Abstract
Background and objectives
Vitamin D deficiency is endemic in children with CKD. We sought to investigate the association of genetic disposition, environmental factors, vitamin D supplementation, and renal function on vitamin D status in children with CKD.
Design, setting, participants, & measurements
Serum 25-hydroxy-vitamin D, 1,25-dihydroxy-vitamin D, and 24,25-dihydroxy-vitamin D concentrations were measured cross-sectionally in 500 children from 12 European countries with CKD stages 3–5. All patients were participants of the Cardiovascular Comorbidity in Children with Chronic Kidney Disease Study, had CKD stage 3–5, and were age 6–18 years old. Patients were genotyped for single-nucleotide polymorphisms in the genes encoding 25-hydroxylase, vitamin D binding protein, 7-dehydrocholesterol reductase, and 24-hydroxylase. Associations of genetic status, season, local solar radiation, oral vitamin D supplementation, and disease-associated factors with vitamin D status were assessed.
Results
Two thirds of patients were vitamin D deficient (25-hydroxy-vitamin D <16 ng/ml). 25-Hydroxy-vitamin D concentrations varied with season and were twofold higher in vitamin D–supplemented patients (21.6 [14.1] versus 10.4 [10.1] ng/ml; P<0.001). Glomerulopathy, albuminuria, and girls were associated with lower 25-hydroxy-vitamin D levels. 24,25-dihydroxy-vitamin D levels were closely correlated with 25-hydroxy-vitamin D and 1,25-dihydroxy-vitamin D (r=0.87 and r=0.55; both P<0.001). 24,25-dihydroxy-vitamin D concentrations were higher with higher c-terminal fibroblast growth factor 23 and inversely correlated with intact parathyroid hormone. Whereas 25-hydroxy-vitamin D levels were independent of renal function, 24,25-dihydroxy-vitamin D levels were lower with lower eGFR. Vitamin D deficiency was more prevalent in Turkey than in other European regions independent of supplementation status and disease-related factors. Single-nucleotide polymorphisms in the vitamin D binding protein gene were independently associated with lower 25-hydroxy-vitamin D and higher 24,25-dihydroxy-vitamin D.
Conclusions
Disease-related factors and vitamin D supplementation are the main correlates of vitamin D status in children with CKD. Variants in the vitamin D binding protein showed weak associations with the vitamin D status.
Keywords: Vitamin D; chronic kidney disease; vitamin D deficiency; vitamin d supplementation; albuminuria; Child; Humans; Polymorphism, Single Nucleotide; Renal Insufficiency, Chronic; 25-hydroxyvitamin D
Introduction
Vitamin D status depends on environmental and endogenous factors and affects a multitude of physiologic processes, such as bone mineral metabolism, the immune system, muscle metabolism, and the cardiovascular phenotype (1–3). Although insufficient vitamin D levels are found in a significant proportion of the general pediatric population (4–6), vitamin D deficiency seems even more common in children with CKD (7–11) and has not declined over the past few decades (8). Vitamin D from nutritional intake and dermal synthesis is converted to 25-hydroxy-vitamin D [25(OH)D] in the liver. Although 1,25-dihydroxy-vitamin D [1,25(OH)2D] can be produced by a variety of tissues, the processing to the active form by 1α-hydroxylase mainly takes place in the kidneys (12). The latter step is controlled by endocrine pathways; in particular, 1α-hydroxylase is activated by parathyroid hormone and inhibited by c-terminal fibroblast growth factor 23, calcium, and phosphorus (13). 25(OH)D is degraded mainly to the inactive metabolite 24,25-dihydroxy-vitamin D [24,25(OH)2D] and 1,25(OH)2D is degraded to 1,24,25-trihydroxy-vitamin D (12,14).
Recent genome–wide association studies have uncovered associations of vitamin D levels with proteins involved in the synthesis, transport, and metabolism of 25(OH)D: 7-dehydrocholesterol reductase (DCHR7), an enzyme reducing the vitamin D3 precursor 7-dehydroxycholesterol to cholesterol; 25-hydroxylase (CYP2R1), the enzyme converting vitamin D3 to 25(OH)D; and vitamin D binding protein (DBP or GC) (15). Single-nucleotide polymorphisms (SNPs) in the genes encoding for DHCR7, CYP2R1, and DBP have been linked to vitamin D deficiency in the general population (15,16) and the risk of developing vitamin D deficiency–associated diseases, including bone fractures (17), malignancy (18), and autoimmune disease (19).
In this study, we comprehensively analyzed the status of vitamin D metabolites in the largest pediatric CKD cohort studied to date, encompassing a wide variation of geographic distribution, nutritional and cultural habits, and vitamin D supplementation practices. Our study focused on the interaction of genetic and environmental factors in the setting of CKD.
Materials and Methods
Patients and Study Design
Vitamin D metabolite levels [25(OH)D, 1,25(OH)2D, and 24,25(OH)2D] were measured in children and adolescents between 6 and 18 years old with CKD stage 3–5 before dialysis or transplantation. All patients were participants of the observational Cardiovascular Comorbidity in Children with Chronic Kidney Disease Study and enrolled at 55 pediatric nephrology centers in 12 European countries (20). Only patients with a complete vitamin D status and genetic information were included in this cross-sectional analysis. By these criteria, 500 of 704 active patients at the baseline visit were eligible for analysis. The study was approved by all local ethical committees and has been performed in accordance with the principles of the Helsinki Declaration. Written informed consent was given by the parents and adolescents, and oral assent was given by younger children.
For this study, clinical information and biospecimens collected at study entry (between September of 2009 and July of 2011) were analyzed.
Documentation and Definitions
The primary renal diagnoses were categorized as congenital anomalies of the kidneys and urinary tract, glomerulopathies, CKD after AKI, inflammatory disorders, metabolic disorders, and other or unknown The GFR was estimated according to the Schwartz equation on the basis of serum creatinine, cystatin C, and urea (21), and patients were categorized to CKD stages 3a, 3b, 4, and 5 by eGFR ranges of 45–60, 30–44, 15–29, and <15 ml/min per 1.73 m2, respectively.
Genome–wide SNP genotyping was performed by hybridization on Illumina 2.5M Omni chips, with subsequent imputation to 10 million SNPs. Imputation was performed using the public 1000 Genomes data as a reference panel (22,23). For this study, SNPs associated with vitamin D insufficiency in the general population were used as candidate SNPs (15,16,24) (i.e., in the genes encoding for CYP2R1 [rs10741657], DBP [rs7041 and rs2282679], DHCR7 [rs12785878], and 24-hydroxylase [CYP24A1; rs6013897]). Results were obtained from imputed data for rs10741657 and rs12785878.
