Insulin resistance indices are inversely associated with vitamin D binding protein concentrations

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

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

Performance of urinary vitamin D-binding protein in diabetic kidney disease: a meta-analysis

Objective: To systematically evaluate the correlation between urinary vitamin D-binding protein (VDBP) and diabetic kidney disease and to evaluate the relationship between urinary VDBP and the albumin-to-creatinine ratio (ACR), renal function indicators [estimate glomerular filtration rate (eGFR), creatinine (CR), blood urea nitrogen (BUN)] and glycaemic control indices [glycated hemoglobin (HbA1c), fasting plasma glucose (FPG)].Methods: We searched the CNKI, Wanfang, VIP, CBM, PubMed, Cochrane Library, Embase and Web of Science databases up to May 31, 2023, for relevant literature. RevMan 5.3 software was used for the meta-analysis.Results: Ultimately, 9 articles were included. Due to heterogeneity in the pooled results, the random-effects model was chosen. Meta-analysis results showed that the urinary VDBP concentrations in the normal albuminuria diabetes group were significantly higher than those in the healthy control group [SMD 1.52, 95% CI (0.84, 2.19), p < 0.00001]. The urinary VDBP concentrations in the microalbuminuria diabetes group were significantly higher than those in the normal albuminuria diabetes group [SMD 1.81, 95% CI (1.40, 2.21), p < 0.00001]. The urinary VDBP concentrations in the macroalbuminuria diabetes group were also significantly higher than those in the microalbuminuria diabetes group [SMD 1.51, 95% CI (1.05, 1.96), p < 0.00001]. In addition, urinary VDBP was positively correlated with the ACR, CR, BUN and HbA1c [Summary r = 0.73, 95% CI (0.54, 0.85), p < 0.0001; Summary r = 0.38, 95% CI (0.10, 0.61), p = 0.009; Summary r = 0.37, 95% CI (0.16, 0.55), p = 0.0008; Summary r = 0.40, 95% CI (0.13, 0.62), p = 0.005, respectively] and tended to be negatively correlated with the eGFR [Summary r = -0.64, 95% CI (-0.92, 0.10), p = 0.08] but was not significantly correlated with the FPG [Summary r = 0.16, 95% CI (-0.03, 0.33), p = 0.10]. Sensitivity analysis showed that our pooled results are robust.Conclusion: Urinary VDBP may be used as a novel biomarker for the early diagnosis of DKD and can be used to assess the severity of DKD.

Consistent Inverse Associations of Total, “Bioavailable”, Free, and “Non-Bioavailable” Vitamin D with Incidence of Diabetes among Older Adults with Lower Baseline HbA1c (≤6%) Levels

Background: Serum 25-hydroxyvitamin (25(OH)D) levels are inversely associated with risk of diabetes. The “free hormone hypothesis” suggests potential effects to be mainly related to concentrations of “bioavailable” and free rather than total 25(OH)D. We assessed associations of serum concentrations of vitamin D-binding protein (VDBP), as well as total “bioavailable”, complementary “non-bioavailable”, and free 25(OH)D, with the risk of developing diabetes among non-diabetic older adults in a large population-based cohort study in Germany. Methods: We included 4841 non-diabetic older adults aged 50–75 years at the baseline exam from the ESTHER cohort conducted in Saarland, Germany, in 2001–2002. Concentrations of “bioavailable” and free 25(OH)D were derived from serum concentrations of VDBP, total 25(OH)D, and albumin. Incidence of diabetes was ascertained during up to 14 years of follow-up. Associations were quantified by multivariable Cox proportional hazards regression models with comprehensive confounder adjustment. Results: During a median follow-up of 10.6 years, 837 non-diabetic participants developed diabetes. We observed similar inverse associations with developing diabetes for VDBP (hazard ratio (HR) for lowest versus highest quintile: 1.37, 95% confidence interval (CI): 1.09, 1.72), total 25(OH)D (HR: 1.31, 95% CI: 1.03, 1.66), and “non-bioavailable” 25(OH)D (HR: 1.30, 95% CI: 1.02, 1.65). Associations were smaller and statistically insignificant for “bioavailable” and free 25(OH)D. However, associations of total “non-bioavailable”, “bioavailable”, and free 25(OH)D with incidence of diabetes were much stronger among, and essentially restricted to, participants with lower baseline HbA_1c (≤6%) levels. Conclusions: This large prospective cohort study of older Caucasian adults, in agreement with results from randomized trials and Mendelian randomization studies, supports a protective effect of vitamin D against development of diabetes. The “free hormone theory” may not be relevant in this context. However, our results underline the importance of adequate vitamin D status among those who have not yet shown any sign of impaired glucose tolerance.

