aim of this study was to determine correlations of 25(OH)D level in CSF with serum total, bioavailable, and free 25(OH)D levels. This observational study enrolled a total of 117 subjects (58 patients with non-neurologic disease and 59 patients with neurologic disease) from 2017 to 2018. CSF and blood samples were collected in pairs. Total 25(OH)D levels in CSF and serum and vitamin D binding protein (VDBP) levels in serum were measured. We also performed GC genotyping for polymorphisms of rs4588 and rs7041 to calculate bioavailable and free 25(OH)D levels. CSF total 25(OH)D levels were compared with serum total, bioavailable, and free 25(OH)D levels. Mean total 25(OH)D concentrations in CSF and serum of all patients were 37.14 ± 7.71 and 25.72 ± 12.37 ng/mL, respectively. The mean total 25(OH)D concentration in CSF was generally 1.4-fold higher than that in the serum. Total 25(OH)D concentrations in CSF showed weakly positive but significant correlations with serum total, bioavailable, and free 25(OH)D concentrations (P = 0.022, P = 0.033, and P = 0.026, respectively). Serum total 25(OH)D concentration was also correlated with serum VDBP concentration (P = 0.017). However, total 25(OH)D levels in CSF of non-neurologic disease group and neurologic disease group were similar. Total 25(OH)D level in CSF has weakly positive but significant correlations with serum total, bioavailable, and free 25-hydroxy vitamin D levels in the Korean population. The distribution of CSF total 25(OH)D in Korean neurologic and non-neurologic disease patients was presented.

Association between Vitamin D Levels and Nonalcoholic Fatty Liver Disease: Potential Confounding Variables

Nonalcoholic fatty liver disease (NAFLD), historically considered to be the hepatic component of the metabolic syndrome, is a spectrum of fat-associated liver conditions, in the absence of secondary causes, that may progress to nonalcoholic steatohepatitis (NASH), fibrosis, and cirrhosis. Disease progression is closely associated with body weight or fatness, dyslipidemia, insulin resistance, oxidative stress, and inflammation. Recently, vitamin D deficiency has been linked to the pathogenesis and severity of NAFLD because of vitamin D "pleiotropic" functions, with roles in immune modulation, cell differentiation and proliferation, and regulation of inflammation. Indeed, several studies have reported an association between vitamin D and NAFLD/NASH. However, other studies have failed to find an association. Therefore, we sought to critically review the current evidence on the association between vitamin D deficiency and NAFLD/NASH, and to analyze and discuss some key variables that may interfere with this evaluation, such as host-, environment-, and heritability-related factors regulating vitamin D synthesis and metabolism; definitions of deficient or optimal vitamin D status with respect to skeletal and nonskeletal outcomes including NAFLD/NASH; methods of measuring 25(OH)D; and methods of diagnosing NAFLD as well as quantifying adiposity, the cardinal link between vitamin D deficiency and NAFLD.

Vitamin D binding protein polymorphisms significantly impact vitamin D status in children

BACKGROUND

Polymorphic alleles of the vitamin D (vitD)-binding protein (VDBP) gene are associated with discriminatory differences in circulating concentrations of 25-hydroxyvitamin D (25-D), the indicator of vitD status (sufficiency defined by the Endocrine Society as ≥75 nmol/L). Within a diverse group of children, we hypothesized that reaching recommended daily allowance (RDA) of vitD intake would have differential impact on vitD status depending on VDBP variability.

METHODS

VDBP alleles (Gc1S, Gc1F, Gc2) in 123 children (1-4 annual visits/child; ages 1-8 years) were compared for relationships with serum 25-D concentrations and daily vitD intake.

RESULTS

In African-American children, reaching the vitD RDA was associated with significantly higher mean serum 25-D concentrations for the 20% carrying the VDBP 1S allele than for the large majority without this allele (77 vs. 61 nmol/L 25-D; p = 0.038). Children with the Gc1S/1S homozygous genotype (30% Caucasians, 24% Hispanics, 2% African-Americans) who met RDA had 51% (39 nmol/L) greater mean serum 25-D than those below RDA (p < 0.0001).

CONCLUSIONS

VDBP genetic variability was a significant factor affecting childhood vitD status when following RDA guidelines. This study may inform public health policy of uniformity in recommended childhood vitD dosage, especially regarding racially/ethnically associated disparities.