A detailed medication history was recorded, including the dosage and application modality of vitamin D. Cholecalciferol or ergocalciferol was used for nutritional vitamin D supplementation, and calcitriol, 1-α-calcidiol, or paricalcitol was used as the active vitamin D analog. Patients were considered as vitamin D supplemented if treatment had started at least 1 week before blood sampling.
Regional solar irradiation was estimated from sunlight radiation maps (www.solargis.info) according to geographic location. Season was classified as winter for November through April and summer for May through October according to the month of blood and urine collection. Patient history was recorded, and a physical examination at the time of blood sampling was performed for anthropometry measurements and pubertal status. Vitamin D status was classified as replete with serum 25(OH)D level >30 ng/ml, insufficient at 16–30 ng/ml, deficient at 5–15.9 ng/ml, and severely deficient at <5 ng/ml in accordance with the Kidney Disease Outcomes Quality Initiative (KDOQI) guidelines (25).
Laboratory Techniques
All biochemical analyses were performed centrally. Standard laboratory techniques were used to measure the serum and urinary concentrations of albumin, creatinine, phosphate, and calcium. Intact parathyroid hormone (iPTH) was measured with the Elecsys ECLIA (Roche Diagnostics, Indianapolis, IN), and C–terminal c-terminal fibroblast growth factor was measured with the Immutopics ELISA (San Clemente). Vitamin D metabolites were measured via liquid chromatography-mass spectrometry/mass spectrometry using an Acquity UPLC coupled to a Quattro Premier Mass Spectrometer (Waters, Milford, MA) as described by Aronov et al. (26). The lower limit of quantification was 25 pg/ml for all vitamin D metabolites. The interassay and intra–assay coefficients of variation were ≤25%.
Statistical Analyses
Serum levels of 25(OH)D and 24,25(OH)2D were below the detection limit (DL) in very few samples, for which concentration was assumed to be DL
. For 1,25(OH)2D, approximately 30% of values were below the DL and handled using the distribution–based multiple imputation method (27). Imputation yielded five complete datasets; these were analyzed separately, and the results were pooled. The influence of potential environmental and disease–associated covariates on vitamin D status was analyzed using multivariable linear regression modeling of log–transformed vitamin D levels. Backward variable selection was performed with a threshold of P=0.15.The influence of genetic polymorphisms on vitamin D levels was tested on the basis of the reduced models. The interaction between albuminuria and glomerulopathy on vitamin D levels was tested in multivariate analysis.
Group comparisons were carried out using chi-squared, t, or Mann–Whitney tests in the case of two groups and chi-squared test, ANOVA, or Kruskal–Wallis test in the case of more than two groups as appropriate. For seasonal and geographic differences, nonparametric Hodges–Lehmann estimators with corresponding 95% confidence intervals were calculated. Descriptive P values <0.05 were considered statistically significant. Data were analyzed using SAS software, version 9.2 (SAS Institute Inc., Cary, NC).
Results
Patient Characteristics
Patient characteristics are given in Table 1. Mean age was 12 years old. Ethnicity was white in 94.6% of the patients. Renal hypodysplasia was the underlying diagnosis in 69.4% of patients, glomerulopathies were the underlying diagnosis in 7.6%, tubulointerstitial and metabolic disorders were the underlying diagnosis in 12%, CKD after AKI was the underlying diagnosis in 4%, and other diagnoses were the underlying diagnosis in 7%. The most common glomerulopathy was FSGS (n=16) followed by rapid progressive GN (n=4) and IgA nephropathy (n=4). Blood samples were obtained in the winter season in 57% of the patients. Annual local solar irradiation at the place of residence was <1200 kW/m2 in 25.0%, 1200–1600 kW/m2 in 40.4%, and >1600 kW/m2 in 34.6% of patients. Age, radiation, and the season of blood sampling were distributed similarly across CKD stages. The genotype frequency distribution of the selected SNPs in DHCR7, GC, CYP2R1, and CYP24A1 is given in Table 2.
Table 1.
Patient characteristics
| Parameter | All | CKD Stage | P Value | |||
|---|---|---|---|---|---|---|
| 3a | 3b | 4 | 5 | |||
| N | 500 | 37 | 162 | 265 | 36 | |
| Age, yr | 12.0±3.3 | 12.9±2.8 | 11.9±3.3 | 12.0±3.4 | 12.2±3.1 | 0.36 |
| Boys, % | 64.2 | 70.3 | 67.3 | 62.6 | 55.6 | 0.44 |
| Biochemistry | ||||||
| eGFR, ml/min per 1.73 m2 | 28.2±10.36 | 51.0±3.96 | 35.9±4.19 | 22.4±4.19 | 13.4±1.32 | |
| Albuminuria, g/g creatinine | 0.37 (1.17) | 0.08 (0.21) | 0.22 (0.78) | 0.46 (1.56) | 1.10 (2.39) | <0.001 |
| Serum albumin, g/L | 38.8±6.10 | 38.8±4.86 | 39.3±6.26 | 39.0±6.00 | 35.9±8.58 | 0.20 |
| Serum calcium,a mg/dl | 8.79±0.99 | 8.72±1.14 | 8.82±0.78 | 8.81±1.02 | 8.60±1.39 | 0.73 |
| Serum phosphorus, mg/dl | 4.76±1.11 | 4.57±1.30 | 4.47±1.04 | 4.83±0.96 | 5.75±1.60 | <0.001 |
| Mineral hormones | ||||||
| Plasma iPTH, ng/ml | 116 (127) | 63 (62) | 88 (83) | 148 (143) | 221 (214) | <0.001 |
| Serum cFGF23, kRU/L | 184 (202) | 134 (115) | 129 (138) | 221 (196) | 599 (901) | <0.001 |
| Serum 25(OH)D, ng/ml | 11.1 (11.3) | 10.2 (9.33) | 13.0 (10.2) | 11.0 (11.6) | 8.86 (11.9) | 0.14 |
| Serum 1,25(OH)2D, pg/ml | 70.0 (92.3) | 60.0 (92.3) | 80.0 (100) | 60 (82.3) | 55.0 (72.3) | 0.07 |
| Serum 24,25(OH)2D, ng/ml | 0.39 (0.53) | 0.36 (0.41) | 0.45 (0.55) | 0.38 (0.47) | 0.38 (0.47) | 0.06 |
| Medications | ||||||
| Vitamin D supplements | 56 (11.2%) | 2 (5.4%) | 17 (10.3%) | 29 (10.9%) | 8 (22.2%) | 0.12 |
| Active vitamin D analog | 254 (50.8%) | 14 (37.8%) | 71 (43.8%) | 149 (56.2%) | 20 (55.6%) | 0.03 |
Data are given as means±SDs, medians (interquartile ranges), or n (%) as appropriate. P values resulted from chi-squared tests, ANOVA, or the nonparametric Kruskal–Wallis test. iPTH, intact parathyroid hormone; cFGF23, c-terminal fibroblast growth factor 23; 25(OH)D, 25-hydroxy-vitamin D; 1,25(OH)2D, 1,25-dihydroxy-vitamin D; 24,25(OH)2D, 24,25-dihydroxy-vitamin D.