Insulin-resistance in paediatric age: Its magnitude and implications

Insulin resistance (IR) is insulin failure in normal plasma levels to adequately stimulate glucose uptake by the peripheral tissues. IR is becoming more common in children and adolescents than before. There is a strong association between obesity in children and adolescents, IR, and the metabolic syndrome components. IR shows marked variation among different races, crucial to understanding the possible cardiovascular risk, specifically in high-risk races or ethnic groups. Genetic causes of IR include insulin receptor mutations, mutations that stimulate autoantibody production against insulin receptors, or mutations that induce the formation of abnormal glucose transporter 4 molecules or plasma cell membrane glycoprotein-1 molecules; all induce abnormal energy pathways and end with the development of IR. The parallel increase of IR syndrome with the dramatic increase in the rate of obesity among children in the last few decades indicates the importance of environmental factors in increasing the rate of IR. Most patients with IR do not develop diabetes mellitus (DM) type-II. However, IR is a crucial risk factor to develop DM type-II in children. Diagnostic standards for IR in children are not yet established due to various causes. Direct measures of insulin sensitivity include the hyperinsulinemia euglycemic glucose clamp and the insulin-suppression test. Minimal model analysis of frequently sampled intravenous glucose tolerance test and oral glucose tolerance test provide an indirect estimate of metabolic insulin sensitivity/resistance. The main aim of the treatment of IR in children is to prevent the progression of compensated IR to decompensated IR, enhance insulin sensitivity, and treat possible complications. There are three main lines for treatment: Lifestyle and behavior modification, pharmacotherapy, and surgery. This review will discuss the magnitude, implications, diagnosis, and treatment of IR in children.

Elevated Plasma Concentrations of Vitamin D-Binding Protein Are Associated with Lower High-Density Lipoprotein and Higher Fat Mass Index in Overweight and Obese Women

(1) Background: Observational studies have established that vitamin D-binding protein (DBP) and 25-hydroxyvitamin D₃ (25(OH)D) concentrations are the major factors affecting the bioavailability of 25(OH)D. It has also been shown that poor 25(OH)D bioavailability elevates the risk of obesity and its related cardio-metabolic disorders. However, the relationship between 25(OH)D and DBP concentrations with cardio-metabolic risk factors in overweight and obese cohorts has not been established. Consequently, we evaluated the association between DBP and 25(OH)D concentrations with lipid profile, blood pressure (BP), and body composition in overweight and obese women. (2) Methods: In this cross-sectional study of 236 overweight and obese women, DBP and 25(OH)D concentrations were measured using an enzyme-linked immunosorbent assay. Body composition was assessed via bioelectrical impedance analysis. Lipid profile and BP were assessed by an auto-analyzer and digital BP monitor, respectively. The associations were examined by multivariate logistic regression. (3) Results: The indicated showed an inverse relationship between DBP and high-density lipoprotein (HDL) (p = 0.010) concentrations (where individuals with higher DBP had lower HDL) which, after adjusting for possible cofounders, remained significant (p = 0.006). Moreover, DBP concentration was positively associated with fat mass index (FMI) after adjustment (p = 0.022). No significant relationships were observed among 25(OH)D and target variables. (4) Conclusions: In conclusion, lower concentrations of HDL and higher values of FMI are associated with higher concentrations of DBP in overweight and obese women. These findings present novel awareness regarding the association of DBP with some metabolic and body composition variables in overweight and obese women. However, a two-way causal relationship between DBP and target variables should be considered.