Vitamin D Binding Protein and the Biological Activity of Vitamin D

Vitamin D has a long-established role in bone health. In the last two decades, there has been a dramatic resurgence in research interest in vitamin D due to studies that have shown its possible benefits for non-skeletal health. Underpinning the renewed interest in vitamin D was the identification of the vital role of intracrine or localized, tissue-specific, conversion of inactive pro-hormone 25-hydroxyvitamin D [25(OH)D] to active 1,25-dihydroxyvitamin D [1,25(OH)₂D]. This intracrine mechanism is the likely driving force behind vitamin D action resulting in positive effects on human health. To fully capture the effect of this localized, tissue-specific conversion to 1,25(OH)₂D, adequate 25(OH)D would be required. As such, low serum concentrations of 25(OH)D would compromise intracrine generation of 1,25(OH)₂D within target tissues. Consistent with this is the observation that all adverse human health consequences of vitamin D deficiency are associated with a low serum 25(OH)D level and not with low 1,25(OH)₂D concentrations. Thus, clinical investigators have sought to define what concentration of serum 25(OH)D constitutes adequate vitamin D status. However, since 25(OH)D is transported in serum bound primarily to vitamin D binding protein (DBP) and secondarily to albumin, is the total 25(OH)D (bound plus free) or the unbound free 25(OH)D the crucial determinant of the non-classical actions of vitamin D? While DBP-bound-25(OH)D is important for renal handling of 25(OH)D and endocrine synthesis of 1,25(OH)₂D, how does DBP impact extra-renal synthesis of 1,25(OH)₂D and subsequent 1,25(OH)₂D actions? Are their pathophysiological contexts where total 25(OH)D and free 25(OH)D would diverge in value as a marker of vitamin D status? This review aims to introduce and discuss the concept of free 25(OH)D, the molecular biology and biochemistry of vitamin D and DBP that provides the context for free 25(OH)D, and surveys in vitro, animal, and human studies taking free 25(OH)D into consideration.

Clinical Usefulness of Bioavailable Vitamin D and Impact of GC Genotyping on the Determination of Bioavailable Vitamin D in a Korean Population

BACKGROUND

Bioavailable 25-hydroxy vitamin D (25(OH)D) has been suggested for the accurate determination of vitamin D status. The purpose of this study was to determine the utility of bioavailable 25(OH)D in assessing vitamin D status when vitamin D-binding protein (VDBP) was significantly altered by pregnancy and liver cirrhosis (LC). The role of genotyping of GC, a gene encoding VDBP, in the determination of bioavailable 25(OH)D concentration in a Korean population was also evaluated.

METHODS

This prospective study enrolled a total of 136 subjects (53 healthy controls, 45 patients with LC, and 38 pregnant women) from 2017 to 2018. The concentrations of total 25(OH)D and VDBP were measured, and bioavailable 25(OH)D concentrations were calculated. GC genotyping was performed to determine rs4588 and rs7041 polymorphisms. Clinical and laboratory data were compared among the three groups of subjects.

RESULTS

Median VDBP and total 25(OH)D concentrations were 165.2 μg/ml and 18.5 ng/ml in healthy controls, 76.9 μg/ml and 10.5 ng/ml in patients with LC, and 368.9 μg/ml and 17.7 ng/ml in pregnant women, respectively. Compared with controls, patients diagnosed with LC had significantly lower VDBP and total 25(OH)D concentrations (all P < 0.001) while pregnant women had significantly higher VDBP concentrations (P < 0.001). Although total 25(OH)D concentrations in pregnant women were similar to those in controls (P = 0.394), their bioavailable 25(OH)D concentrations were significantly lower (1.2 vs. 3.0 ng/ml; P < 0.001). Among all the three groups combined, the genotype-specific bioavailable 25(OH)D and the genotype-independent bioavailable 25(OH)D concentrations did not differ significantly (P = 0.299).

CONCLUSIONS

Our study has demonstrated that bioavailable 25(OH)D concentration reflects vitamin D status more accurately than the total 25(OH)D concentration, especially in pregnant women. In addition, GC genotyping did not significantly affect bioavailable 25(OH)D concentration. Therefore, if VDBP concentration is significantly altered, the measurement of bioavailable 25(OH)D concentration might facilitate the accurate determination of vitamin D status. However, GC genotyping might be unnecessary.