Corrected for serum albumin concentration.
Table 2.
Genetic and environmental correlates of vitamin D levels
| Patients, % | 25(OH)D, ng/ml | P Value | Vitamin D Deficient, % | 1,25(OH)2D, pg/ml | P Value | 24,25(OH)2D, ng/ml | P Value | |
|---|---|---|---|---|---|---|---|---|
| Environmental correlates | ||||||||
| Region | ||||||||
| Western Europe | 27.4 | 18.1 (14.3) | <0.001 | 41.6 | 70 (85) | 0.004 | 0.72 (0.70) | <0.001 |
| Central Europe | 9.4 | 14.2 (9.0) | 59.6 | 80 (130) | 0.60 (0.59) | |||
| Southern Europe | 7.6 | 13.1 (8.1) | 68.4 | 70 (70) | 0.38 (0.51 | |||
| Turkey | 55.6 | 9.1 (8.4) | 82.7 | 50 (80) | 0.31 (0.32) | |||
| Local solar radiation | ||||||||
| Low, <1200 kW/m2 | 25.0 | 16.0 (15.0) | <0.001 | 51.2 | 76 (40) | <0.001 | 0.67 (0.75) | <0.001 |
| Medium, 1200–1600 kW/m2 | 40.4 | 11.4 (11.0) | 67.3 | 75 (107) | 0.40 (0.57) | |||
| High, >1600 kW/m2 | 34.6 | 9.0 (8.5) | 81.5 | 50 (80) | 0.29 (0.32) | |||
| Glomerulopathy | ||||||||
| Yes | 7.6 | 6.8 (9.4) | <0.001 | 86.8 | 50 (87) | 0.21 | 0.67 (0.71) | 0.17 |
| No | 92.4 | 11.6 (11.3) | 66.7 | 70 (88) | 0.31 (0.32) | |||
| Season | ||||||||
| Winter | 57 | 8.9 (9.3) | <0.001 | 77.9 | 50 (62) | <0.001 | 0.31 (0.39) | <0.001 |
| Summer | 43 | 14.6 (10.9) | 55.4 | 90 (90) | 0.55 (0.53) | |||
| Vitamin D supplementation | ||||||||
| Yes | 11.2 | 21.6 (14.1) | <0.001 | 28.1 | 100 (85) | <0.001 | 0.90 (0.60) | <0.001 |
| No | 88.8 | 10.4 (10.1) | 73.2 | 60 (90) | 0.35 (0.44) | |||
| Genetic correlates | ||||||||
| GC (rs7041) | ||||||||
| Major homozygotes | 36.4 | 12.4 (12.9) | 0.43 | 63.7 | 70 (99) | 0.27 | 0.38 (0.57) | 0.59 |
| Heterozygotes | 45.0 | 11.0 (10.3) | 70.7 | 70 (79) | 0.39 (0.50) | |||
| Minor homozygotes | 18.6 | 10.7 (10.3) | 71.0 | 60 (79) | 0.43 (0.52) | |||
| GC (rs4588) | ||||||||
| Major homozygotes | 54.0 | 11.8 (12.8) | 0.74 | 66.3 | 70 (100) | 0.18 | 0.38 (0.52) | 0.05 |
| Heterozygotes | 37.8 | 10.6 (9.65) | 71.4 | 60 (78) | 0.37 (0.50) | |||
| Minor homozygotes | 8.2 | 12.5 (8.58) | 65.9 | 70 (96) | 0.61 (0.73) | |||
| GC (rs2282679) | ||||||||
| Major homozygotes | 53.8 | 11.5 (12.8) | 0.74 | 66.2 | 70 (100) | 0.21 | 0.38 (0.52) | 0.06 |
| Heterozygotes | 38.0 | 10.7 (10.2) | 71.6 | 60 (78) | 0.38 (0.48) | |||
| Minor homozygotes | 8.2 | 12.5 (10.8) | 65.8 | 70 (96) | 0.61 (0.74) | |||
| CYP2R1 (rs10741657) | ||||||||
| Major homozygotes | 44.6 | 10.6 (9.8) | 0.27 | 72.7 | 70 (87) | 0.70 | 0.37 (0.52) | 0.25 |
| Heterozygotes | 44.0 | 12.8 (11.9) | 65.0 | 60 (89) | 0.4 (0.52) | |||
| Minor homozygotes | 11.4 | 12.8 (14.3) | 63.2 | 50 (82) | 0.45 (0.67) | |||
| DHCR7 (rs12785878) | ||||||||
| Major homozygotes | 45.6 | 12.6 (12.2) | 0.30 | 65.4 | 60 (90) | 0.99 | 0.44 (0.62) | 0.08 |
| Heterozygotes | 40.4 | 11.1 (10.4) | 69.8 | 65 (89) | 0.38 (0.45) | |||
| Minor homozygotes | 14.0 | 9.7 (10.7) | 72.9 | 70 (75) | 0.35 (0.36) | |||
| CYP24A1 (rs6013897) | ||||||||
| Major homozygotes | 63.0 | 11.8 (12.8) | 0.17 | 34.3 | 60 (89) | 0.09 | 0.4 (0.56) | 0.13 |
| Heterozygotes | 29.4 | 10.6 (9.65) | 25.2 | 60 (90) | 0.35 (0.47) | |||
| Minor homozygotes | 7.6 | 12.5 (8.58) | 36.8 | 80 (80) | 0.47 (0.42) |
Concentrations are given as medians and interquartile ranges. P values resulted from the nonparametric Wilcoxon rank sum test (two groups) or the Kruskal–Wallis test (more than two groups). Genetic correlates: GC, vitaminD binding protein; CYP2R1, 25-hydroxylase; DCHR7, 7-dehydrocholesterol reductase; CYP24A1, 24-hydroxylase. 25(OH)D, 25-hydroxy-vitamin D; 1,25(OH)2D, 1,25-dihydroxy-vitamin D; 24,25(OH)2D, 24,25-dihydroxy-vitamin D.