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.

Association of vitamin D-binding protein and vitamin D3 with insulin and homeostatic model assessment (HOMA-IR) in overweight and obese females

OBJECTIVE

Equivocal association the contribution of 25-hydroxyvitamin D (25(OH)D) and the well-accepted role of vitamin D-binding protein (VDBP) on bioavailability of 25(OH)D or its independent roles, has led to possible association of the VDBP in glucose metabolism. This study was conducted to evaluate the relationships among 25(OH)D, VDBP, glucose/insulin metabolism and homeostatic model assessment (HOMA-IR). Blood samples were collected from 236 obese and overweight women. VDBP and 25(OH)D levels, and biochemical parameters were measured using an enzyme-linked immunosorbent assay (ELISA). An impedance fat analyzer was utilized to acquire the body composition.

RESULTS

Using the multivariate linear regression, a reverse relationship was observed between VDBP and (HOMA-IR), such that women with higher VDBP displayed lower insulin resistance. The relationship was independent of age, body mass index, standardized energy intake and physical activity (p = 0.00). No significant relationship between 25(OH)D levels, FBS, body composition or insulin resistance were observed (p > 0.2). Current study observed that higher level of VDBP may be associated with lower levels of insulin and HOMA-IR, thus the evaluation of VDBP in diverse population groups seems to have significant clinical value in evaluating the prevalence of DM or early stage of glucose intolerance.

Hyperketonemia GWAS and parity-dependent SNP associations in Holstein dairy cows intensively sampled for blood β-hydroxybutyrate concentration

Hyperketonemia (HYK) is a metabolic disorder that affects early postpartum dairy cows; however, there has been limited success in identifying genomic variants contributing to HYK susceptibility. We conducted a genome-wide association study (GWAS) using HYK phenotypes based on an intensive screening protocol, interrogated genotype interactions with parity group (GWIS), and evaluated the enrichment of annotated metabolic pathways. Holstein cows were enrolled into the experiment after parturition, and blood samples were collected at four timepoints between 5 and 18 days postpartum. Concentration of blood β-hydroxybutyrate (BHB) was quantified cow-side via a handheld BHB meter. Cows were labeled as a HYK case when at least one blood sample had BHB ≥ 1.2 mmol/L, and all other cows were considered non-HYK controls. After quality control procedures, 1,710 cows and 58,699 genotypes were available for further analysis. The GWAS and GWIS were performed using the forward feature select linear mixed model method. There was evidence for an association between ARS-BFGL-NGS-91238 and HYK susceptibility, as well as parity-dependent associations to HYK for BovineHD0600024247 and BovineHD1400023753. Candidate genes annotated to these single nuclear polymorphism associations have been previously associated with obesity, diabetes, insulin resistance, and fatty liver in humans and rodent models. Enrichment analysis revealed focal adhesion and axon guidance as metabolic pathways contributing to HYK etiology, while genetic variation in pathways related to insulin secretion and sensitivity may affect HYK susceptibility in a parity-dependent matter. In conclusion, the present work proposes several novel marker associations and metabolic pathways contributing to genetic risk for HYK susceptibility.

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.

Vitamin D binding protein is related to cardiac autonomic function and metabolic status in prediabetes

