A comparison of measured and calculated free 25(OH) vitamin D levels in clinical populations

Development and Validation of Novel Free Vitamin D Equations: The Health Aging and Body Composition Study

Vitamin D deficiency is prevalent in 25% of Americans. However, 25(OH)D may not be an accurate measure of vitamin D because the majority (85%-90%) of 25(OH)D is bound to vitamin D binding protein (VDBP), which varies by over 30% across individuals. Free 25(OH)D may be a better measure, but it is difficult to measure accurately and precisely. The existing free 25(OH)D estimating equation does not include VDBP phenotypes; therefore, new equations that include this variable may be more accurate. A total of 370 participants in the Health, Aging, and Body Composition Study, a cohort of healthy community-dwelling individuals aged 70-79 years old, underwent VDBP and vitamin D metabolite [25(OH)D, 24,25(OH)2D, 1,25(OH)2D, free 25(OH)D] measurements and were randomly allocated into equation development (two out of three) and internal validation (one out of three) groups. New equations were developed with multiple linear regression and were internally validated with Bland-Altman plots. The mean age was 75 ± 3 years, 53% were female, and the mean measured free 25(OH)D was 5.37 ± 1.81 pg/mL. Three equations were developed. The first equation included albumin, 25(OH)D3, 25(OH)D2, VDBP, 1,25(OH)2D3, and 24,25(OH)2D3. The second equation included all variables in Eq. (1) plus VDBP phenotypes. The third equation included albumin, 25(OH)D3, intact parathyroid hormone, and 1,25(OH)2D3. In internal validation, all three new equations predicted free 25(OH)D values within 30% and 15% of the measured free 25(OH)D concentrations in 76%-80% and 48%-52% of study participants, respectively. Equation (2) was the most precise, with a mean bias of 0.06 (95% limits of agreement -2.41 to 2.30) pg/mL. The existing equation estimated free 25(OH)D within 30% and 15% of measured free 25(OH)D in 43% and 22% of participants, respectively. Free 25(OH)D can be estimated with clinically available biomarkers as well as with more laboratory-intensive biomarkers with moderate precision. © 2023 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

Association of circulating serum free bioavailable and total vitamin D with cathelicidin levels among active TB patients and household contacts

The free hormone hypothesis postulates that the estimation of free circulating 25 (OH)D may be a better marker of vitamin D status and is of clinical importance compared to total vitamin D fraction. The unbound fraction is involved in biological activities since it is able to penetrate into the cell. Studies have shown that cathelicidin/LL-37 inhibits the growth of Mycobacterium tuberculosis in a vitamin D-dependent manner and therefore adequate vitamin D is required for its expression. The study aimed to determine the association between serum bioavailable and total vitamin D with LL-37 levels in ATB patients, LTBI, and individuals with no TB infection. This was a cross-sectional study in which bioavailable vitamin D and LL-37 levels were measured using competitive ELISA kits and total vitamin D was measured using electrochemilumiscence and consequently determined their association. The mean (SD) bioavailable vitamin D levels of the study participants were 3.8 ng/mL (2.6) and the median (IQR) of LL-37 levels were 320 ng/mL (160, 550 ng/mL). The mean (SD) of total vitamin D levels was 19.0 ng/mL (8.3) ng/mL. Similar weak correlations were observed between the bioavailable and total vitamin D with LL-37 levels, therefore, deviating from our hypothesis.

The clinical relevance of native vitamin D in pediatric kidney disease

Hypovitaminosis D has been reported to be common in chronic kidney disease (CKD) as well as in proteinuric disorders. We reviewed available evidence to assess clinically relevant effects of low vitamin D status and native vitamin D (NVD) therapy, in pediatric renal diseases. Online medical databases were searched for articles related to vitamin D status, associations of hypovitaminosis D and effects of NVD therapy in kidney disease. Hypovitaminosis D was associated with worse skeletal, cardiovascular, inflammatory, and renal survival outcomes in CKD. Low serum 25 hydroxy-vitamin D (25[OH]D) levels correlated positively with glomerular filtration rate and negatively with serum parathyroid (PTH) levels. However, to date, evidence of benefit of NVD supplementation is restricted mainly to improvements in serum PTH, and biochemical 25[OH]D targets form the basis of clinical practice recommendations for NVD therapy. In nephrotic syndrome (NS) relapse, studies indicate loss of 25[OH]D along with vitamin D binding protein in urine, and serum total 25[OH]D levels are low. Preliminary evidence indicates that free 25[OH]D may be a better guide to the biologically active fraction. NVD therapy in NS does not show consistent results in improving skeletal outcomes and hypercalciuria has been reported when total 25[OH]D levels were considered as indication for therapy. NVD formulations should be regularised, and therapy monitored adequately to avoid adverse effects.

Serum 25-Hydroxyvitamin D Levels and Youth-Onset Type 2 Diabetes: A Two-Sample Mendelian Randomization Study

Observational studies have linked vitamin D insufficiency to pediatric type 2 diabetes (T2D), but evidence from vitamin D supplementation trials is sparse. Given the rising prevalence of pediatric T2D in all ethnicities, determining the protective role of vitamin D has significant public health importance. We tested whether serum 25-hydroxyvitamin D (25OHD) levels are causally linked to youth-onset T2D risk using Mendelian randomization (MR). We selected 54 single-nucleotide polymorphisms (SNPs) associated with 25OHD in a European genome-wide association study (GWAS) on 443,734 individuals and obtained their effects on pediatric T2D from the multi-ethnic PRODIGY GWAS (3006 cases/6061 controls). We applied inverse variance weighted (IVW) MR and a series of MR methods to control for pleiotropy. We undertook sensitivity analyses in ethnic sub-cohorts of PRODIGY, using SNPs in core vitamin D genes or ancestry-informed 25OHD SNPs. Multivariable MR accounted for the mediating effects of body mass index. We found that a standard deviation increase in 25OHD in the logarithmic scale did not affect youth-onset T2D risk (IVW MR odds ratio (OR) = 1.04, 95% CI = 0.96-1.13, p = 0.35) in the multi-ethnic analysis, and sensitivity, ancestry-specific and multivariable MR analyses showed consistent results. Our study had limited power to detect small/moderate effects of 25OHD (OR of pediatric T2D < 1.39 to 2.1). In conclusion, 25OHD levels are unlikely to have significant effects on the risk of youth-onset T2D across different ethnicities.

Correlation of Vitamin D Level and Bone Mineral Density in Epilepsy Children Who Received Oral Antiepileptic Drug

BACKGROUND: Vitamin D plays an essential role in achieving adequate bone mineralization. Antiepileptic drug (AED) will cause a change in calcium serum levels and bone mineral density (BMD) through increase catabolism of Vitamin D in the liver, as well as having a direct effect on the bone. AIM: The aimed of this study was to know the correlation of Vitamin D [25(OH)D] level and BMD in epilepsy children who received oral AED. METHODS: This cross-sectional study was conducted from May to September 2016. Subjects were collected using consecutive sampling in 1−11-year-old epilepsy children who went to the pediatric Neurology and Endocrinology outpatient clinic at Sanglah Hospital. Age, sex, body weight, body height, type and number of AED used, and duration of treatment were recorded. Measurement of 25(OH)D level and BMD was performed. Pearson test was used to analyze the strength of correlation based on normality test result. RESULTS: Thirty subjects were collected, male (19/63.33%), mean age was 7.22 years, mean treatment duration was 1.71 years. Type of AED was inducer AED (18/60%) and non-inducer AED (12/40%). Mean Vitamin D level was 27.19 ng/mL and mean BMD was 0.78 g/cm2. Partial correlation test of Vitamin D level with BMD after controlling age found r = 0.118 with p = 0.54. CONCLUSION: There was weak correlation between Vitamin D level and BMD in epilepsy children who received oral AED, but the correlation analysis was not sufficient to prove the relationship.

Assessment of Vitamin D Status in Slovenian Premenopausal and Postmenopausal Women, Using Total, Free, and Bioavailable 25-Hydroxyvitamin D (25(OH)D)

The objective of our study was to evaluate vitamin D status and its predictors in Slovenian premenopausal and postmenopausal women. A cross-sectional study was carried out between 1 March 2021 and 31 May 2021. A total of 319 healthy women from the Central Slovenian region aged between 44 and 65 were recruited; 176 were included in the final analysis. The vitamin D status was determined by measuring the total 25-Hydroxycholecalciferol (25(OH)D) concentration, vitamin D binding protein (DBP), and albumin and calculating the bioavailable 25(OH)D and free 25(OH)D. For the calculation of bioavailable and free 25(OH)D, we developed a new online calculator. The Endocrine Society’s thresholds for vitamin D deficiency and insufficiency were used; 29.0% of premenopausal and 24.4% of postmenopausal subjects were found to be vitamin D deficient (total 25(OH)D < 50 nmol/L); 76.8% of the premenopausal and 61.7% of postmenopausal subjects were found to have insufficient levels (total 25(OH)D < 75 nmol/L). Premenopausal women had 11.8% lower total 25(OH)D, 32.2% lower bioavailable 25(OH)D, and 25.2% higher DBP than postmenopausal women. The most important predictors of vitamin D status were vitamin D supplementation and time spent in the sun. Contrary to similar studies, the vitamin D status in Slovenian postmenopausal women was significantly better than in premenopausal women. In postmenopausal women, the measurement of free or bioavailable 25(OH)D instead of the total 25(OH)D could be advantageous.