Vitamin D Metabolite Concentrations
Descriptive statistics of the vitamin D metabolite concentrations are given in Tables 1 and 2 and by region of origin in Supplemental Table 1. The vast majority of patients (94.2%) showed 25(OH)D levels below the repletion cutoff (30 ng/ml), and 68.2% were considered 25(OH)D deficient [25(OH)D<16 ng/ml]. Serum 1,25(OH)2D levels were highly correlated with 25(OH)D (r=0.56; P<0.001). In multivariate analysis, 25(OH)D levels showed the strongest association with 1,25(OH)2D [Table 3, correlates of serum 1,25(OH)2D concentration]. Serum 24,25(OH)2D levels correlated with both 25(OH)D (r=0.87; P<0.001) and 1,25(OH)2D concentrations (r=0.55; P<0.001) (Figure 1).
Table 3.
Environmental and genetic correlates of (log–transformed) serum 25-hydroxy-vitamin D, 1,25-dihydroxy-vitamin D, and 24,25-dihydroxy-vitamin D concentrations
| Parameter | Estimate | 95% CI | P Value |
|---|---|---|---|
| Correlates of serum 25(OH)D concentration | |||
| Intercept | 2.48 | 2.25 to 2.72 | <0.001 |
| Age, 10 yr | −0.16 | −0.31 to −0.00 | 0.05 |
| Sex, girl | −0.13 | −0.24 to −0.03 | 0.02 |
| Season, summer | 0.37 | 0.26 to 0.48 | <0.001 |
| Albuminuria, g/g | −0.07 | −0.10 to −0.04 | <0.001 |
| Vitamin D supplementation | 0.61 | 0.44 to 0.79 | <0.001 |
| Region of origin: Turkey | −0.32 | −0.43 to −0.21 | <0.001 |
| Glomerulopathy | −0.45 | −0.65 to −0.25 | <0.001 |
| Glomerulopathy × albuminuria, g/g | −0.02 | −0.03 to −0.00 | 0.02 |
| GC1s-1sa | 0.13 | 0.02 to 0.24 | 0.02 |
| Correlates of serum 1,25(OH)2D concentration | |||
| Intercept | 2.27 | 1.63 to 2.90 | <0.001 |
| 25(OH)D, log | 0.73 | 0.62 to 0.83 | <0.001 |
| Sex, girl | −0.21 | −0.37 to −0.09 | <0.01 |
| Active vitamin D analog | 0.20 | 0.02 to 0.37 | 0.03 |
| Active vitamin D analog × dosage | −0.31 | −0.54 to 0.08 | <0.01 |
| cFGF23, kRU/L log | −0.05 | −0.14 to 0.03 | 0.21 |
| Metabolic | 0.29 | −0.10 to 0.68 | 0.14 |
| Correlates of serum 24,25(OH)2D concentration | |||
| Intercept | −4.11 | −4.55 to −3.66 | <0.001 |
| 1,25(OH)2D, log | 0.06 | 0.03 to 0.10 | <0.001 |
| 25(OH)D, log | 1.04 | 0.99 to 1.10 | <0.001 |
| Serum albumin, 10 g/L | −0.05 | −0.12 to 0.02 | 0.16 |
| Sex, girl | 0.05 | −0.01 to 0.11 | 0.11 |
| eGFR, 10 ml/min per 1.73 m2 | 0.08 | 0.05 to 0.11 | <0.001 |
| cFGF23, kRU/L, log | 0.11 | 0.07 to 0.15 | <0.001 |
| iPTH, ng/ml log | −0.04 | −0.06 to −0.02 | <0.001 |
| Glomerulopathy | 0.78 | 0.23 to 1.33 | <0.01 |
| Albuminuria, g/g | 0.02 | 0.00 to 0.05 | 0.04 |
| Glomerulopathy × albuminuria, g/g | −0.01 | −0.03 to 0.00 | 0.07 |
| Region of origin: Turkey | −0.17 | −0.23 to −0.11 | <0.001 |
| GC rs4588, major allele no. | −0.05 | −0.10 to −0.01 | 0.04 |
95% CI, 95% confidence interval; 25(OH)D, 25-hydroxy-vitamin D; GC, vitamin D binding protein gene; 1,25(OH)2D, 1,25-dihydroxy-vitamin D; cFGF23, c-terminal fibroblast growth factor 23; 24,25(OH)2D, 24,25-dihydroxy-vitamin D; iPTH, intact parathyroid hormone.
Major homozygous for both rs7041 and rs4588.
Figure 1.
Correlation of vitamin D metabolites. 1,25(OH)2D, 1,25-dihydroxy-vitamin D; 24,25(OH)2D, 24,25-dihydroxy-vitamin D; 25(OH)D, 25-hydroxy-vitamin D.
Vitamin D Supplementation
Oral vitamin D supplements were administered in 11.2% of all patients at a median daily dose of 1000 IU/m2 body surface area. Vitamin D supplementation prevalence increased with declining eGFR from 5.4% in CKD stage 3a to 22.2% in CKD stage 5. In the vitamin D–replete group, 41.4% of patients were vitamin D supplemented compared with 21.5% in the insufficient group and 4.7% in the deficient group. Vitamin D supplementation was associated with twofold higher 25(OH)D (P<0.001) and almost threefold higher 24,25(OH)2D levels (P<0.001) (Table 2). Treatment with active vitamin D analogs was associated with significantly higher 1,25(OH)2D (70 [102] ng/ml versus 60 [82] ng/ml; P=0.05) and 24,25(OH)2D (0.41 [0.58] ng/ml versus 0.37 [0.47] ng/ml; P=0.03) levels in univariate analysis. In multivariate analysis, 25(OH)D levels showed a stronger association than treatment with an active vitamin D analog. Patients with a higher dosage of active vitamin D had relatively lower 1,25(OH)2 levels.
Seasonal and Geographic Factors
25(OH)D levels from blood samples collected in the winter were a median of 4.8 ng/ml lower (95% confidence interval, 3.5 to 6.1) compared with those in the summer season. Among nonsupplemented patients, children in regions with solar radiation >1600 kW/m2 per year showed in median 3.8 ng/ml lower 25(OH)D levels compared with those in regions with lower sunlight intensity (95% confidence interval, −5.2 to −2.5; P<0.001) in univariate analysis. In this regard, analysis of 25(OH)D levels by treatment center revealed a highly significant inverse U–shaped relationship: 25(OH)D levels are positively associated with local solar radiation in regions with <1300 kW/m2 per year but sharply declined with higher radiation above this cutoff level (Figure 2). Patients from Turkey exhibited significantly lower 25(OH)D and 24,25(OH)2D levels than those from other European regions, including patients from Italy and Portugal living at comparable geographic latitude (Table 2) (P<0.001). During summer time, nonsupplemented Turkish boys had significantly higher 25(OH)D levels than nonsupplemented Turkish girls, whereas no sex difference was observed in children from the other European regions. During wintertime, no sex differences were evident in any region. In the multivariate analysis of the total cohort, the lower 25(OH)D and 24,25(OH)2D levels of Turkish patients seemed independent of vitamin D supplementation, solar radiation, season, and underlying renal disease.