A putative causal relationship between vitamin D status and glucose metabolism and a direct effect of vitamin D on cardiac autonomic function (CAF) have been suggested. We hypothesized that vitamin D binding protein (DBP), as a transporter of vitamin D, might also influence CAF and the overall metabolic risk. The present study aims to assess the relationship between DBP and CAF and metabolic status in a high-risk population with prediabetes. A total of 174 subjects (mean age 49.1±12.9 years, mean body mass index 30.2±6.2 kg/m²) were divided into 2 groups according to glucose tolerance: 48 with normal glucose tolerance and 126 with prediabetes. Glucose tolerance was assessed during oral glucose tolerance test, applying 2006 World Health Organization criteria. Fasting and postload glucose and immunoreactive insulin were measured (homeostatic model assessment of insulin resistance and homeostatic model assessment of β-cell function were calculated). Anthropometric indexes, blood pressure, hemoglobin A1c, creatinine, lipids, high-sensitivity C-reactive protein, total 25-hydroxyvitamin D, DBP (free 25-hydroxyvitamin D was calculated), and intact parathormone were measured. Body composition was estimated by impedance analysis (InBody 720), whereas tissue advanced glycation end products were assessed by skin autofluorescence (AGE Reader, DiagnOptics, the Netherlands). CAF was evaluated by АNX-3.0 system, applying standard autonomic tests. DBP was found to be elevated in women, as well as in the presence of cardiac autonomic dysfunction and metabolic syndrome. DBP was related to parasympathetic activity in both sexes and in prediabetes; to body fat in women and in prediabetes; and to age and high-density lipoprotein cholesterol in men. Vitamin D deficiency was established in 40.8% of the studied cohort. These results support the hypothesis that DBP is associated with CAF and some metabolic parameters in prediabetes.

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

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

Prevalence of vitamin D deficiency in women from southern Brazil and association with vitamin D-binding protein levels and GC-DBP gene polymorphisms

Vitamin D deficiency is highly prevalent worldwide, and vitamin D-binding protein (DBP) a major regulator of serum vitamin D levels. The rs4588 and rs7041 polymorphisms of the GC gene constitute the genetic basis of the three major isoforms of circulating DBP (GC1s, GC1f, and GC2), while the rs2282679 variant is located in an important regulatory region of the GC gene. The aim of this study was to assess the prevalence of 25-hydroxyvitamin D [25(OH)D] deficiency and to ascertain whether it is associated with DBP levels and with GC gene variants. Biorepository samples of 443 women aged 20 to 72 years, with no evidence of clinical disease, were analyzed. Circulating levels of 25(OH)D were considered sufficient if ≥20 ng/mL and deficient if <20 ng/mL. Genotype analysis was performed by RT-PCR. Mean age was 53.4±9.4 years; mean BMI was 27.8±5.8 kg/m2. The overall sample had mean 25(OH)D levels of 22.8±8.3 ng/mL; 39.7% of participants had deficient circulating 25(OH)D levels. Higher prevalence ratios (PR) of 25(OH)D deficiency were found for the CC genotype of rs2282679 (PR 1.74; 95%CI 1.30 to 2.24; p<0.001), GC2 isoform (PR 1.66; 95%CI 1.17 to 2.38; p = 0.005), time since menopause (PR 1.02; 95%CI 1.003 to 1.03, p = 0.016), and HOMA-IR (PR 1.02; 95%CI 1.01 to 1.03, p = 0.004). DBP levels (per 30 μg/mL increase in DBP) were associated with lower PR for 25(OH)D deficiency (PR 0.89; 95%CI 0.80;0.99; p = 0.027). Except for HOMA-IR, these prevalence ratios remained significant after adjustment for age and BMI. In conclusion, the rs2282679 polymorphism and the GC2 isoform of DBP were associated with lower serum DBP levels and with susceptibility to 25(OH)D deficiency in Brazilian women with no evidence of clinical disease.

Upregulation of vitamin D-binding protein is associated with changes in insulin production in pancreatic beta-cells exposed to p,p'-DDT and p,p'-DDE