Analytical, experimental and clinical aspects of total 25-hydroxyvitamin D measurement by two ligand assays are differentially biased by disease-related factors

BACKGROUND

Pathological concentrations of plasma proteins may confound the results of binding assays. We compared two routinely used total 25-hydroxyvitamin D [t-25(OH)D] methods: a chemiluminescence-immuno-(CLIA) and an electro-chemiluminescence-protein-binding-(ECLPBA) assay.

METHODS

Two sub-studies were performed: 1) In an "in vitro" study, exogenous albumin was added to pools of patients' sera with low albumin levels; and 2) In "ex vivo" studies of Cohort_1: sera of hospitalized patients with low albumin levels, and of healthy controls; and of Cohort_2: outpatients with chronic kidney disease in pre-dialysis stage, or on peritoneal dialysis and hemodialysis were investigated by the routine and LC_MS/MS methods.

RESULTS

When increasing albumin concentrations were "in vitro" added, t-25(OH)D levels were overestimated by ECLPBA, and underestimated by CLIA. In patients' sera, positive correlations were detected between t-25(OH)D-vitamin D binding protein (DBP) values by both routine methods, and between t-25(OH)D-albumin values by all three methods. Much higher t-25(OH)D was measured by LC_MS/MS in all subgroups. When altering albumin levels with constant DBP concentration, the "in vitro" experiment revealed a higher sensitivity of ECLPBA. The "ex vivo" measurements demonstrated clinically relevant differences between the routine methods.

CONCLUSION

Both routine methods are dependent of the matrix effect in hospitalized patients, which is predicted by the DBP/Albumin ratios. In hemodialysis, ECLPBA is recommended because its outcomes differ less from those of LC_MS/MS. The results of LC_MS/MS are reliable, but not routinely available. A guidance would be valuable on how levels measured by the binding methods differ from those by LC-MS/MS in various clinical conditions.

100 YEARS OF VITAMIN D: Combined hormonal contraceptives and vitamin D metabolism in adolescent girls

OBJECTIVE

Combined hormonal contraceptive (CHC) use has been associated with higher total 25-hydroxyvitamin D (25(OH)D) levels. Here, we investigate the relation between CHC use and vitamin D metabolism to elucidate its clinical interpretation.

METHODS

The cross-sectional Fit Futures 1 included 1038 adolescents. Here, a subgroup of 182 girls with available 25(OH)D, 1,25-dihydroxyvitamin D (1,25(OH)2D), 24,25-dihydroxyvitamin D (24,25(OH)2D), vitamin D-binding protein (DBP) and measured free 25(OH)D levels, in addition to parathyroid hormone (PTH) and fibroblast growth factor 23 (FGF23), was investigated. Vitamin D metabolites were compared between girls using (CHC+) and not using CHC (CHC-). Further, the predictability of CHC on 25(OH)D levels was assessed in a multiple regression model including lifestyle factors. The ratios 1,25(OH)2D/25(OH)D and 24,25(OH)2D/25(OH)D (vitamin D metabolite ratio (VMR)) in relation to 25(OH)D were presented in scatterplots.

RESULTS

CHC+ (n = 64; 35% of the girls) had higher 25(OH)D levels (mean ± s.d., 60.3 ± 22.2) nmol/L) than CHC- (n = 118; 41.8 ± 19.3 nmol/L), P -values <0.01. The differences in 25(OH)D levels between CHC+ and CHC- were attenuated but remained significant after the adjustment of lifestyle factors. CHC+ also had higher levels of 1,25(OH)2D, 24,25(OH)2D, DBP and calcium than CHC-, whereas 1,25(OH)2D/25(OH)D, PTH, FGF23 and albumin were significantly lower. Free 25(OH)D and VMR did not statistically differ, and both ratios appeared similar in relation to 25(OH)D, irrespective of CHC status.

CONCLUSION

This confirms a clinical impact of CHC on vitamin D levels in adolescents. Our observations are likely due to an increased DBP-concentration, whereas the free 25(OH)D appears unaltered.

Vitamin D-Binding Protein and the Free Hormone Hypothesis for Vitamin D in Bio-Naïve Patients with Psoriasis

High levels of vitamin D-binding protein (DBP) have been reported in patients with psoriasis and the possibility of DBP as a marker of inflammation has been discussed. Furthermore, high DBP levels might negatively affect free 25(OH)D concentrations. According to the free hormone hypothesis, only the free fraction of a steroid hormone is capable of exerting biological action. Thus, free 25(OH)D level could be a better biomarker of vitamin D status than total 25(OH)D level. The objectives of this study were to identify the strongest determinants for DBP levels and to test the free hormone hypothesis for vitamin D in psoriasis. Additionally, we also aimed to investigate correlations between directly measured free 25(OH)D levels in serum and psoriasis disease severity compared to total 25(OH)D levels. This was a retrospective cross-sectional study including 40 bio-naïve patients with mild to severe plaque psoriasis. Psoriasis disease severity was evaluated using high sensitivity C-reactive protein (hsCRP), Psoriasis Area Severity Index (PASI) and visual analogue scale (VAS). Vitamin D metabolites including directly measured free 25(OH)D and serum DBP levels were measured. DBP levels were higher in patients with self-reported arthropathy than those without irrespective of confounding factors like sex, age and body weight. Total and free 25(OH)D levels correlated well (ρ = 0.77, p < 0.0001) and both were inversely correlated to intact parathyroid hormone (iPTH) (ρ = −0.33, p = 0.038 for total 25(OH)D and ρ = −0.40, p = 0.010 for free 25(OH)D). Only total 25(OH)D correlated to serum calcium levels (ρ = 0.32, p = 0.047). No correlations between any of the vitamin D metabolites and psoriasis disease severity were observed. In conclusion, DBP might be a new inflammatory biomarker in psoriasis, especially in psoriatic arthritis. Total 25(OH)D was a reliable measure for vitamin D status in this psoriasis cohort. However, evaluation of free 25(OH)D in patients with psoriatic disease and multiple co-morbidities and/or ongoing biologic treatment should be considered.

Haplotypes in the GC, CYP2R1 and CYP24A1 Genes and Biomarkers of Bone Mineral Metabolism in Older Adults

Candidate gene studies have analyzed the effect of specific vitamin D pathway genes on vitamin D availability; however, it is not clear whether genetic variants also affect overall bone metabolism. This study evaluated the association between genetic polymorphisms in GC, CYP2R1 and CYP24A1 and serum levels of total 25(OH)D, iPTH and other mineral metabolism biomarkers (albumin, total calcium and phosphorus) in a sample of 273 older Spanish adults. We observed a significant difference between CYP2R1 rs10741657 codominant model and total 25(OH)D levels after adjusting them by gender (p = 0.024). In addition, the two SNPs in the GC gene (rs4588 and rs2282679) were identified significantly associated with iPTH and creatinine serum levels. In the case of phosphorus, we observed an association with GC SNPs in dominant model. We found a relationship between haplotype 2 and 25(OH)D levels, haplotype 4 and iPTH serum levels and haplotype 7 and phosphorus levels. In conclusion, genetic variants in CYP2R1 and GC could be predictive of 25(OH)D and iPTH serum levels, respectively, in older Caucasian adults. The current study confirmed the role of iPTH as one of the most sensitive biomarkers of vitamin D activity in vivo.

Directly measured free and total 25-hydroxyvitamin D levels in relation to metabolic health in multi-ethnic postmenopausal females in Saudi Arabia

BACKGROUND

Measurement of free 25-hydroyvitamin D (25(OH)D) status has been suggested as a more representative marker of vitamin D status than that of total 25(OH)D. Previously, free 25(OH)D could only be calculated indirectly; however, a newly developed direct assay for the measurement of free 25(OH)D is now available. The aim of this study therefore was to investigate directly measured total and free vitamin D levels association with metabolic health in postmenopausal healthy women living in Saudi Arabia.

METHODS

A sample of 302 postmenopausal women aged ≥50 years (n = 302) living in Saudi Arabia were recruited in a cross-sectional study design. Blood samples were collected from subjects for measurement of serum levels of total 25(OH)D, directly measured free 25(OH)D, metabolic bone parameters, lipid profile, and other biochemical tests.

RESULTS

A positive correlation was found between directly measured free and total 25(OH)D (r = 0.64, P< 0.0001). Total but not free 25(OH)D showed significant association with serum intact parathyroid hormone (P = 0.004), whilst free 25(OH)D but not total 25(OH)D showed a significant association with total cholesterol and LDL-C (P = 0.032 and P = 0.045, respectively).

CONCLUSIONS

Free 25(OH)D and total 25(OH)D were found to be consistently correlated but with different associations to metabolic health parameters. Further research is needed to determine which marker of vitamin D status would be the most appropriate in population studies.