Figure 2.
25-Hydroxy-vitamin D [25(OH)D] levels for each center by solar radiation. Each circle represents one center and is sized according to patient number per center; 25(OH)D levels are given as median per center.
Renal Diagnosis and Function
25(OH)D levels were significantly lower in patients with glomerulopathies compared with in patients with other diagnoses (P<0.001) (Table 2). This difference was confirmed in the multivariate analysis [Table 3, correlates of serum 25(OH)D concentration, Supplemental Table 2, full model]. There was a significant interaction between albuminuria and glomerulopathies, indicating an even stronger association of albuminuria with 25(OH)D levels in patients with glomerulopathies.
In patients with glomerulopathies, 24,25(OH)2D was relatively higher than in other patients, and albuminuria was positively associated with 24,25(OH)2D levels. In this regard, there was no significant interaction between the presence of a glomerulopathy and the extent of albuminuria (P=0.07) [Table 3, correlates of serum 24,25(OH)2D concentration].
Whereas 25(OH)D and 24,25(OH)2D levels did not differ across CKD stages in the population as a whole (Table 1), significantly lower levels were observed in nonvitamin D–supplemented patients with CKD stage 5 compared with those with stage 3 or 4 for both 25(OH)D (6.99 [5.25] versus 10.7 [10.7]; P<0.01) and 24,25(OH)2D (0.2 [0.19] versus 0.37 [0.46] ng/ml; P<0.003). In the multivariate analysis, eGFR was confirmed as being significantly associated with 24,25(OH)2D but not with 25(OH)D levels.
Endocrine Factors
By univariate analysis, 25(OH)D and 24,25(OH)2D were negatively associated with iPTH (r=−0.24; P<0.001 and r=−0.3; P<0.001, respectively) but not with c-terminal fibroblast growth factor (cFGF23). In the multivariable regression, iPTH was associated with lower 24,25(OH)2D levels, and cFGF23 was associated with higher 24,25(OH)2D levels [Table 3, correlates of serum 24,25(OH)2D concentration, Supplemental Table 3, full model].
Genetic Variants
Vitamin D levels by genetic polymorphisms of the whole cohort are listed descriptively in Table 2, genetic correlates, and another subdivision by region of origin is shown in Supplemental Table 1. Among the variants selected for multivariable analysis in CYP2R1, CYP24A1, GC, and DHCR7, only polymorphisms in the GC gene exhibited a weak association with vitamin D status. Patients carrying one or two minor alleles of rs7041 showed approximately 10% lower 25(OH)D concentrations than major allele homozygotes. In multivariate analysis, 25(OH)D levels were significantly higher for patients who were homozygous for the major allele of both rs7041 and rs4588 of the GC gene when correcting for age, glomerular disease, albuminuria, season, region, and vitamin D supplementation [Table 3, correlates of serum 25(OH)D concentration]. Conversely, 24,25(OH)2D levels were significantly lower for patients with at least one major allele of rs4588 [Table 3, correlates of serum 1,25(OH)2D concentration].
Discussion
This study provides a comprehensive analysis of modifiable and nonmodifiable correlates of the vitamin D status of children with CKD. The study cohort covered a large geographic area with substantial variation of climatic conditions, ethnic composition, nutritional and cultural attitudes, and health care policies. To our knowledge, our study is the first to address the role of common variants in vitamin D–regulating genes on vitamin D status in patients with CKD.
Our study across 55 pediatric nephrology centers impressively shows the high current prevalence of vitamin D deficiency in the European pediatric CKD population. Only one in 17 children had a 25(OH)D level considered optimal according to the KDOQI definition (25), and two thirds of the cohort were classified as deficient. Our findings confirm previous reports of the prevalence of vitamin D insufficiency and deficiency in children with CKD ranging between 16% and 82% (6,7,9,10).
CKD
Conflicting evidence has been reported regarding a potentially increasing prevalence of vitamin D deficiency with advancing CKD (6,9,28,29). When adjusting for genetic, environmental, and supplementation variables, we did not observe a major association of renal function on 25(OH)D levels, suggesting that disease-associated factors other than declining eGFR per se contribute to the high prevalence of vitamin D deficiency in CKD.
Vitamin D Supplementation
Patients with vitamin D supplementation had twice as high serum 25(OH)D levels and a 70% lower prevalence of vitamin D deficiency. The positive association of supplementation was independent of age and the severity of renal failure. This observation is in keeping with the results of controlled pediatric trials in children with mild to moderate CKD (7,30), whereas previous observational studies, probably because of their smaller sample size, had been unable to link supplementation to vitamin D levels (6,9,29). However, on the background of vitamin D deficiency in 73% of nonsupplemented patients, the overall supplementation rate of 11% seems inappropriately low.
Season and Ethnic Factors
Although lower vitamin D levels during the winter season were expected, the relationship of vitamin D concentrations with the region of origin was surprising. This result prevailed in the multivariate analysis and when only patients without vitamin D supplementation were analyzed. We speculate that cultural factors, such as clothing habits with higher skin cover in girls, and genetic diversity contribute to this finding. Previous studies in Turkish children found sex differences in 25(OH)D levels and associations to concealing clothing in healthy children (31–33). An additional factor may be the slightly darker skin among Turkish patients compared with central European populations, because skin pigmentation has been shown to negatively influence ultraviolet B–induced vitamin D synthesis (34). In Europe and Turkey, dairy milk products usually are not fortified with vitamin D. Therefore, the main dietary sources of vitamin D are fish products. Surveys of the general population actually show high disparities in the daily fish consumption in Europe and Turkey, with a significantly lower consumption of fish in Turkey (about one third of the consumption in Europe). Southern European countries (e.g., Italy and Greece) are reported to have higher rates of fish consumption compared with northern regions (Germany and The Netherlands) (35,36). Although this could further explain the very low vitamin D levels in Turkey, it may play a pivotal role in the explanation of the U-shaped curve of vitamin D levels throughout Europe and Turkey.