Persistent organochlorine pollutants (POPs) gradually accumulate in the human organism due to their presence in the environment. Some studies have described a correlation between the level of POPs in the human body and the incidence of diabetes, but we know little about the direct effect of POPs on pancreatic beta-cells. We exposed pancreatic beta-cells INS1E to non-lethal concentrations of p,p′-DDT (1,1′-(2,2,2-Trichloroethane-1,1-diyl)bis(4-chlorobenzene)) and p,p′-DDE (1,1′-(2,2-dichloroethene-1,1-diyl)bis(4-chlorobenzene)) for 1 month, and assessed changes in protein expression and the intracellular insulin level. 2-D electrophoresis revealed 6 proteins with changed expression in cells exposed to p,p′-DDT or p,p′-DDE. One of the detected proteins – vitamin D-binding protein (VDBP) – was upregulated in both cells exposed to p,p′-DDT, and cells exposed to p,p′-DDE. Both exposures to pollutants reduced the intracellular level of insulin mRNA, proinsulin, and insulin monomer; p,p′-DDT also slightly reduced the level of hexameric insulin. Overexpression of VDBP caused by the stable transfection of beta-cells with the gene for VDBP decreased both the proinsulin and hexameric insulin level in beta-cells similarly to the reduction detected in cells exposed to p,p′-DDT. Our data suggest that in the cells exposed to p,p′-DDT and p,p′-DDE, the increased VDBP protein level decreased the proinsulin expression in an unknown mechanism.

Free Vitamin D: Relationship to Insulin Sensitivity and Vascular Health in Youth

OBJECTIVE

To evaluate the relationship of free 25 hydroxy vitamin D [free 25(OH)D] or bioavailable vitamin D (BioD) concentrations to insulin sensitivity and cardiovascular disease risk markers in normal weight and overweight youth.

STUDY DESIGN

Cross-sectional study of 79 adolescents 15.4 ± 0.2 years, 18 normal weight, 30 overweight, and 31 overweight with prediabetes who underwent peripheral arterial tonometry, dual-energy x-ray absorptiometry, and hyperinsulinemic-euglycemic clamp in subset (n = 71) for determination of reactive hyperemia index (RHI), body composition, and insulin sensitivity. 25(OH)D and vitamin D binding protein were measured; free 25(OH)D and BioD were calculated.

RESULTS

Across tertiles of free 25(OH)D concentrations (4.0 ± 0.2, 7.5 ± 0.3, and 17.0 ± 2.1 pg/mL, P < .001), the group in the lowest tertile had significantly higher percent body fat (37.8 ± 1.1, 35.2 ± 1.5 and 25.3 ± 2.1%, P < .001), lower insulin sensitivity (4.4 ± 0.4, 6.7 ± 1.2, and 8.2 ± 0.9 mg/kg fat-free mass/minute per μu/mL, P = .03), lower RHI (1.42 ± 0.06, 1.54 ± 0.06, and 1.77 ± 0.09, P = .002), higher high-sensitivity C-reactive protein (3.4 ± 0.6, 1.7 ± 0.3, and 1.6 ± 0.4 mg/L, P = .015) compared with the second and third tertiles, respectively. Free 25(OH)D levels were inversely related to percent body fat and high-sensitivity C-reactive protein, and positively related to RHI and insulin sensitivity. The relationships of free 25(OH)D to RHI and to insulin sensitivity were no longer significant after adjusting for %body fat. Similar relationships were observed for BioD.

CONCLUSIONS

Youth with low free 25(OH)D or BioD concentrations have lower insulin sensitivity and worse endothelial function and inflammatory biomarkers compared with those with more sufficient 25(OH)D. However, the effects of vitamin D on these biomarkers may not be independent of the effect of adiposity.

The Association between Vitamin D Deficiency and variants of Vitamin D Binding protein gene among Healthy Iranian Adults