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.

Different threshold levels of circulating total and free 25-hydroxyvitamin D for the diagnosis of vitamin D deficiency in obese adolescents

The total serum 25-hydroxyvitamin D [25(OH)D_T] level is lower in obese individuals than in their nonobese peers, despite similar bone turnover markers and bone mineral density. This study aimed to investigate whether the threshold level of 25(OH)D for the diagnosis of vitamin D deficiency (VDD) in obese adolescents was lower than that in controls and to compare 25(OH)D_T, free [25(OH)D_F] and bioavailable [25(OH)D_B] vitamin D with VDBP levels in obese individuals and their controls. A total of 173 adolescents (90 obese individuals and 83 controls) aged 12-18 years were included in the study. The metabolic and anthropometric parameters of the participants were recorded, the 25(OH)D_T, 25(OH)D_F, and VDBP levels were measured, and the 25(OH)D_B levels were calculated. The cutoff values for VDD were estimated according to the level of 25(OH)D below which parathyroid hormone begins to rise. The obese subjects had lower 25(OH)D_T (12.1 ± 5.8 vs. 16.4 ± 9.3 ng/mL, p < 0.001), 25(OH)D_F (12.6 ± 4.2 vs. 16.7 ± 7.6 pg/mL, p < 0.001), 25(OH)D_B [4.8 (2.3) vs. 6.1 (5.2) ng/mL, p = 0.012], and VDBP [112.2 (51.3) vs. 121.9 (95.5) μg/mL, p < 0.001] levels than the controls. The cutoff values for 25(OH)D_T and 25(OH)D_F levels for VDD were lower in the obese group than in the control group (9.4 vs. 14.1 ng/mL; 12.2 vs. 16.8 pg/mL, respectively).Conclusion: The vitamin D cutoff values for the diagnosis of VDD were different in the obese and control groups. Using the same cutoff value for VDD may cause overtreatment in obese adolescents. What is Known: • Vitamin D deficiency is more prevalent in obese children than nonobese controls, despite the same bone turnover markers and bone mineral density • The cutoff value of vitamin D level for the diagnosis of VDD is based on the PTH elevation What is New: • In obese adolescents, total and free vitamin D cutoff value for the diagnosis of VDD was lower than nonobese peers • Using the same cutoff value for vitamin D deficiency in both obese and nonobese adolescents may cause overtreatment.

High levels of serum vitamin D-binding protein in patients with psoriasis: A case-control study and effects of ultraviolet B phototherapy

The role of vitamin D in psoriasis remains contradictory despite the fact that vitamin D analogues constitute an established treatment for psoriasis. It has been proposed that the ability of vitamin D to exert anti-inflammatory effects might not depend solely on the concentration of serum 25(OH)D but also on the concentration of vitamin D-binding protein (DBP). High concentrations of DBP might diminish vitamin D's biologic action. The aims of this study were (i) to analyze the serum levels of DBP, total and calculated free 25(OH)D in patients with psoriasis and compare the results with healthy controls and (ii) to study the effect of ultraviolet B (UVB) phototherapy on DBP levels. Caucasian subjects (n = 68) with active plaque psoriasis were compared with a population-based sample of men and women (n = 105), matched for age and sex. Season of enrollment was taken into consideration. The patients were also studied before and after UVB phototherapy. The severity of the disease was calculated as Psoriasis Area Severity Index (PASI). DBP, free 25(OH)D index and total 25(OH)D were higher in patients with psoriasis compared with controls (P= 0.004, P = 0.045 and P < 0.0001, respectively). DBP did not change after phototherapy, whereas 25(OH)D increased and intact parathyroid hormone (iPTH) decreased (P < 0.001 for both). Psoriasis improved and PASI decreased after phototherapy (P < 0.001). There was no correlation between DBP and 25(OH)D or between DBP and PASI. Measurement of DBP is recommended when evaluating vitamin D status in patients with psoriasis. High DBP levels in psoriasis imply a disturbed vitamin D pathway that warrants further investigation. Direct measurement of free 25(OH)D, instead of total 25(OH)D that circumvents abnormally high levels of DBP, could be considered.

total and free vitamin D in type 2 diabetes mellitus patients in Baghdad city

Background: The free-form of vitamin D has been used by many researchers as an index of vitamin D status in health and disease. Several methods are there to estimate free, total, and even bioavailable vitamin D. Objective: The present work was carried out to measure free vitamin D using a special formula suggested by Bikle and Schwartz in 2019, which includes the vitamin D binding protein (VDBP). The results will be used to evaluate the vitamin D status in patients with type 2 diabetes mellitus (T2DM), and its relation to the disease progression. Methods: Sixty-four patients with T2DM and 73 healthy subjects, all from Baghdad city, were enrolled in the current study from March to October 2020. For each participant, fasting blood glucose, hemoglobin (HbA1c), insulin resistance HOMA-IR, and body mass index (BMI) were measured in addition to the total vitamin D and VDBP. Moreover, free vitamin D was calculated by the formula of Bikle &amp; Schwartz. Results: There were highly significant correlations between total vitamin D and absolute values of free vitamin D or its percentage. The difference in total vitamin D was significant between patients and healthy controls with no significant change in VDBP, free and bio-available vitamin D, while free vitamin D% was higher in the patient’s group. Correlations between vitamin D and each of BMI, fasting glucose, HbA1c, and HOMA-IR were not significant; however, there was a negative correlation with BMI and fasting glucose in the healthy control subjects only. The Receiver Operating Characteristic (ROC) curve analysis of vitamin D in the diagnosis of diabetes mellitus was poor. Conclusion: Total vitamin D can represent vitamin D status, but it cannot be used as a factor for diagnosing T2DM. However, it could be of importance to change the glycemic status.

Association of blood lead level with vitamin D binding protein, total and free 25-hydroxyvitamin D levels in middle-school children

A negative association between blood Pb level (BPbL) and vitamin D metabolites in occupationally exposed populations has been reported, but data from the general population are scarce. Furthermore, the association between BPbL and vitamin D binding protein (DBP) and free 25-hydroxyvitamin D (25(OH)D) has not been reported. We investigated the association of BPbL with DBP, total and free 25(OH)D in healthy adolescents (n 1347; age range 11-16 years) cross-sectionally selected from all Governorates of Kuwait, utilising multi-stage cluster random sampling. Pb in whole blood was analysed by inductively coupled plasma MS, and DBP with ELISA. Plasma 25(OH)D was analysed by LC-MS/MS, and free 25(OH)D was calculated utilising the levels and binding affinities of DBP and albumin for 25(OH)D. DBP was positively associated with BPbL (β = 0·81; 95 % CI 0·14, 0·22; P < 0·001). A negative association between BPbL and total 25(OH)D was non-significant (P = 0·24) when BPbL was used as a continuous variable but was significant when used as quartiles (P = 0·02). The negative association between BPbL and free 25(OH)D was significant whether BPbL was used as continuous, as quartiles or as cut-off point of <5 µg/dl (0·24 µmol/l). In multinomial logistic regression, the odds of vitamin D insufficiency and deficiency were more than two-fold higher in the upper quartiles of BPbL compared with the lowest quartile. The negative correlation of BPbL with free 25(OH)D was more robust than its correlation with total 25(OH)D. Future studies must consider the levels of DBP when assessing the association between Pb and vitamin D metabolites.

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

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.

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.

Total, Bioavailable, and Free 25(OH)D Relationship with Indices of Bone Health in Elderly: A Randomized Controlled Trial

CONTEXT

Questions regarding the superiority of free and bioavailable 25-hydroxyvitamin D [25(OH)D] in predicting health outcomes remain unresolved.

OBJECTIVE

This study investigates the impact of vitamin D variables-total, bioavailable, or free 25(OH)D-on indices of bone and mineral metabolism, at baseline and in response to 2 vitamin D doses.

DESIGN

Our objectives are implemented as exploratory analyses on data collected in a 1-year, double-blind, randomized controlled trial completed in July 2014.

SETTING

Participants were recruited from 3 major hospitals in an ambulatory setting.

PARTICIPANTS

Participants were >65 years of age, overweight, and had a baseline serum 25(OH)D between 10 and 30 ng/mL. A total of 221 participants completed the study.

INTERVENTION

Subjects were randomized to receive calcium and oral vitamin D3 (600 IU/day or 3750 IU/day) supplementation.

RESULTS

Participants who received the higher vitamin D dose had levels that were 1.3- to 1.4-fold higher than those taking the lower dose, for all variables (P value < 0.001). Serum values of bioavailable and free 25(OH)D were associated with total 25(OH)D, with r values of 0.942 and 0.943, respectively (P value < 0.001). Parathyroid hormone (PTH) was negatively associated with all vitamin D variables, with correlation coefficients ranging from -0.22 to -0.25, while calcium and bone turnover markers (carboxy-terminal collagen crosslinks and osteocalcin) did not. Only total 25(OH)D had a positive relationship with % change bone mineral density (BMD) at the femoral neck at 12 months, while only free and bioavailable 25(OH) had a positive relationship with % change total body BMD at 12 months.