Impact of genetic influence on serum total- and free 25-hydroxyvitamin-D in humans

Serum 25-hydroxyvitamin D /25OHD/ levels in humans are determined primarily by environmental factors such as UV-B radiation and diet, including vitamin D intake. Although some genetic determinants of 25OHD levels have been shown, the magnitude of this association has not yet been clarified. The present study evaluates the genetic contribution to total- /t-25OHD/ and free-25OHD /f-25OHD/ in a representative sample of the Hungarian population (n = 462). The study was performed at the end of winter to minimize the effect of sunlight, which is a major determinant of serum vitamin D levels. Single nucleotide polymorphisms (SNPs) of five genes playing major roles in vitamin D metabolism were investigated (NADSYN1, DHCR7, GC, CYP2R1 and CYP24A1). The selected SNPs account for 13.1% of the variance of t-25OHD levels. More than half of the genetic effect on t-25OHD levels was explained by two polymorphisms (rs7935125 in NADSYN1 and rs2762941 in CYP24A1), which had not previously been investigated with respect to vitamin D metabolism. No SNPs exhibited association with f-25OHD levels. Unexpectedly, SNPs that showed univariate associations with vitamin D binding protein (DBP) levels were not associated with f-25OHD levels questioning the biological significance of these polymorphisms. The present study shows that t-25OHD levels are significantly influenced by genetic factors, however, the clinical significance of this observation remains to be defined, as variation in f-25OHD levels are marginally explained by genetic effects.

Sunlight exposure is just one of the factors which influence vitamin D status

Studies on the determinants of vitamin D status have tended to concentrate on input - exposure to ultraviolet B radiation and the limited sources in food. Yet, vitamin D status, determined by circulating concentrations of 25-hydroxyvitamin D (25(OH)D), can vary quite markedly in groups of people with apparently similar inputs of vitamin D. There are small effects of polymorphisms in the genes for key proteins involved in vitamin D production and metabolism, including 7-dehydrocholesterol reductase, which converts 7-dehydrocholesterol, the precursor of vitamin D, to cholesterol, CYP2R1, the main 25-hydroxylase of vitamin D, GC, coding for the vitamin D binding protein which transports 25(OH)D and other metabolites in blood and CYP24A1, which 24-hydroxylates both 25(OH)D and the hormone, 1,25-dihydroxyvitamin D. 25(OH)D has a highly variable half-life in blood. There is evidence that the half-life of 25(OH)D is affected by calcium intake and some therapeutic agents. Fat tissue seems to serve as a sink for the parent vitamin D, which is released mainly when there are reductions in adiposity. Some evidence is presented to support the proposal that skeletal muscle provides a substantial site of sequestration of 25(OH)D, protecting this metabolite from degradation by the liver, which may help to explain why exercise, not just outdoors, is usually associated with better vitamin D status.

Vitamin D and critical illness outcomes

PURPOSE OF REVIEW

Although low vitamin D levels have been shown to be a risk factor for adverse outcomes in critical care, it is not clear to date if supplementation can alter such outcomes in all ICU patients. The focus of vitamin D research now is on interventional trials to identify a critically ill patient subset who may benefit from high-dose vitamin D supplementation.

RECENT FINDINGS

The VITdAL-ICU trial, a randomized, double-blind, placebo-controlled, single center trial of 475 heterogeneous critically ill patients, did not show improvement in hospital length of stay or overall mortality but did demonstrate in a secondary outcome that high-dose oral vitamin D3 improved mortality in patients with severe vitamin D deficiency.

SUMMARY

Vitamin D supplementation may represent a personalized and targeted therapy for critical illness. Vitamin D regulates over 1000 genes in the human genome, and the mechanism of action is influenced by gene polymorphisms and epigenetics. The study of the metabolomics, transcriptomics and epigenetics of vitamin D status and supplementation holds promise generating insights into critical illness outcomes.

Association of vitamin D deficiency and VDBP gene polymorphism with the risk of AMI in a Pakistani population

Objective

To investigate the relationship of vitamin D deficiency and risk of AMI in a Pakistani population, and to find out any association between vitamin D binding protein (VDBP) genotypes and risk of AMI in this population.