Background: The high prevalence of vitamin D deficiency may be due to both genetic and environment factors. The aim of this study was to demonstrate that vitamin D deficiency may be due to variants of vitamin D binding protein (DBP) among otherwise healthy Iranian adults. Methods: This cross-sectional study was conducted on 265 healthy adults in Tehran. Anthropometric and biochemical parameters were assessed. Dietary vitamin D intake was assessed with a Food Frequency Questionnaire (FFQ), and participant DBP genotypes were determined by polymerase chain reactions - restriction fragment length polymorphism. Results: Significant associations were found between vitamin D status and low-density lipoprotein cholesterol (P < 0.001), total cholesterol (P < 0.001), and fasting blood sugar (P < 0.001), after adjustment for confounder factors. This study demonstrated that "rs7041" gene was associated with vitamin D deficiency (OR = 0.63, β ± SE = -0.46 ± 0.14, P < 0.0001). After considering the "GG" genotype of the "rs7041" polymorphism as a reference, the prevalence of vitamin D deficiency was found to be higher in the individuals with "TT" genotype from the "rs7041" polymorphism. Conclusion: It was found that the prevalence of vitamin D deficiency was higher in individuals with T allele carriers in the "rs7041" polymorphism.

Data Independent Acquisition Mass Spectrometry Can Identify Circulating Proteins That Predict Future Weight Loss with a Diet and Exercise Programme

We investigated biological determinants that would associate with the response to a diet and weight loss programme in impaired glucose regulation (IGR) people using sequential window acquisition of all theoretical fragment ion spectra (SWATH) mass spectrometry (MS), a data acquisition method which complement traditional mass spectrometry-based proteomics techniques. Ten women and 10 men with IGR underwent anthropometric measurements and fasting blood tests. SWATH MS was carried out with subsequent immunoassay of specific peptide levels. After a six-month intervention, 40% of participants lost 3% or more in weight, 45% of patients remained within 3% of their starting weight and 15% increased their weight by 3% or more. Hemoglobin A1c (HbA1C) level was reduced with weight loss with improvements in insulin sensitivity. SWATH MS on pre-intervention samples and subsequent principal component analysis identified a cluster of proteins associated with future weight loss, including insulin-like growth factor-II (IGF-II) and Vitamin D binding protein. Individuals who lost 3% in weight had significantly higher baseline IGF-II levels than those who did not lose weight. SWATH MS successfully discriminated between individuals who were more likely to lose weight and potentially improve their sensitivity to insulin. A higher IGF-II baseline was predictive of success with weight reduction, suggesting that biological determinants are important in response to weight loss and exercise regimes. This may permit better targeting of interventions to prevent diabetes in the future.

Attenuated serum 25-hydroxyvitamin D and vitamin D binding protein associated with cognitive impairment in patients with type 2 diabetes

PURPOSE

Clinical studies suggest that 25-hydroxyvitamin D (25[OH]D) deficiency plays a pivotal role in both type 2 diabetes mellitus (T2DM) and cognitive impairment. However, it is unclear if 25(OH)D deficiency could be a possible cause of cognitive impairment in T2DM. Vitamin-D binding protein (VDBP) acts as a major 25(OH)D transporter. Preclinical study has demonstrated improvement in cognitive function by VDBP via inhibiting synaptic degeneration. The aim of the study was to assess the association between serum 25(OH)D, VDBP and cognitive impairment in T2DM patients.

PATIENTS AND METHODS

In this case-control study, cognitive function was assessed using the Mini-Mental State Examination (MMSE) and serum 25(OH)D and VDBP levels were estimated using ELISA kits.

RESULTS

A total of 88 subjects were included in the study. T2DM patients had lower serum 25(OH)D (p=0.02), VDBP levels (p=0.04) and MMSE scores (p<0.0001) than controls. T2DM patients had higher prevalence of 25(OH)D deficiency and insufficiency, aOR 0.322 (0.128-0.809), p=0.016 and cognitive impairment, aOR 4.405 (1.617-12.002); p=0.004. Cognitive impairment was associated with serum 25(OH)D, aOR 0.131 (0.027-0.638); p=0.014 and VDBP, aOR 1.008 (1.001-1.015), p=0.029. A general linear model showed a significant association of MMSE with serum 25(OH)D (p=0.022).

CONCLUSION

Serum 25(OH)D deficiency and cognitive impairment was higher in T2DM patients. Routine assessment of cognitive function is suggested to prevent further behavioral complications. The association of VDBP and cognitive impairment in T2DM needs further exploration.