CONCLUSION

Calculated free and bioavailable 25(OH)D do not appear to be superior to total 25(OH)D in predicting indices of bone health in an elderly population.

Opposite correlation of 25-hydroxy-vitamin D- and 1,25-dihydroxy-vitamin D-metabolites with gestational age, bone- and lipid-biomarkers in pregnant women

25-Hydroxyvitamin D (25OHD) and 1,25-dihydroxyvitamin D (1,25(OH)₂D) need to be bound to carrier proteins to be transported to their target cells. The majority of either 25OHD or 1,25(OH)₂D is bound to vitamin D-binding protein (DBP), a smaller fraction is bound to albumin and only very small amounts of 25OHD or 1,25(OH)₂D are free. Albumin-bound 25OHD or 1,25(OH)₂D is relatively easily available after dissociation from albumin. Thus, the sum of free and albumin-bound forms is called bioavailable 25OHD and bioavailable 1,25(OH)₂D. Total 25OHD and 1,25(OH)₂D are defined as the sum of free, albumin-bound and DBP-bound 25OHD and 1,25(OH)₂D, respectively. This cross-sectional study in 427 pregnant women compared the correlation of the six vitamin D compounds with biomarkers of bone health, lipid metabolism, kidney function, endocrine parameters, and group B water-soluble vitamins. Among the 25OHD metabolites analysed, total 1,25(OH)₂D showed clearly the best correlation with calcium, bone-specific alkaline phosphatase, adiponectin, LDL, HDL, urea, thyroxine, and group B water-soluble vitamins. When comparing the three 25OHD metabolites, both free 25OHD and bioavailable 25OHD showed overall good correlations with calcium, bone-specific alkaline phosphatase, adiponectin, LDL, HDL, urea, thyroxine, triiodothyronine, and group B water-soluble vitamins, The correlations of 1,25(OH)₂D and 25OHD metabolites went always in opposite directions. Only PTH correlates always inversely with all six vitamin D compounds. In conclusion, free 25(OH)D and bioavailable 25(OH)D are more precise determinants of the vitamin D status than total 25(OH)D in normal pregnancy, whereas total 1,25(OH)₂D is superior to free and bioavailable 1,25(OH)₂D. Except for PTH, correlations of 25(OH)D and 1,25(OH)₂D metabolites with typical clinical chemistry readouts go in opposite directions.

Evaluation of Vitamin D Metabolism in Patients with Type 1 Diabetes Mellitus in the Setting of Cholecalciferol Treatment

In this prospective controlled study, we examined 25 adults with adequately controlled (HbA1c level < 8.0%) type 1 diabetes mellitus (T1DM) and 49 conditionally healthy adults, intending to reveal the diversity of vitamin D metabolism in the setting of cholecalciferol intake at a therapeutic dose. All patients received a single dose (150,000 IU) of cholecalciferol aqueous solution orally. Laboratory assessments including serum vitamin D metabolites (25(OH)D₃, 25(OH)D₂, 1,25(OH)₂D₃, 3-epi-25(OH)D₃ and 24,25(OH)₂D₃), free 25(OH)D, vitamin D-binding protein (DBP) and parathyroid hormone (PTH) as well as serum and urine biochemical parameters were performed before the intake and on Days 1, 3 and 7 after the administration. The studied groups had no significant differences in baseline parameters except that the patients with diabetes showed higher baseline levels of free 25(OH)D (p < 0.05). They also lacked a correlation between the measured and calculated free 25(OH)D in contrast to the patients from the control group (r = 0.41, p > 0.05 vs. r = 0.88, p < 0.05), possibly due to the glycosylation of binding proteins, which affects the affinity constant for 25(OH)D. The elevation of vitamin D levels after the administration of cholecalciferol was comparable in both groups, with slightly higher 25(OH)D₃ levels observed in the diabetes group throughout the study since Day 1 (p < 0.05). Overall, our data indicate that in patients with adequately controlled T1DM 25(OH)D₃ levels and the therapeutic response to cholecalciferol is similar to that in healthy individuals.

The Free Hormone Hypothesis: When, Why, and How to Measure the Free Hormone Levels to Assess Vitamin D, Thyroid, Sex Hormone, and Cortisol Status

The free hormone hypothesis postulates that only the nonbound fraction (the free fraction) of hormones that otherwise circulate in blood bound to their carrier proteins is able to enter cells and exert biologic effects. In this review, I will examine four hormone groups-vitamin D metabolites (especially 25OHD), thyroid hormones (especially thyroxine [T4]), sex steroids (especially testosterone), and glucocorticoids (especially cortisol)-that are bound to various degrees to their respective binding proteins-vitamin D-binding protein (DBP), thyroid-binding globulin (TBG), sex hormone-binding globulin (SHBG), and cortisol-binding globulin (CBG)-for which a strong case can be made that measurement of the free hormone level provides a better assessment of hormonal status than the measurement of total hormonal levels under conditions in which the binding proteins are affected in levels or affinities for the hormones to which they bind. I will discuss the rationale for this argument based on the free hormone hypothesis, discuss potential exceptions to the free hormone hypothesis, and review functions of the binding proteins that may be independent of their transport role. I will then review the complications involved with measuring the free hormone levels and the efforts to calculate those levels based on estimates of binding constants and levels of both total hormone and total binding protein. In this review, the major focus will be on DBP and free 25OHD, but the parallels and differences with the other binding proteins and hormones will be highlighted. Vitamin D and its metabolites, thyroid hormones, sex steroids, and glucocorticoids are transported in blood bound to serum proteins. The tightness of binding varies depending on the hormone and the binding protein such that the percent free varies from 0.03% for T4 and 25OHD to 4% for cortisol with testosterone at 2%. Although the major function of the primary carrier proteins (DBP, TBG, SHBG, and CBG) may be to transport their respective lipophilic hormones within the aqueous media that is plasma, these proteins may have other functions independent of their transport function. For most tissues, these hormones enter the cell as the free hormone presumably by diffusion (the free hormone hypothesis), although a few tissues such as the kidney and reproductive tissues express megalin/cubilin enabling by endocytosis protein-bound hormone to enter the cell. Measuring the free levels of these protein-bound hormones is likely to provide a better measure of the true hormone status than measuring the total levels in situations where the levels and/or affinities of the binding proteins are altered. Methods to measure free hormone levels are problematic as the free levels can be quite low, the methods require separation of bound and free that could disturb the steady state, and the means of separating bound and free are prone to error. Calculation of free levels using existing data for association constants between the hormone and its binding protein are likewise prone to error because of assumptions of linear binding models and invariant association constants, both of which are invalid. © 2020 The Author. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

Direct detection of free vitamin D as a tool to assess risk conditions associated with chronic plaque psoriasis

Introduction

Psoriasis is a major public health problem that results in high social and health costs. New approaches and methods are required to identify any conditions related to the disease and comorbidity development. The vitamin D deficiency is associated to psoriasis and could play an important role in its pathogenesis. However, the serum level of vitamin D is currently measured as total vitamin D, which is affected by wide variability. Therefore, the determination of the free form could be more significant, since it is independent of confounding factors. A cross-sectional study was performed to assess the association between chronic plaque psoriasis and serum level of free vitamin D, detected by a direct analytical method.

Methods

The levels of bioavailable vitamin D, total vitamin D and other metabolic parameters whose homeostasis is regulated by vitamin D were evaluated in 72 psoriasis patients and in 48 healthy controls. A direct immunoassay method was used to measure serum free vitamin D level. Analysis of covariance was performed to calculate estimated marginal means (EMM) and 95% confidence interval (CI), after adjustment for age, sex and BMI, within the two groups.

Results

Patients showed an EMM of 5.526 ± 0.271pg/ml, 95% CI 4.989-6.063; while controls an EMM of 6.776 ± 0.271 pg/ml, 95% CI 6.115-7.437.

Conclusions

Chronic plaque psoriasis patients exhibited a serum level of free vitamin D lower than controls. The direct immunoassay method could represent a useful tool to assess vitamin D status and identify a risk condition associated with the onset of the pathology.

Free and Bioavailable Fractions of Vitamin D: Association with Maternal Characteristics in Brazilian Pregnant Women

Normal pregnancy is characterized by many changes in vitamin D metabolism, challenging the assessment of vitamin D status based exclusively on serum total 25-hydroxyvitamin D (25(OH)D). We hypothesized that measuring free and bioavailable fractions contributes to a better vitamin D status assessment in late pregnancy. Our aim was to evaluate a broad set of biomarkers of vitamin D status in Brazilian women in the third trimester of pregnancy. This cross-sectional study was conducted in women (n = 123, 18–44 y, 27–41 wk gestation) attended in a public maternity in Rio de Janeiro (2016–2018). Biomarkers included serum concentrations of total 25(OH)D₃, parathyroid hormone (PTH), vitamin D-binding protein (DBP), and free and bioavailable fractions of 25(OH)D₃. Vitamin D insufficiency (<50 nmol/L) was prevalent in 47.9% of the pregnant women. Serum 25(OH)D₃ was inversely associated with the gestational week (β = −0.71, 95% confidence interval (CI): −1.31 to −0.16) and season, being lower in autumn (β = −9.90, 95% CI: −16.14 to -3.64) and winter (β = −16.74, 95%CI: −23.13 to −10.34). Concentrations of DBP, and free and bioavailable 25(OH)D₃ were also inversely associated with winter months (P < 0.05). DBP was directly associated with prepregnancy BMI (β = 5.84, 95% CI: 0.62 to 11.06). The recognized season-effect on total 25(OH)D₃ appeared to also occur on free and bioavailable fractions. Although advanced gestational age was associated with lower total 25(OH)D₃, our results suggest an adaptive mechanism responsible for maintaining free fraction during the 3^rd trimester. We also suggest that starting pregnancy in obese condition may have an impact on vitamin D bioavailability, which deserves further investigation.