Methods

In a comparative cross-sectional study, 246 patients (age: 20-70 years; 171 males and 75 females) with first AMI were enrolled with informed consent. Similarly, 345 healthy adults (230 males and 115 females) were enrolled as controls. Their fasting serum samples were analyzed for 25 (OH) vitamin D, lipids and other biomarkers using kit methods, while DNA was analyzed for VDBP genotypes using PCR-RFLP based methods. Chi-squared test and logistic regression were used for association of vitamin D deficiency and VDBP genotypes with AMI.

Results

Mean serum concentration of 25(OH) vitamin D was significantly lower in AMI patients compared to healthy subjects (p=0.015) and percent vitamin D deficiency was higher in AMI patients compared to healthy subjects (p=0.003). VDBP IF-IF genotype was positively associated with the risk of AMI in subject above 45 years after adjusting for potential confounders [OR = 9.86; 95% CI=1.16 to 83.43].

Conclusion

Vitamin D deficiency and VDBP IF-IF genotype are associated with AMI in Pakistani adults.

Vitamin D-Binding Protein Polymorphisms, 25-Hydroxyvitamin D, Sunshine and Multiple Sclerosis

Blacks have different dominant polymorphisms in the vitamin D-binding protein (DBP) gene that result in higher bioavailable vitamin D than whites. This study tested whether the lack of association between 25-hydroxyvitamin D (25OHD) and multiple sclerosis (MS) risk in blacks and Hispanics is due to differences in these common polymorphisms (rs7041, rs4588). We recruited incident MS cases and controls (blacks 116 cases/131 controls; Hispanics 183/197; whites 247/267) from Kaiser Permanente Southern California. AA is the dominant rs7041 genotype in blacks (70.0%) whereas C is the dominant allele in whites (79.0% AC/CC) and Hispanics (77.1%). Higher 25OHD levels were associated with a lower risk of MS in whites who carried at least one copy of the C allele but not AA carriers. No association was found in Hispanics or blacks regardless of genotype. Higher ultraviolet radiation exposure was associated with a lower risk of MS in blacks (OR = 0.06), Hispanics and whites who carried at least one copy of the C allele but not in others. Racial/ethnic variations in bioavailable vitamin D do not explain the lack of association between 25OHD and MS in blacks and Hispanics. These findings further challenge the biological plausibility of vitamin D deficiency as causal for MS.

Vitamin D binding protein rs7041 genotype alters vitamin D metabolism in pregnant women

Research has identified reduced circulating 25-hydroxyvitamin D [25(OH)D] in individuals with the rs7041 (c.1296T>G) T allele in the vitamin D binding protein gene ( GC); however, the effects of the T allele on vitamin D biomarkers during pregnancy and lactation are unknown. Thus, we examined the metabolic effects of GC rs7041 on vitamin D biomarkers among third-trimester pregnant ( n = 26), lactating ( n = 28), and nonpregnant/nonlactating ( n = 21) women consuming a single amount of vitamin D (511 IU/d) and related nutrients for 10-12 wk. T allele carriers had less circulating 25(OH)D, regardless of reproductive state [thymine-thymine (TT): 80% of guanine-guanine (GG), P = 0.05; guanine-thymine (GT): 85% of GG, P = 0.1]. Among pregnant women, the T allele attenuated the expected increase in vitamin D binding protein (DBP). Specifically, although GG pregnant women exhibited greater DBP (216%, P < 0.0001) than did GG nonpregnant women, that difference was lessened among GT women, and TT pregnant women did not exhibit greater DBP than TT nonpregnant women. Furthermore, TT pregnant women had greater placental 25(OH)D₃ to 24,25-dihydroxyvitamin D ratios (251% of GG, P = 0.07) and less osteocalcin, a bone formation marker, in the cord blood of their neonates (24% of GT, P = 0.02). Overall, the GC rs7041 genotype modified the effects of pregnancy on maternal and placental vitamin D metabolism, with possible functional consequences for fetal bone development and infant health.-Ganz, A. B., Park, H., Malysheva, O. V., Caudill, M. A. Vitamin D binding protein rs7041 genotype alters vitamin D metabolism in pregnant women.

Vitamin D Binding Protein and Vitamin D Levels in Multi-Ethnic Population

INTRODUCTION

Low levels of 25-hydroxyvitamin D (25(OH)D) has been associated with many negative health outcomes including falls and fractures. 25(OH)D is largely bound to vitamin D binding protein (VDBP). There is increasing evidence that free or bioavailable 25(OH)D may be a better measure of vitamin D deficiency.