Serum Calcitriol Concentrations and Kidney Function Decline, Heart Failure, and Mortality in Elderly Community-Living Adults: The Health, Aging, and Body Composition Study

RATIONALE & OBJECTIVES

Lower 25-hydroxyvitamin D concentrations have been associated with risk for kidney function decline, heart failure, and mortality. However, 25-hydroxyvitamin D requires conversion to its active metabolite, calcitriol, for most biological effects. The associations of calcitriol concentrations with clinical events have not been well explored.

STUDY DESIGN

Case-cohort study.

SETTING & PARTICIPANTS

Well-functioning community-living older adults aged 70 to 79 years at inception who participated in the Health, Aging, and Body Composition (Health ABC) Study.

PREDICTOR

Serum calcitriol measured using positive ion electrospray ionization-tandem mass spectrometry.

OUTCOMES

Major kidney function decline (≥30% decline in estimated glomerular filtration rate from baseline), incident heart failure (HF), and all-cause mortality during 10 years of follow-up.

ANALYTIC APPROACH

Baseline calcitriol concentrations were measured in a random subcohort of 479 participants and also in cases with major kidney function decline [n=397]) and incident HF (n=207) during 10 years of follow-up. Associations of serum calcitriol concentrations with these end points were evaluated using weighted Cox regression to account for the case-cohort design, while associations with mortality were assessed in the subcohort alone using unweighted Cox regression.

RESULTS

During 8.6 years of mean follow-up, 212 (44%) subcohort participants died. In fully adjusted models, each 1-standard deviation lower calcitriol concentration was associated with 30% higher risk for major kidney function decline (95% CI, 1.03-1.65; P=0.03). Calcitriol was not significantly associated with incident HF (HR, 1.16; 95% CI, 0.94-1.47) or mortality (HR, 1.01; 95% CI, 0.81-1.26). We observed no significant interactions between calcitriol concentrations and chronic kidney disease status, baseline intact parathyroid or fibroblast factor 23 concentrations.

LIMITATIONS

Observational study design, calcitriol measurements at a single time point, selective study population of older adults only of white or black race.

CONCLUSIONS

Lower calcitriol concentrations are independently associated with kidney function decline in community-living older adults. Future studies will be needed to clarify whether these associations reflect lower calcitriol concentrations resulting from abnormal kidney tubule dysfunction or direct mechanisms relating lower calcitriol concentrations to more rapid loss of kidney function.

Why should we measure free 25(OH) vitamin D?

Vitamin D, either in its D₂ or D₃ form, is essential for normal human development during intrauterine life, kidney function and bone health. Vitamin D deficiency has also been linked to cancer development and some autoimmune diseases. Given this huge impact of vitamin D on human health, it is important for daily clinical practice and clinical research to have reliable tools to judge on the vitamin D status. The major circulating form of vitamin D is 25-hydroxyvitamin D (25(OH)D), although it is not the most active metabolite, the concentrations of total 25-hydroxyvitamin D in the serum are currently routinely used in clinical practice to assess vitamin D status. In the circulation, vitamin D - like other steroid hormones - is bound tightly to a special carrier - vitamin D-binding protein (DBP). Smaller amounts are bound to blood proteins - albumin and lipoproteins. Only very tiny amounts of the total vitamin D are free and potentially biologically active. Currently used vitamin D assays do not distinguish between the three forms of vitamin D - DBP-bound vitamin D, albumin-bound vitamin D and free, biologically active vitamin D. Diseases or conditions that affect the synthesis of DBP or albumin thus have a huge impact on the amount of circulating total vitamin D. DBP and albumin are synthesized in the liver, hence all patients with an impairment of liver function have alterations in their total vitamin D blood concentrations, while free vitamin D levels remain mostly constant. Sex steroids, in particular estrogens, stimulate the synthesis of DBP. This explains why total vitamin D concentrations are higher during pregnancy as compared to non-pregnant women, while the concentrations of free vitamin D remain similar in both groups of women. The vitamin D-DBP as well as vitamin D-albumin complexes are filtered through the glomeruli and re-uptaken by megalin in the proximal tubule. Therefore, all acute and chronic kidney diseases that are characterized by a tubular damage, are associated with a loss of vitamin D-DBP complexes in the urine. Finally, the gene encoding DBP protein is highly polymorphic in different human racial groups. In the current review, we will discuss how liver function, estrogens, kidney function and the genetic background might influence total circulating vitamin D levels and will discuss what vitamin D metabolite is more appropriate to measure under these conditions: free vitamin D or total vitamin D.