Relationships between Total, Free and Bioavailable Vitamin D and Vitamin D Binding Protein in Early Pregnancy with Neonatal Outcomes: A Retrospective Cohort Study

Maternal vitamin D deficiency has been associated with adverse neonatal outcomes, however, existing results are inconsistent. Current data focus on total 25-hydroxyvitamin D (25(OH)D) as the common measure of vitamin D status, while additional measures including vitamin D-binding protein (VDBP) and free and bioavailable metabolites have not been explored in relation to neonatal outcomes. We examined whether VDBP and total, free, and bioavailable vitamin D metabolites in early pregnancy are associated with subsequent neonatal outcomes. In this retrospective analysis of 304 women in early pregnancy (<20 weeks gestation), demographic and anthropometric data were collected and total 25(OH)D (chemiluminescent assay), VDBP (polyclonal enzyme-linked immunosorbent assay (ELISA)) and albumin (automated colorimetry) were measured in bio-banked samples. Free and bioavailable 25(OH)D were calculated using validated formulae. Neonatal outcomes were derived from a medical record database. Higher maternal total and free 25(OH)D concentrations were associated with higher neonatal birthweight (β = 5.05, p = 0.002 and β = 18.06, p = 0.02, respectively), including after adjustment for maternal covariates including age, body mass index (BMI) and ethnicity (all p ≤ 0.04). Higher total 25(OH)D and VDBP concentrations were associated with a lower likelihood of neonatal jaundice (odds ratio [OR] [95%CI] = 0.997 [0.994, 1.000], p = 0.04 and 0.98 [0.96, 0.99], p = 0.03, respectively), but these were attenuated after adjustment for the above maternal covariates (both p = 0.09). Our findings suggest a novel association between free 25(OH)D and neonatal birthweight. Total 25(OH)D concentrations were also associated with birthweight, and both total 25(OH)D and VDBP were associated with jaundice, but the latter were not significant after adjustment. These results suggest a potential link between these metabolites and neonatal outcomes; however, further large-scale prospective studies are warranted.

Cord Blood Vitamin D Levels and Early Childhood Blood Pressure: The Healthy Start Study

Background Vitamin D deficiency is associated with cardiovascular events among adults, but it is unclear whether early-life vitamin D deficiency influences cardiovascular risk factors in children. Methods and Results We measured total and bioavailable 25-dihydroxyvitamin D (25OHD) in cord blood and in blood from 4- to 6-year-old children, and we assessed cardiovascular risk factors (blood pressure, arterial stiffness, body size, and adiposity) at 4 to 6 years. We tested for racial/ethnic differences in total and bioavailable 25OHD (n=715) and modeled the adjusted association between cord blood 25OHD and childhood cardiovascular risk factors (n=171). We observed racial/ethnic differences in total and bioavailable 25OHD levels in both cord and child blood samples (all P<0.05). Each 25-nmol/L increase in cord blood total 25OHD was associated with a 2.5-mm Hg (SE 0.8) decrease in systolic blood pressure ( P=0.002) and a 1.7-mm Hg (SE 0.6) decrease in diastolic blood pressure ( P=0.01), independent of childhood 25OHD levels, race/ethnicity, and other covariates. There was no association between cord blood total 25OHD and any other cardiovascular risk factors. Cord blood levels of bioavailable and free 25OHD were not associated with any cardiovascular risk factor in childhood. Conclusions In this diverse prebirth cohort, we observed lower systolic and diastolic blood pressure among children with higher total 25OHD levels at birth. Our findings suggest that intrauterine exposure to vitamin D may contribute to early-life programming of offspring blood pressure. Intervention studies are needed to determine whether increasing fetal vitamin D exposure can reduce the risk of elevated blood pressure in childhood.

Early Pregnancy Vitamin D Binding Protein Is Independently Associated with the Development of Gestational Diabetes: A Retrospective Cohort Study

BACKGROUND

Vitamin D-binding protein (VDBP) has been implicated in several adverse pregnancy outcomes either directly or indirectly via influencing the concentrations of biologically active vitamin D metabolites. However, human studies exploring these metabolites in pregnancy remain sparse. Here, we examine whether VDBP and total, free, and bioavailable 25-hydroxyvitamin D (25(OH)D) metabolites in early pregnancy are associated with subsequent adverse pregnancy outcomes.

METHODS

We conducted a retrospective analysis of 304 pregnant women in early pregnancy (<20 weeks gestation). The demographic characteristics, anthropometric data, and total 25(OH)D were measured and plasma or serum samples were collected and bio-banked. Using these samples, we measured VDBP (polyclonal ELISA) and albumin (automated colorimetry), and calculated free and bioavailable 25(OH)D using validated formulae. Pregnancy outcomes were derived from scanned medical records. Regression models were used to analyse the relationships between vitamin D metabolites in early pregnancy and subsequent pregnancy outcomes (gestational diabetes mellitus (GDM), pre-eclampsia, preterm birth), with adjustment for predetermined clinically relevant maternal factors including age, body mass index (BMI), and ethnicity.

RESULTS

Lower VDBP concentrations were associated with higher glucose levels and a greater likelihood of developing GDM at 26-28 weeks gestation (odds ratio [OR] (95% CI) = 0.98 (0.97,0.99), p = 0.015). This finding remained significant after adjustment for maternal covariates including age, BMI, and ethnicity (β = -0.003, p = 0.03). Lower total, free and bioavailable 25(OH)D, but not VDBP, were associated with a shorter length of gestation, but only the relationship with total 25(OH)D remained significant after adjustment for the above maternal covariates (β = 0.02, p = 0.006).

CONCLUSIONS

This is the first study to examine VDBP, and total, free and bioavailable 25(OH)D in relation to pregnancy outcomes in a well characterised multi-ethnic cohort of pregnant women. Our findings show that VDBP and total 25(OH)D are associated with GDM and length of gestation, respectively; however, further investigations using large-scale prospective studies are needed to confirm our findings.

Vitamin D and stress urinary incontinence in pregnancy: a cross-sectional study

OBJECTIVE

To assess the association between levels of vitamin D and urinary incontinence (UI) in pregnancy.

DESIGN

A cross-sectional study. Secondary analysis of a randomised controlled trial.

SETTING

Two university hospitals in Norway.

POPULATION

A total of 851 healthy, pregnant women >18 years in gestational weeks 18-22 with a singleton live fetus.

METHODS

Data on UI were collected from a questionnaire at inclusion and serum analysis of 25-hydroxy vitamin D (25(OH)D) was performed. Univariable and multivariable logistic regression analyses were applied to study associations between exposure and outcomes.

MAIN OUTCOME MEASURES

Prevalence of self-reported UI, stress (SUI) and urge (UUI) or mixed UI.

RESULTS

In total, 230/851 (27%) of the participants were vitamin D insufficient (25(OH)D <50 nmol/l) and 42% reported to have any UI. Women with 25(OH)D <50 nmol/l were more likely to report any UI (P = 0.03) and SUI (P < 0.01) compared with women with 25(OH)D ≥50 nmol/l. In a univariable logistic regression analysis, serum levels of 25(OH)D <50 nmol/l was associated with increased risk of any UI (odds ratio [OR] 1.5 with 95% CI 1.0-2.1), SUI only (OR 1.7, 95% CI 1.2-2.4), but not mixed UI or UUI only (OR 0.8, 95% CI 0.5-1.5). In a multivariable logistic regression model, serum levels of 25(OH)D <50 nmol/l were associated with a higher risk of experiencing SUI only (OR 1.5, 95% CI 1.1-2.2).

CONCLUSIONS

Serum 25(OH)D <50 nmol/l was associated with increased risk of any UI, and SUI in particular.

TWEETABLE ABSTRACT

Low levels of vitamin D are associated with increased risk of urinary incontinence in pregnancy.

Serum Bioavailable, Rather Than Total, 25-hydroxyvitamin D Levels Are Associated With Hepatocellular Carcinoma Survival

BACKGROUND AND AIMS

Free and bioavailable 25-hydroxyvitamin D (25OHD) are emerging measurements of vitamin D status. It remains unclear whether circulating free or bioavailable 25OHD are relevant to hepatocellular carcinoma (HCC) prognosis. Our aim was to test the hypothesis that bioavailable 25OHD may be a better serum biomarker of vitamin D status than total 25OHD on the association with HCC survival.