OBJECTIVE

To determine the prevalence of 25(OH)D deficiency and VDBP levels in multi-ethnic population, and its impact on muscle strength.

DESIGN AND METHODS

Cross-sectional study of older adults in Western region of Singapore. 295 participants from three ethnic groups were selected from the Healthy Older People Everyday (HOPE) cohort for measurements of total 25(OH)D and VDBP levels. Total 25(OH)D, VDBP, frailty status, Timed-Up-and-Go (TUG) and grip strength (GS) were assessed. Albumin, free and bioavailable 25(OH)D were only available for 256 participants.

RESULTS

53% of Malay and 55% of Indians were deficient in 25(OH)D compared with 18.2% of ethnic Chinese participants. Chinese also had higher total 25(OH)D concentrations with a mean of 29.1 ug/l, (p = <0.001). Chinese had the lowest level of VDBP (169.6ug/ml) followed by Malay (188.8 ug/ml) and Indian having the highest (220.1 ug/ml). Calculated bioavailable and free 25(OH)D levels were significantly higher in Chinese, followed by Malays and Indians, which also correlated with better grip strength measures amongst the Chinese.

CONCLUSION

The Malays and Indians had overall lower free, bioavailable and total 25(OH)D compared with ethnic Chinese. Chinese ethnic group also had the lowest VDBP and better overall grip strength.

Genetic and Racial Differences in the Vitamin D Endocrine System

Twin studies indicate that genetic factors may explain about 50% of the variation of serum 25-hydroxyvitamin D (25OHD). Polymorphisms of 3 genes, delta-7-sterol-reductase, CYP2R1, and DBP/GC (and maybe CYP24A1) combined, can explain about 5% to 10% of the variation in serum 25OHD. These polymorphisms are found in nearly all populations. The variation in serum 25OHD found in different areas and populations in the world is mainly due to environmental and lifestyle factors, not truly dependent on racial differences. One genetic variant of DBP, (GC2), is associated with a modest (∼10%) decrease in serum DBP and 25OHD concentrations for unexplained reasons.

Vitamin D in adolescents: Are current recommendations enough?

Vitamin D is essential for bone development during adolescence and low vitamin D status during this critical period of growth may impact bone mineralization, potentially reducing peak bone mass and consequently increasing the risk of osteoporosis in adulthood. Therefore, the high prevalence of vitamin D inadequacy and deficiency in adolescent populations is of great concern. However, there is currently a lack of consensus on the 25-hydroxyvitamin D [25(OH)D] concentration, the widely accepted biomarker of vitamin D status, that defines adequacy, and the vitamin D intake requirements to maintain various 25(OH)D thresholds are not well established. While the current intake recommendations of 10-15μg/day may be sufficient to prevent vitamin D deficiency (25(OH)D<25-30nmol/l), greater intakes may be needed to achieve the higher threshold levels proposed to represent adequacy (25(OH)D>50nmol/l). This review will address these concerns and consider if the current dietary recommendations for vitamin D in adolescents are sufficient.

Diurnal rhythms of vitamin D binding protein and total and free vitamin D metabolites

Vitamin D binding protein (DBP) concentration is known to influence the availability and bioactivity of vitamin D metabolites but its diurnal rhythm (DR), its inter-relationships with the DRs of vitamin D metabolites and its influence on free vitamin D metabolite concentrations are not well described. The DRs of plasma total 25(OH)D, total 1,25(OH)2D, DBP, albumin and calculated free 25(OH)D and free 1,25(OH)2D were measured in men and women aged 60-75 years and resident in the UK (n 30), Gambia (n 31) and China (n 30) with differences in lifestyle, dietary intake and vitamin D status. Blood samples were collected every 4h for 24h and DRs statistically analysed with Fourier regression. Gambians had significantly higher plasma concentrations of vitamin D metabolites and lower albumin concentration compared to the British and Chinese. Significant DRs were observed for all analytes and calculated free vitamin D metabolites (P<0.01). The pattern of DRs was similar between countries. The magnitude of the DRs of free 1,25(OH)2D was attenuated compared to that of total 1,25(OH)2D whereas it was not different between total and free 25(OH)D. Relationships between the DRs were generally weak. There was no phase shift between 1,25(OH)2D and DBP with the strongest cross correlation at 0h time lag (r=0.15, P=<0.001). In comparison, 25(OH)D correlated less well with DBP (1h time lag, r=0.07, P=0.12). These data demonstrate a relationship between the DRs of 1,25(OH)2D and DBP, possibly to maintain free 1,25(OH)2D concentrations. In contrast, the DRs of total and free 25(OH)D appeared to be less influenced by DBP, suggesting that DBP has comparatively less effect on 25(OH)D concentration and 25(OH)D availability. This work highlights the importance of standardisation in timing of sample collection particularly for the assessment of plasma protein concentrations.