Albuminuria and Glomerulopathies
Our observation of the association between glomerulopathies and low 25(OH)D levels confirms the findings of previous studies (6,29). Loss of DBP in patients who are proteinuric has been assumed as the likely underlying mechanism (29). Notably, 24,25(OH)2D levels were relatively higher in patients with glomerulopathies and positively correlated with albuminuria. Lower 25(OH)D levels in patients with glomerulopathies may, therefore, partially be explained by more rapid degradation of 25(OH)D secondary to DBP loss and thus, reduced vitamin D binding capacity in patients with gross proteinuria (29).
24,25(OH)2D
We observed strong independent associations of both 25(OH)D and 1,25(OH)2D with 24,25(OH)2D levels, confirming the role of 1,25(OH)2D as the main inducer of CYP24A1. In addition, iPTH and cFGF23 showed independent opposite associations with 24,25(OH)2D, pointing to their important regulatory roles in vitamin D metabolism. The positive, highly significant relationship of cFGF23 with 24,25(OH)2D levels supports its role as an antagonist to 1,25(OH)2D (29,37,38) and agonist of CYP24A1, serving to protect the organism from calcium overload (39). However, in the literature, evidence is not consistent in this regard (40).
Finally, although 25(OH)D levels were largely independent of eGFR, we noted an independent association of borderline significance of 24,25(OH)2D with eGFR, similar to observations in both the pediatric and adult CKD population (29,41).
Genetic Influence
We systematically studied the role of common genetic variants on vitamin D status in our pediatric CKD cohort. Common variants in GC are associated with lower DBP serum levels in the general population (16,42,43). In this cohort, only a weak association of lower 25(OH)D levels and higher 24,25(OH)2D with a polymorphism in the GC gene was found.
Additional polymorphisms (e.g., in genes involved in cholesterol synthesis or hydroxylation of vitamin D metabolites) have been associated with vitamin D deficiency (15,24). However, contrary to these findings in healthy cohorts, we could not verify the influence of the described polymorphisms in our cohort. This may partly be attributed to the fact that our cohort was an ethnically rather homogenous group, and differences were not sufficiently great to detect genetic variabilities. We hypothesize that, in this cohort, environmental and disease-associated factors are more relevant in a cohort of CKD and may render negligible the influence of small genetic variations.
Although the large cohort assessed in a wide geographic area and the first exploration of the role of common genetic variants on vitamin D metabolism in pediatric CKD are the most important strengths of this study, our work is limited by its cross–sectional, descriptive character, which precludes demonstration of cause and effect relationships. Also, in the absence of DBP measurements, we can only speculate about genotype–dependent quantitative and/or qualitative differences in DBP and their effects on vitamin D status in children with different types of kidney disease and stages of CKD. Because the participants of this study were mainly white, the results should not be generalized to other ethnic groups, although our data warrant further investigation in other ethnic populations.
In conclusion, we present a unique comprehensive study describing the vitamin D status and its correlates in children with CKD. Vitamin D deficiency is widespread in European children with CKD, and current supplementation practices are only partially efficient in repleting vitamin D stores. Vitamin D levels are influenced by a complex interplay of environmental, disease- and treatment-associated, and ethnic and genetic factors. Vitamin D levels should be monitored, and supplementation practices should be reviewed and adapted accordingly.
Disclosures
None.
Supplementary Material
Acknowledgments
This study has been made possible by grants from the Kuratorium für Heimdialyse Foundation for Preventive Medicine, the European Renal Association–European Dialysis and Transplant Association (www.era-edta.org), the German Federal Ministry of Education and Research (reference no. 01EO0802), and Pfizer Deutschland GmbH. M.W. and A.K. were supported by the Emmy Noether Programme of the German Research Foundation (grant KO3598/2-1 to A.K.). Genotyping of the Cardiovascular Comorbidity in Children with Chronic Kidney Disease (4C) Study cohort was supported by National Institutes for Diabetes and Digestive and Kidney Diseases grant R01DK082394.
An abstract of this work was presented at the Kidney Week of the American Society of Nephrology in Philadelphia, PA on November 7, 2014.
A list of the investigators contributing to the 4C Study is in the Supplemental Appendix.
Footnotes
Published online ahead of print. Publication date available at www.cjasn.org.
This article contains supplemental material online at http://cjasn.asnjournals.org/lookup/suppl/doi:10.2215/CJN.10210915/-/DCSupplemental.
References
- 1.Holick MF: Vitamin D deficiency. N Engl J Med 357: 266–281, 2007 [DOI] [PubMed] [Google Scholar]
- 2.Bischoff-Ferrari H: Health effects of vitamin D. Dermatol Ther (Heidelb) 23: 23–30, 2010 [DOI] [PubMed] [Google Scholar]
- 3.Ovesen L, Andersen R, Jakobsen J: Geographical differences in vitamin D status, with particular reference to European countries. Proc Nutr Soc 62: 813–821, 2003 [DOI] [PubMed] [Google Scholar]
- 4.Gordon CM, DePeter KC, Feldman HA, Grace E, Emans SJ: Prevalence of vitamin D deficiency among healthy adolescents. Arch Pediatr Adolesc Med 158: 531–537, 2004 [DOI] [PubMed] [Google Scholar]