APPROACH AND RESULTS

We included 1,031 newly diagnosed, previously untreated patients with HCC from the Guangdong Liver Cancer Cohort enrolled between September 2013 and April 2017. Serum total 25OHD levels were measured using an electrochemiluminescence immunoassay. Serum-free 25OHD levels were measured using a two-step enzyme-linked immunosorbent assay. Bioavailable 25OHD levels were calculated from measured free 25OHD and albumin using a previously validated equation. Primary outcomes were liver cancer-specific (LCSS) and overall survival (OS). Cox proportional hazards models were performed to calculate the multivariable hazard ratios (HRs) and 95% confidence intervals (CIs). During a median follow-up of 726 days, 430 patients had deceased, including 393 deaths from HCC. In multivariable analyses, higher bioavailable 25OHD levels were significantly associated with better survival, independent of nonclinical and clinical prognostic factors including serum C-reactive protein, Barcelona Clinic Liver Cancer stage, and cancer treatment. The multivariable-adjusted HRs in the highest versus lowest quartile of bioavailable 25OHD levels were 0.69 (95% CI: 0.51, 0.93; P for trend = 0.014) for LCSS and 0.71 (95% CI: 0.53, 0.94; P for trend = 0.013) for OS. In contrast, neither total nor free 25OHD levels were associated with LCSS or OS.

CONCLUSIONS

Higher bioavailable, rather than total, 25OHD levels were independently associated with improved survival in a population-based HCC cohort, suggesting a potential utility of bioavailable 25OHD in HCC prognosis.

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.

The measurement of vitamin D metabolites part II-the measurement of the various vitamin D metabolites

Today, the possibility exists to measure a number of different vitamin D metabolites with accurate and precise methods. The most abundant vitamin D metabolite, 25(OH)D, is considered the best marker for estimating vitamin D status and is therefore the most commonly measured in clinical practice. There is no consensus on the added value of measuring other metabolites beyond 25-hydroxyvitamin D, although, in some special clinical scenarios and complicated cases, these metabolites may provide just the information needed for an accurate diagnosis. The problem this review addresses is which metabolite to measure and when and how to measure it.

Vitamin D-Binding Protein in Pregnancy and Reproductive Health

Vitamin D-binding protein (VDBP), the main carrier of vitamin D, has recently been implicated in reproductive health and pregnancy outcomes including endometriosis, polycystic ovary syndrome (PCOS), pre-eclampsia, and gestational diabetes mellitus (GDM). Improved methods for measuring VDBP and an increased understanding of its role in biological processes have led to a number of newly published studies exploring VDBP in the context of pregnancy. Here, we synthesize the available evidence regarding the role of VDBP in reproductive health and pregnancy, and we highlight areas requiring further study. Overall, low levels of maternal serum VDBP concentrations have been associated with infertility, endometriosis, PCOS and spontaneous miscarriage, as well as adverse pregnancy outcomes including GDM, pre-eclampsia, preterm birth and fetal growth restriction. However, increased VDBP concentration in cervicovaginal fluid has been linked to unexplained recurrent pregnancy loss and premature rupture of membranes. Some genetic variants of VDBP have also been associated with these adverse outcomes. Further studies using more accurate VDBP assays and accounting for ethnic variation and potential confounders are needed to clarify whether VDBP is associated with reproductive health and pregnancy outcomes, and the mechanisms underlying these relationships.

Differential Impact of Serum 25-Hydroxyvitamin D3 Levels on the Prognosis of Patients with Liver Cirrhosis According to MELD and Child-Pugh Scores

BACKGROUND

Prognosis of patients with diverse chronic diseases is reportedly associated with 25-hydroxyvitamin D levels. In this study, we investigated the potential role of 25-hydroxyvitamin D3 (25[OH]D3) levels in improving the predictive power of conventional prognostic models for patients with liver cirrhosis.

METHODS

We investigated clinical findings, including serum 25(OH)D3 levels at admission, of 155 patients with cirrhosis who were followed up for a median of 16.9 months.

RESULTS

Median 25(OH)D3 levels were significantly different among patients exhibiting Child-Pugh grades A, B, and C. Mortality, including urgent transplantation, was significantly associated with 25(OH)D3 levels in univariate analysis. Severe vitamin-D deficiency (serum 25[OH]D3 level < 5.0 ng/mL) was significantly related to increased mortality, even after adjusting for Child-Pugh and Model for End-stage Liver Disease (MELD) scores. In particular, the presence of severe vitamin D deficiency clearly defined a subgroup with significantly poorer survival among patients with Child-Pugh scores of 5-10 or MELD scores ≤ 20. A new combination model of MELD score and severe vitamin D deficiency showed significantly more accurate predictive power for short- and long-term mortality than MELD scores alone. Additionally, serum 25(OH)D3 levels and new model scores were significantly associated with the development of spontaneous bacterial peritonitis, overt encephalopathy, and acute kidney injury.

CONCLUSION

Serum 25(OH)D3 level is an independent prognostic factor for patients with liver cirrhosis and has a differential impact on disease outcomes according to MELD and Child-Pugh scores.

Effects of Supplemental Vitamin D on Bone Health Outcomes in Women and Men in the VITamin D and OmegA-3 TriaL (VITAL)

Although supplemental vitamin D is used to promote bone health in the general population, data from randomized controlled trials (RCTs) have been inconsistent. We determined whether daily, vitamin D₃ supplementation improves bone mineral density (BMD) and/or structure. VITamin D and OmegA-3 TriaL (VITAL) is a double-blind, placebo-controlled RCT of supplemental vitamin D₃ (2000 IU/d) and/or omega-3 fatty acids (1 g/d) in 25,871 adults nationwide. This ancillary study included a subcohort of 771 participants (men ≥50 and women ≥55 years; not taking bone active medications) evaluated at baseline and at 2-year follow-up (89% retention). Total 25(OH)D levels were measured by liquid chromatography tandem mass spectrometry (Quest Diagnostics, San Juan Capistrano, CA, USA). Free 25(OH)D (FVD) levels were measured using the ELISA assay by Future Diagnostics Solutions BV (Wijchen, Netherlands). Primary endpoints were 2-year changes in areal (a) BMD at the spine, hip, and whole body determined by dual-energy X-ray absorptiometry (DXA). Secondary endpoints were 2-year changes in volumetric (v) BMD and cortical thickness at the radius and tibia assessed by peripheral quantitative computed tomography. Supplemental vitamin D₃ versus placebo had no effect on 2-year changes in aBMD at the spine (0.33% versus 0.17%; p = 0.55), femoral neck (-0.27% versus -0.68%; p = 0.16), total hip (-0.76% versus -0.95%; p = 0.23), or whole body (-0.22% versus -0.15%; p = 0.60), or on measures of bone structure. Effects did not vary by sex, race/ethnicity, body mass index, or 25(OH)D levels. Among participants with baseline FVD levels below the median (<14.2 pmol/L), there was a slight increase in spine aBMD (0.75% versus 0%; p = 0.043) and attenuation in loss of total hip aBMD (-0.42% versus -0.98%; p = 0.044) with vitamin D₃ . Whether baseline FVD levels help to identify those more likely to benefit from supplementation warrants further study. Supplemental vitamin D₃ versus placebo for 2 years in general healthy adults not selected for vitamin D insufficiency did not improve BMD or structure. © 2020 The Authors. Journal of Bone and Mineral Research published by American Society for Bone and Mineral Research.

New aspects of vitamin D metabolism and action - addressing the skin as source and target

Vitamin D has a key role in stimulating calcium absorption from the gut and promoting skeletal health, as well as many other important physiological functions. Vitamin D is produced in the skin. It is subsequently metabolized to its hormonally active form, 1,25-dihydroxyvitamin D (1,25(OH)₂D), by the 1-hydroxylase and catabolized by the 24-hydroxylase. In this Review, we pay special attention to the effect of mutations in these enzymes and their clinical manifestations. We then discuss the role of vitamin D binding protein in transporting vitamin D and its metabolites from their source to their targets, the free hormone hypothesis for cell entry and HSP70 for intracellular transport. This is followed by discussion of the vitamin D receptor (VDR) that mediates the cellular actions of 1,25(OH)₂D. Cell-specific recruitment of co-regulatory complexes by liganded VDR leads to changes in gene expression that result in distinct physiological actions by 1,25(OH)₂D, which are disrupted by mutations in the VDR. We then discuss the epidermis and hair follicle, to provide a non-skeletal example of a tissue that expresses VDR that not only makes vitamin D but also can metabolize it to its hormonally active form. This enables vitamin D to regulate epidermal differentiation and hair follicle cycling and, in so doing, to promote barrier function, wound healing and hair growth, while limiting cancer development.

Free vitamin D correlate better with bone mineral density and thoracolumbar junction osteoporotic vertebral fractures than serum vitamin D

Background

Vitamin D deficiency has long been studied as a risk factor for osteoporosis. However, the association between serum vitamin D status, bone mineral density (BMD) and the incidence of vertebral fractures (OVFs) remain controversial. It is believed that free portion of the circulating vitamin D carries the metabolic activities of vitamin D. Therefore, the aim of the present study is to analyse if free vitamin D correlates with BMD and osteoporotic fragile vertebral fractures in the elderly population.