Associations of Vitamin D-Binding Globulin and Bioavailable Vitamin D Concentrations With Coronary Heart Disease Events: The Multi-Ethnic Study of Atherosclerosis (MESA)

CONTEXT

Low 25-hydroxyvitamin D [25(OH)D] is associated with coronary heart disease (CHD) in people who are white and Chinese but not black or Hispanic. Vitamin D binding globulin (VDBG) avidly binds 25(OH)D, reducing its bioavailability, and differs in isoform and concentration by race.

OBJECTIVE

Evaluate associations of VDBG with CHD and whether accounting for VDBG or estimating bioavailable 25(OH)D explains the heterogeneity of the association of 25(OH)D with CHD.

DESIGN AND SETTING

We conducted a case-cohort study within the Multi-Ethnic Study of Atherosclerosis. Participants with an incident CHD event over 12 years of follow-up (n = 538) and a randomly assigned subcohort (n = 999) were included. We measured baseline 25(OH)D, VDBG, and isoforms using mass spectrometry and estimated bioavailable 25(OH)D from published equations.

RESULTS

VDBG was associated with an increased risk of CHD [hazard ratio, 1.77 (95% confidence interval, 1.46 to 2.14) per standard deviation increment, P < 0.0001], without evidence of heterogeneity by race or isoform (each P for interaction > 0.1). Low total 25(OH)D was differentially associated with CHD events, by race, with or without adjustment for VDBG (P for interaction = 0.04 or 0.05, respectively). Associations of 25(OH)D with CHD were strengthened with adjustment for VDBG among participants who were white or Chinese, and bioavailable 25(OH)D was associated with CHD events only among white participants.

CONCLUSIONS

High VDBG concentration was associated with CHD events in all racial and ethnic groups. Incorporation of VDBG strengthened existing associations of 25(OH)D with CHD but did not explain racial heterogeneity in associations of 25(OH)D with CHD.

Clinical Utility of Measurement of Vitamin D-Binding Protein and Calculation of Bioavailable Vitamin D in Assessment of Vitamin D Status

Background

The associations of vitamin D deficiency with various clinical conditions highlighted the importance of vitamin D testing. Currently, clinicians measure only the total 25-hydroxyvitamin D [25(OH)D] concentration, regardless of its bioavailability. We aimed to determine the effect of vitamin D-binding protein (VDBP) on 25(OH)D bioavailability.

Methods

Serum samples were collected from 60 healthy controls, 50 pregnant women, and 50 patients in intensive care units (ICUs). Total 25(OH)D was quantified by liquid chromatography with tandem mass spectrometry, and VDBP levels were determined by using an ELISA kit (R&D Systems, USA). The bioavailable 25(OH)D levels were calculated by using total 25(OH)D, VDBP, and albumin concentrations.

Results

In comparison with healthy controls, the total 25(OH)D concentration was significantly lower in ICU patients (median, 11.65 vs 18.25 ng/mL; P<0.00001), but no significant difference was noted between pregnant women (18.25 ng/mL) and healthy controls. The VDBP level was significantly lower in ICU patients (95.58 vs 167.18 µg/mL, P=0.0002) and higher in pregnant women (225.01 vs 167.18 µg/mL, P=0.008) compared with healthy controls. Nonetheless, the calculated bioavailable 25(OH)D levels of ICU patients and pregnant women were significantly lower than those of healthy controls (1.97 and 1.93 ng/mL vs 2.56 ng/mL; P=0.0073 and 0.0027).

Conclusions

A single marker of the total 25(OH)D level is not sufficient to accurately evaluate vitamin D status, especially in pregnant women. In cases where VDBP concentrations may be altered, VDBP measurements and bioavailable 25(OH)D calculations may help to determine vitamin D status accurately.