- 5.Weng FL, Shults J, Leonard MB, Stallings VA, Zemel BS: Risk factors for low serum 25-hydroxyvitamin D concentrations in otherwise healthy children and adolescents. Am J Clin Nutr 86: 150–158, 2007 [DOI] [PubMed] [Google Scholar]
- 6.Mansbach JM, Ginde AA, Camargo CA Jr.: Serum 25-hydroxyvitamin D levels among US children aged 1 to 11 years: Do children need more vitamin D? Pediatrics 124: 1404–1410, 2009 [DOI] [PMC free article] [PubMed] [Google Scholar]
- 7.Kalkwarf HJ, Denburg MR, Strife CF, Zemel BS, Foerster DL, Wetzsteon RJ, Leonard MB: Vitamin D deficiency is common in children and adolescents with chronic kidney disease. Kidney Int 81: 690–697, 2012 [DOI] [PMC free article] [PubMed] [Google Scholar]
- 8.Hari P, Gupta N, Hari S, Gulati A, Mahajan P, Bagga A: Vitamin D insufficiency and effect of cholecalciferol in children with chronic kidney disease. Pediatr Nephrol 25: 2483–2488, 2010 [DOI] [PubMed] [Google Scholar]
- 9.Ali FN, Arguelles LM, Langman CB, Price HE: Vitamin D deficiency in children with chronic kidney disease: Uncovering an epidemic. Pediatrics 123: 791–796, 2009 [DOI] [PubMed] [Google Scholar]
- 10.Stein DR, Feldman HA, Gordon CM: Vitamin D status in children with chronic kidney disease. Pediatr Nephrol 27: 1341–1350, 2012 [DOI] [PMC free article] [PubMed] [Google Scholar]
- 11.Belostotsky V, Mughal MZ, Berry JL, Webb NJA: Vitamin D deficiency in children with renal disease. Arch Dis Child 93: 959–962, 2008 [DOI] [PubMed] [Google Scholar]
- 12.Bikle DD: Vitamin D metabolism, mechanism of action, and clinical applications. Chem Biol 21: 319–329, 2014 [DOI] [PMC free article] [PubMed] [Google Scholar]
- 13.Quarles LD: Role of FGF23 in vitamin D and phosphate metabolism: Implications in chronic kidney disease. Exp Cell Res 318: 1040–1048, 2012 [DOI] [PMC free article] [PubMed] [Google Scholar]
- 14.DeLuca HF: Evolution of our understanding of vitamin D. Nutr Rev 66[Suppl 2]: S73–S87, 2008 [DOI] [PubMed] [Google Scholar]
- 15.Wang TJ, Zhang F, Richards JB, Kestenbaum B, van Meurs JB, Berry D, Kiel DP, Streeten EA, Ohlsson C, Koller DL, Peltonen L, Cooper JD, O’Reilly PF, Houston DK, Glazer NL, Vandenput L, Peacock M, Shi J, Rivadeneira F, McCarthy MI, Anneli P, de Boer IH, Mangino M, Kato B, Smyth DJ, Booth SL, Jacques PF, Burke GL, Goodarzi M, Cheung C-L, Wolf M, Rice K, Goltzman D, Hidiroglou N, Ladouceur M, Wareham NJ, Hocking LJ, Hart D, Arden NK, Cooper C, Malik S, Fraser WD, Hartikainen AL, Zhai G, Macdonald HM, Forouhi NG, Loos RJ, Reid DM, Hakim A, Dennison E, Liu Y, Power C, Stevens HE, Jaana L, Vasan RS, Soranzo N, Bojunga J, Psaty BM, Lorentzon M, Foroud T, Harris TB, Hofman A, Jansson JO, Cauley JA, Uitterlinden AG, Gibson Q, Järvelin MR, Karasik D, Siscovick DS, Econs MJ, Kritchevsky SB, Florez JC, Todd JA, Dupuis J, Hyppönen E, Spector TD: Common genetic determinants of vitamin D insufficiency: A genome-wide association study. Lancet 376: 180–188, 2010 [DOI] [PMC free article] [PubMed] [Google Scholar]
- 16.Ahn J, Yu K, Stolzenberg-Solomon R, Simon KC, McCullough ML, Gallicchio L, Jacobs EJ, Ascherio A, Helzlsouer K, Jacobs KB, Li Q, Weinstein SJ, Purdue M, Virtamo J, Horst R, Wheeler W, Chanock S, Hunter DJ, Hayes RB, Kraft P, Albanes D: Genome-wide association study of circulating vitamin D levels. Hum Mol Genet 19: 2739–2745, 2010 [DOI] [PMC free article] [PubMed] [Google Scholar]
- 17.Trummer O, Schwetz V, Walter-Finell D, Lerchbaum E, Renner W, Gugatschka M, Dobnig H, Pieber TR, Obermayer-Pietsch B: Allelic determinants of vitamin d insufficiency, bone mineral density, and bone fractures. J Clin Endocrinol Metab 97: E1234–E1240, 2012 [DOI] [PubMed] [Google Scholar]
- 18.Lange CM, Miki D, Ochi H, Nischalke H-D, Bojunga J, Bibert S, Morikawa K, Gouttenoire J, Cerny A, Dufour J-F, Gorgievski-Hrisoho M, Heim MH, Malinverni R, Müllhaupt B, Negro F, Semela D, Kutalik Z, Müller T, Spengler U, Berg T, Chayama K, Moradpour D, Bochud P-Y Hiroshima Liver Study Group Swiss Hepatitis C Cohort Study Group : Genetic analyses reveal a role for vitamin D insufficiency in HCV-associated hepatocellular carcinoma development. PLoS One 8: e64053, 2013 [DOI] [PMC free article] [PubMed] [Google Scholar]
- 19.Cooper JD, Smyth DJ, Walker NM, Stevens H, Burren OS, Wallace C, Greissl C, Ramos-Lopez E, Hyppönen E, Dunger DB, Spector TD, Ouwehand WH, Wang TJ, Badenhoop K, Todd JA: Inherited variation in vitamin D genes is associated with predisposition to autoimmune disease type 1 diabetes. Diabetes 60: 1624–1631, 2011 [DOI] [PMC free article] [PubMed] [Google Scholar]
- 20.Querfeld U, Anarat A, Bayazit AK, Bakkaloglu AS, Bilginer Y, Caliskan S, Civilibal M, Doyon A, Duzova A, Kracht D, Litwin M, Melk A, Mir S, Sözeri B, Shroff R, Zeller R, Wühl E, Schaefer F 4C Study Group : The Cardiovascular Comorbidity in Children with Chronic Kidney Disease (4C) study: Objectives, design, and methodology. Clin J Am Soc Nephrol 5: 1642–1648, 2010 [DOI] [PMC free article] [PubMed] [Google Scholar]
- 21.Schwartz GJ, Muñoz A, Schneider MF, Mak RH, Kaskel F, Warady BA, Furth SL: New equations to estimate GFR in children with CKD. J Am Soc Nephrol 20: 629–637, 2009 [DOI] [PMC free article] [PubMed] [Google Scholar]
- 22.Wuttke M, Schaefer F, Wong CS, Köttgen A: Genome-wide association studies in nephrology: Using known associations for data checks. Am J Kidney Dis 65: 217–222, 2015 [DOI] [PMC free article] [PubMed] [Google Scholar]
- 23.Gupta J, Kanetsky PA, Wuttke M, Köttgen A, Schaefer F, Wong CS: Genome-wide association studies in pediatric chronic kidney disease [published online ahead of print October 21, 2015]. Pediatr Nephrol [DOI] [PMC free article] [PubMed] [Google Scholar]