Methods

A total of 90 consecutive patients, including 81 female and 9 male patients, aged > 48 years, were included in this cross sectional study between March and July of 2018. Total vitamin D (total 25(OH)D), free vitamin D (free 25(OH)D), calcium and phosphorus were measured. BMD was measured using dual energy X-ray absorptiometry (DEXA) and osteoporotic vertebral fracture was assessed using plain radiograph. Multiple linear regression was performed to find out the association between total vitamin D, free vitamin D and BMD at various sites. To evaluate the association with osteoporotic vertebral multivariate logistic regression model was used.

Results

The mean total vitamin D and free vitamin D were 25.1 ± 10.2 and 6.1 ± 1.7 respectively. Free vitamin D had a linear correlation with total vitamin D (R² = 0.69). While free vitamin D had a positive correlation with lumbar BMD roles (p < 0.05), total vitamin D didn’t have any association with BMD at any site. Of the total patients, 62 patients (68.9%) had thoracolumbar junction OVFs. Free vitamin D level correlated with the prevalence of OVFs as well as lumbar osteoporosis (p < 0.05). However, there was no statistical correlation between serum vitamin D status and the OVFs.

Conclusions

Free vitamin D was significantly related to the occurrence of thoracolumbar junction OVFs and lumbar BMD, which assumed to be a positive predictor for fracture and osteoporosis prevention. However, total serum vitamin D levels did not have any association with BMD at different sites as well as fragile vertebral fracture.

Trial registration

The study is registered at clinicaltrials.gov NCT03605173.

Comparing directly measured versus mathematically calculated free serum 25-hydroxy vitamin D level in children

INTRODUCTION

25-Hydroxy vitamin D (25(OH)D) is essential for calcium homeostasis and bone metabolism. The majority of serum 25(OH)D is bound to vitamin D-binding protein (VDBP) (~ 85%) and to albumin (~ 15%), with only a miniscule amount circulating as free 25(OH)D. Free 25(OH)D can be calculated mathematically by Bikle method from the concentrations of total 25(OH)D, VDBP, and albumin or measured directly by ELISA. A direct head-to-head comparison between the two methods has not been done in children.

MATERIALS AND METHODS

The objective of the study was to compare the mathematically calculated versus directly measured free 25(OH) vitamin D in children. Serum samples from 74 children (ages 1-19 years) were simultaneously analyzed for total 25(OH)D, serum albumin, VDBP, and free 25(OH)D. Pearson correlation analysis and Bland-Altman plot were used to evaluate agreement between the two methods.

RESULTS

The mean age was 9.1 ± 5.1 years, with 61% boys, 76% Caucasians, and 24% African-Americans. The mean ± SD for total 25(OH)D was 38.7 ± 12.8 ng/mL, bioavailable 25(OH)D 3.1 ± 1.1 ng/mL, mathematically calculated free 25(OH)D 8.4 ± 3.2 pg/mL, and directly measured free 25(OH)D 8.9 ± 3.6 pg/mL. Pearson correlation reflected a significant correlation between mathematically calculated and directly measured free 25(OH)D (r = 0.66, p < 0.0005). Bland-Altman plot reflected a tight agreement within a 95% limit of agreement (mean = - 0.026 ± 2SD).

CONCLUSIONS

The directly measured and mathematically calculated free 25(OH)D are in close agreement and are interchangeable. Depending on the local availability of instruments and methods, free 25(OH)D can be either directly measured or mathematically calculated.

Free vitamin D levels in steroid-sensitive nephrotic syndrome and healthy controls

INTRODUCTION

Body stores of vitamin D are measured as "total" serum 25-hydroxy vitamin D (25(OH)D). Its largest component is protein bound and lost in urine in nephrotic syndrome (NS). Our study investigates whether "free" 25(OH)D levels are a better guide to bone health and need for vitamin D supplementation in patients with steroid-sensitive NS (SSNS).

METHODS

A cross-sectional study was performed in children with SSNS and healthy controls. Blood was tested for albumin, creatinine, calcium, phosphate, ALP, total and free (by direct ELISA) 25(OH)D, iPTH, and urine for protein-creatinine ratio.

RESULTS

Seventy-nine NS patients (48 in relapse, 31 in remission) and 60 healthy controls were included. The levels of total 25(OH)D were significantly different (lowest in NS relapse and highest in controls) (p < 0.001). Corrected calcium and phosphate levels were normal, and there were no differences in free 25(OH)D, ALP, or iPTH levels between groups. Only total and not free 25(OH)D correlated significantly and negatively with urinary protein creatinine ratios (r_s = - 0.42 vs. 0.04). Free 25(OH)D values of 3.75 and 2.85 pg/ml corresponded to total 25(OH)D levels of 20 and 12 ng/ml, respectively, in healthy controls.

CONCLUSION

These results confirm that total 25(OH)D levels are low in NS and related to degree of proteinuria. However levels of free 25(OH)D, ALP, and iPTH did not change in relapse or remission in comparison with healthy controls. Our results suggest that in proteinuric renal diseases, free 25(OH)D rather than total 25(OH)D levels should be used to diagnose vitamin D deficiency and guide therapy.

Vitamin D Measurement, the Debates Continue, New Analytes Have Emerged, Developments Have Variable Outcomes

The demand for measurement of vitamin D metabolites for clinical diagnosis and to advance our understanding of the role of vitamin D in human health has significantly increased in the last decade. New developments in technologies employed have enabled the separation and quantification of additional metabolites and interferences. Also, developments of immunoassays have changed the landscape. Programmes and materials for assay standardisation, harmonisation and the expansion of the vitamin D external quality assurance scheme (DEQAS) with the provision of target values as measured by a reference measurement procedure have improved standardisation, quality assurance and comparability of measurements. In this article, we describe developments in the measurement of the commonly analysed vitamin D metabolites in clinical and research practice. We describe current analytical approaches, discuss differences between assays, their origin, and how these may be influenced by physiological and experimental conditions. The value of measuring metabolites beyond 25 hydroxyvitamin D (25(OH)D), the marker of vitamin D status, in routine clinical practice is not yet confirmed. Here we provide an overview of the value and application of the measurement of 1,25 dihydroxyvitamin D, 24,25 dihydroxyvitamin D and free 25OHD in the diagnosis of patients with abnormalities in vitamin D metabolism and for research purposes.

Free 25-hydroxyvitamin-D concentrations are lower in children with renal transplant compared with chronic kidney disease

BACKGROUND

Total serum 25-hydroxyvitamin D [25(OH)D] is considered the best marker of vitamin D status and used routinely in clinical practice. However, 25(OH)D is predominantly bound to vitamin D-binding protein (VDBP), and it has been reported that the free-25(OH)D and 25(OH)D loosely bound to albumin fraction correlates better with clinical outcomes.

METHODS

We assessed total-25(OH)D, measured free-25(OH)D, and calculated free-25(OH)D and their relationship with VDBP and biomarkers of mineral metabolism in 61 children (22 CKD 2-3, 18 dialysis, and 21 post-transplant).

RESULTS

Total-25(OH)D concentrations were comparable across the three groups (p = 0.09), but free- and bioavailable-25(OH)D (free- and albumin-25(OH)D) were significantly lower in the transplant group (both: p = 0.01). Compared to CKD and dialysis patients, the transplant group had significantly higher VDBP concentrations (p = 0.03). In all three groups, total-25(OH)D concentrations were positively associated with measured free-, calculated free-, and bioavailable-25(OH)D. Multivariable regression analysis showed that total-25(OH)D was the only predictor of measured free-25(OH)D concentrations in the dialysis group (β = 0.9; R² = 90%). In the transplant group, measured free-25(OH)D concentrations were predicted by both total-25(OH)D and VDBP concentrations (β = 0.6, - 0.6, respectively; R² = 80%). Correlations between parathyroid hormone with total-25(OH)D and measured and calculated free-25(OH)D were only observed in the transplant group (all: p < 0.001).

CONCLUSIONS

In transplanted patients, VDBP concentrations were significantly higher compared to CKD and dialysis patients, and consequently, free-25(OH)D concentrations were lower, despite a comparable total-25(OH)D concentration. We suggest that free-25(OH)D measures may be required in children with CKD, dialysis, and transplant, with further research required to understand its association with markers of mineral metabolism.

Serum 25-hydroxyvitamin D in the VITamin D and OmegA-3 TriaL (VITAL): Clinical and demographic characteristics associated with baseline and change with randomized vitamin D treatment

BACKGROUND

The VITamin D and OmegA-3 TriaL (VITAL) is a completed randomized, placebo-controlled trial of vitamin D3 (2000 IU/day) and marine omega-3 (1 g/day) supplements in the primary prevention of cancer and cardiovascular disease. Here we examine baseline and change in 25-hydroxyvitamin D (25(OH)D) and related biomarkers with randomized treatment and by clinical factors.

METHODS

Baseline 25(OH)D was measured in 15,804 participants (mean age 68 years.; 50.8% women; 15.7% African Americans) and in 1660 1-year follow-up samples using liquid chromatography-tandem mass spectrometry and chemiluminescence. Calcium and parathyroid hormone (iPTH) were measured by chemiluminescence and spectrophotometry respectively.

RESULTS

Mean baseline total 25(OH)D (ng/mL ± SD) was 30.8 ± 10.0 ng/mL, and correlated inversely with iPTH (r = -0.28), p < .001. After adjusting for clinical factors, 25(OH)D (ng/mL ± SE) was lower in men vs women (29.7 ± 0.30 vs 31.4 ± 0.30, p < .0001) and in African Americans vs whites (27.9 ± 0.29 vs 32.5 ± 0.22, p < .0001). It was also lower with increasing BMI, smoking, and latitude, and varied by season. Mean 1-year 25(OH)D increased by 11.9 ng/mL in the active group and decreased by 0.7 ng/mL in placebo. The largest increases were noted among individuals with low baseline and African Americans. Results were similar for chemiluminescent immunoassay. Mean calcium was unchanged, and iPTH decreased with treatment.

CONCLUSION

In VITAL, baseline 25(OH)D varied by clinical subgroups, was lower in men and African Americans. Concentrations increased with vitamin D supplementation, with the greatest increases in those with lower baseline 25(OH)D. The seasonal trends in 25(OH)D, iPTH, and calcium may be relevant when interpreting 25(OH)D levels for clinical treatment decisions.

CLINICAL TRIAL REGISTRATION

VITAL ClinicalTrials.gov number NCT01169259.

Vitamin D and Acute Respiratory Infections-The PODA Trial

Background

There is considerable heterogeneity in clinical trials examining the role of vitamin D in the prevention of acute respiratory infections (ARIs).

Methods

The primary aim of the Physical Performance, Osteoporosis, and Vitamin D in Older African-American Women (PODA) trial was the prevention of bone loss and decline in physical performance. A questionnaire about ARIs was administered every 3 months for 3 years to 260 black American women in a double-blind randomized clinical trial that had a placebo group and a vitamin D supplementation group. The serum 25(OH)D level was maintained >30 ng/mL in the vitamin D group.

Results

Serum 25(OH)D was maintained >30 ng/mL in 90% of the active group, whereas levels approximated those associated with the recommended dietary allowance (20 ng/mL) in the placebo group. There was no difference in occurrence of ARIs in the treatment group vs the placebo group. ARIs were not related to total or free 25(OH)D, which were measured at baseline and annually for 36 months.

Conclusions

Vitamin D supplementation sufficient to maintain serum 25(OH)D >30 ng/mL does not prevent ARIs in older African American women.

ClinicalTrialsgov Registration Number

NCT01153568.

Changes of Vitamin D-Binding Protein, and Total, Bioavailable, and Free 25-Hydroxyvitamin D in Transgender People

CONTEXT

Total 25-hydroxyvitamin D [25(OH)D] is mainly bound to vitamin d-binding protein (DBP). Bioavailable 25(OH)D consists of albumin-bound and free 25(OH)D and is available for metabolic processes. As sex steroids influence DBP, hormonal treatment (HT) in transgender people might affect DBP and consequently the available 25(OH)D. Total 25(OH)D might therefore not well represent bioavailable and free 25(OH)D.

OBJECTIVE

To investigate the effects of HT on DBP, and total, bioavailable, and free 25(OH)D, and to assess whether total 25(OH)D well represents bioavailable and free 25(OH)D.

DESIGN

A prospective study.

SETTING

A university hospital.

PARTICIPANTS

Twenty-nine transwomen and 30 transmen.

INTERVENTION

Estradiol and cyproterone acetate in transwomen, and testosterone in transmen.

MAIN OUTCOME MEASURES

DBP, total 25(OH)D, free 25(OH)D, and albumin were measured at baseline and after 3 months of HT, and deseasonalized total 25(OH)D and bioavailable 25(OH)D were calculated.

RESULTS

DBP changed with +5% (95% CI, -0% to 10%; P = 0.06) in transwomen and with -3% (95% CI: -9% to 3%; P = 0.34) in transmen. No significant changes were found in total 25(OH)D, free, and bioavailable 25(OH)D concentrations. Total 25(OH)D was well correlated with bioavailable (R2, 0.75) and free (R2, 0.76) 25(OH)D.

CONCLUSIONS

DBP tended to increase in transwomen, but did not change in transmen. HT did not influence free 25(OH)D, total 25(OH)D, and bioavailable 25(OH)D concentrations in transwomen and transmen. As total 25(OH)D represents bioavailable and free 25(OH)D well, HT in transgender people does not interfere with the assessment of vitamin D status.

Comparison of free and total 25-hydroxyvitamin D in normal human pregnancy

Vitamin D status correct monitoring during pregnancy is critically important for both maternal and fetal health. 25-Hydroxyvitamin D (25(OH)D) - a prohormone of a biologically active 1,25-dihydroxyvitamin D (1,25(OH)₂D), despite the lack of biological activity, during the past decades has been routinely used as a main biomarker characterizing vitamin D status. About 85% of 25(OH)D in the bloodstream is bound to its specific carrier - vitamin D-binding protein (DBP), the remaining 15% are loosely bound to albumin, and only less than 0.1% are free in the circulation ("free 25(OH)D"). Total 25(OH)D is the sum of DBP-bound, albumin-bound and free 25(OH)D. According to a "free hormone hypothesis", only free 25(OH)D is able to induce a biological effect. Normal pregnancy is characterized by elevated serum DBP levels, and due to this fact the diagnostic strength of serum total 25(OH)D has been questioned. Free 25(OH)D might be a better characteristic of vitamin D status in this settings. We aimed to compare the diagnostic strength of a routine total 25(OH)D with directly measured free 25(OH)D in normal pregnancy by comparing the association strength between free and total 25(OH)D with biomarkers of bone health (PTH, calcium, bone-specific alkaline phosphatase (BSAP)), lipid metabolism (adiponectin, LDL, HDL), kidney function (urea), endocrine parameters (T4, T3, TSH), and group B water-soluble vitamins. The study was conducted in 368 healthy white pregnant women - residents of north-east Germany. Free 25(OH)D showed an overall better associations with gestational age, markers of bone metabolism (calcium (rho = 0.141, p = 0.007 with free 25(OH)D; rho = 0.060, p = 0.251 with total 25(OH)D) and BSAP (rho = -0.203, p < 0.001 with free 25(OH)D; rho = -0.108, p = 0.038 with total 25(OH)D), lipid metabolism parameters (adiponectin (rho = 0.142, p = 0.008 with free 25(OH)D; rho = 0.054, p = 0.307 with total 25(OH)D), LDL cholesterol (rho = -0.191, p < 0.001 with free 25(OH)D; rho = 0.033, p = 0.539 with total 25(OH)D)) and a kidney function marker (urea (rho = 0.114, p = 0.032 with free 25(OH)D; rho = 0.008, p = 0.887 with total 25(OH)D)) than total 25(OH)D. In conclusion, the current study revealed that free 25(OH)D is a more precise determinant of the vitamin D status during normal human pregnancy than total 25(OH)D. In the settings of normal pregnancy, free 25(OH)D revealed better associations with markers of bone metabolism (calcium, BSAP), lipid metabolism (adiponectin, LDL cholesterol, LDL/HDL ratio) and kidney function (urea) than total 25(OH)D.

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.

Current vitamin D status in European and Middle East countries and strategies to prevent vitamin D deficiency: a position statement of the European Calcified Tissue Society

Vitamin D deficiency (serum 25-hydroxyvitamin D (25(OH)D) <50 nmol/L or 20 ng/mL) is common in Europe and the Middle East. It occurs in <20% of the population in Northern Europe, in 30-60% in Western, Southern and Eastern Europe and up to 80% in Middle East countries. Severe deficiency (serum 25(OH)D <30 nmol/L or 12 ng/mL) is found in >10% of Europeans. The European Calcified Tissue Society (ECTS) advises that the measurement of serum 25(OH)D be standardized, for example, by the Vitamin D Standardization Program. Risk groups include young children, adolescents, pregnant women, older people (especially the institutionalized) and non-Western immigrants. Consequences of vitamin D deficiency include mineralization defects and lower bone mineral density causing fractures. Extra-skeletal consequences may be muscle weakness, falls and acute respiratory infection, and are the subject of large ongoing clinical trials. The ECTS advises to improve vitamin D status by food fortification and the use of vitamin D supplements in risk groups. Fortification of foods by adding vitamin D to dairy products, bread and cereals can improve the vitamin D status of the whole population, but quality assurance monitoring is needed to prevent intoxication. Specific risk groups such as infants and children up to 3 years, pregnant women, older persons and non-Western immigrants should routinely receive vitamin D supplements. Future research should include genetic studies to better define individual vulnerability for vitamin D deficiency, and Mendelian randomization studies to address the effect of vitamin D deficiency on long-term non-skeletal outcomes such as cancer.