- 24.Cheung C-L, Lau K-S, Sham P-C, Tan KCB, Kung AWC: Genetic variant in vitamin D binding protein is associated with serum 25-hydroxyvitamin D and vitamin D insufficiency in southern Chinese. J Hum Genet 58: 749–751, 2013 [DOI] [PubMed] [Google Scholar]
- 25.KDOQI Work Group: KDOQI Clinical Practice Guideline for Nutrition in Children with CKD: 2008 update. Executive summary. Am J Kidney Dis 53[3 Suppl 2]: S11–S104, 2009 [DOI] [PubMed]
- 26.Aronov PA, Hall LM, Dettmer K, Stephensen CB, Hammock BD: Metabolic profiling of major vitamin D metabolites using Diels-Alder derivatization and ultra-performance liquid chromatography-tandem mass spectrometry. Anal Bioanal Chem 391: 1917–1930, 2008 [DOI] [PMC free article] [PubMed] [Google Scholar]
- 27.Baccarelli A, Pfeiffer R, Consonni D, Pesatori AC, Bonzini M, Patterson DG Jr., Bertazzi PA, Landi MT: Handling of dioxin measurement data in the presence of non-detectable values: Overview of available methods and their application in the Seveso chloracne study. Chemosphere 60: 898–906, 2005 [DOI] [PubMed] [Google Scholar]
- 28.Seeherunvong W, Abitbol CL, Chandar J, Zilleruelo G, Freundlich M: Vitamin D insufficiency and deficiency in children with early chronic kidney disease. J Pediatr 154: 906–911.e1, 2009 [DOI] [PubMed] [Google Scholar]
- 29.Denburg MR, Kalkwarf HJ, de Boer IH, Hewison M, Shults J, Zemel BS, Stokes D, Foerster D, Laskin B, Ramirez A, Leonard MB: Vitamin D bioavailability and catabolism in pediatric chronic kidney disease. Pediatr Nephrol 28: 1843–1853, 2013 [DOI] [PMC free article] [PubMed] [Google Scholar]
- 30.Shroff R, Wan M, Gullett A, Ledermann S, Shute R, Knott C, Wells D, Aitkenhead H, Manickavasagar B, van’t Hoff W, Rees L: Ergocalciferol supplementation in children with CKD delays the onset of secondary hyperparathyroidism: A randomized trial. Clin J Am Soc Nephrol 7: 216–223, 2012 [DOI] [PubMed] [Google Scholar]
- 31.Hatun S, Islam O, Cizmecioglu F, Kara B, Babaoglu K, Berk F, Gökalp AS: Subclinical vitamin D deficiency is increased in adolescent girls who wear concealing clothing. J Nutr 135: 218–222, 2005 [DOI] [PubMed] [Google Scholar]
- 32.Aypak C, Türedi O, Yüce A: The association of vitamin D status with cardiometabolic risk factors, obesity and puberty in children. Eur J Pediatr 173: 367–373, 2014 [DOI] [PubMed] [Google Scholar]
- 33.Karagüzel G, Dilber B, Çan G, Ökten A, Değer O, Holick MF: Seasonal vitamin D status of healthy schoolchildren and predictors of low vitamin D status. J Pediatr Gastroenterol Nutr 58: 654–660, 2014 [DOI] [PubMed] [Google Scholar]
- 34.Libon F, Cavalier E, Nikkels AF: Skin color is relevant to vitamin D synthesis. Dermatology 227: 250–254, 2013 [DOI] [PubMed] [Google Scholar]
- 35.Welch AA, Lund E, Amiano P, Dorronsoro M, Brustad M, Kumle M, Rodriguez M, Lasheras C, Janzon L, Jansson J, Luben R, Spencer EA, Overvad K, Tjønneland A, Clavel-Chapelon F, Linseisen J, Klipstein-Grobusch K, Benetou V, Zavitsanos X, Tumino R, Galasso R, Bueno-De-Mesquita HB, Ocké MC, Charrondière UR, Slimani N: Variability of fish consumption within the 10 European countries participating in the European Investigation into Cancer and Nutrition (EPIC) study. Public Health Nutr 5: 1273–1285, 2002 [DOI] [PubMed] [Google Scholar]
- 36.Can MF, Günlü A, Can HY: Fish consumption preferences and factors influencing it. Food Sci Technol Int 35: 339–346, 2015 [Google Scholar]
- 37.Jones G, Prosser DE, Kaufmann M: 25-Hydroxyvitamin D-24-hydroxylase (CYP24A1): Its important role in the degradation of vitamin D. Arch Biochem Biophys 523: 9–18, 2012 [DOI] [PubMed] [Google Scholar]
- 38.Shimada T, Urakawa I, Isakova T, Yamazaki Y, Epstein M, Wesseling-Perry K, Wolf M, Salusky IB, Jüppner H: Circulating fibroblast growth factor 23 in patients with end-stage renal disease treated by peritoneal dialysis is intact and biologically active. J Clin Endocrinol Metab 95: 578–585, 2010 [DOI] [PMC free article] [PubMed] [Google Scholar]
- 39.Shalhoub V, Shatzen EM, Ward SC, Davis J, Stevens J, Bi V, Renshaw L, Hawkins N, Wang W, Chen C, Tsai M-M, Cattley RC, Wronski TJ, Xia X, Li X, Henley C, Eschenberg M, Richards WG: FGF23 neutralization improves chronic kidney disease-associated hyperparathyroidism yet increases mortality. J Clin Invest 122: 2543–2553, 2012 [DOI] [PMC free article] [PubMed] [Google Scholar]
- 40.Dai B, David V, Alshayeb HM, Showkat A, Gyamlani G, Horst RL, Wall BM, Quarles LD: Assessment of 24,25(OH)2D levels does not support FGF23-mediated catabolism of vitamin D metabolites. Kidney Int 82: 1061–1070, 2012 [DOI] [PMC free article] [PubMed] [Google Scholar]
- 41.Bosworth CR, Levin G, Robinson-Cohen C, Hoofnagle AN, Ruzinski J, Young B, Schwartz SM, Himmelfarb J, Kestenbaum B, de Boer IH: The serum 24,25-dihydroxyvitamin D concentration, a marker of vitamin D catabolism, is reduced in chronic kidney disease. Kidney Int 82: 693–700, 2012 [DOI] [PMC free article] [PubMed] [Google Scholar]
- 42.Wang L, Song Y, Manson JE, Pilz S, März W, Michaëlsson K, Lundqvist A, Jassal SK, Barrett-Connor E, Zhang C, Eaton CB, May HT, Anderson JL, Sesso HD: Circulating 25-hydroxy-vitamin D and risk of cardiovascular disease: A meta-analysis of prospective studies. Circ Cardiovasc Qual Outcomes 5: 819–829, 2012 [DOI] [PMC free article] [PubMed] [Google Scholar]
- 43.Powe CE, Evans MK, Wenger J, Zonderman AB, Berg AH, Nalls M, Tamez H, Zhang D, Bhan I, Karumanchi SA, Powe NR, Thadhani R: Vitamin D-binding protein and vitamin D status of black Americans and white Americans. N Engl J Med 369: 1991–2000, 2013 [DOI] [PMC free article] [PubMed] [Google Scholar]
Associated Data
This section collects any data citations, data availability statements, or supplementary materials included in this article.