Vitamin D3 supplementation in obese, African-American, vitamin D deficient adolescents

Objectives

Obese, African-American (AA) adolescents are at increased risk for vitamin D deficiency. The primary objective of this pilot study was to examine the effect of vitamin D supplementation upon 25-hydroxy vitamin D (25OHD) levels in obese, AA adolescents.

Methods

A randomized, double-blinded, controlled pilot study included 26 obese (BMI ≥ 95%ile), vitamin D deficient (25OHD < 20 ng/mL), pubertal AA adolescents (ages 12-17). Subjects received cholecalciferol 1000 IU or 5000 IU daily for 3 months. Serum 25OHD, vitamin D binding protein, parathyroid hormone, and cardiometabolic risk markers were obtained at baseline and post-treatment.

Results

Of 39 subjects enrolled, 26 (67%) were vitamin D deficient (mean 25OHD 12.0 ± 3.8 ng/mL) at baseline and were randomized, with 22 completing the study. Sex, age, season, pubertal stage, BMI, insulin resistance (HOMA-IR) and 25OHD were similar at baseline between the 1000 IU and 5000 IU groups. Post-treatment, 25OHD increased less in the 1000 IU group (5.6 ng/mL, p = 0.03) vs. the 5000 IU group (15.6 ng/mL, p = 0.002). 83% of the 5000 IU group and 30% of the 1000 IU group reached post-treatment 25OHD ≥ 20 ng/mL (p = 0.01); 50% of the 5000 IU group, but no subject from the 1000 IU group, achieved 25OHD ≥ 30 ng/mL (p = 0.009). We detected no group differences in mineral metabolites or cardiometabolic risk markers following supplementation.

Conclusions

Cholecalciferol dosing in excess of the current Institute of Medicine dietary reference intakes was required to achieve 25OHD levels ≥20 ng/mL in obese, AA adolescents. Supplementation of 5000 IU may be required to achieve the desired goal.

Vitamin D Deficiency and Supplementation in Cardiovascular Disorders

Cardiovascular disease (CVD) is the leading cause of death in developed countries. Similarly, the frequency of vitamin D deficiency is increasing, and a number of epidemiologic and clinical studies have suggested that there is an increased risk of CVD among people with depletion of this vitamin. This has raised much interest in the potential pathogenic and therapeutic role of vitamin D in CVD. However, randomized trials and meta-analyses have not shown a clear benefit of vitamin D supplementation with respect to cardiovascular events. Herein, we provide a comprehensive review of the most relevant evidence to date regarding vitamin D deficiency and supplementation, and their respective impact on CVD prevention and treatment.

Bioavailable and free 25-hydroxyvitamin D and vitamin D binding protein in polycystic ovary syndrome: Relationships with obesity and insulin resistance

Polycystic ovary syndrome (PCOS) is a common condition characterised by both reproductive and metabolic features (obesity, insulin resistance, diabetes risk). Some evidence suggests that women with PCOS have lower vitamin D levels compared to healthy controls. Vitamin D binding protein (DBP) is the main carrier of vitamin D in circulation and plays an important role in regulating vitamin D concentration and bioavailability for target tissues. To our knowledge, no previous studies have examined DBP, bioavailable and free 25-hydroxyvitamin D (25(OH)D) in women with PCOS. The primary aim of this study was to compare DBP, bioavailable and free 25(OH)D concentrations in women with PCOS and controls. The secondary aim was to investigate relationships between DBP, bioavailable and free 25(OH)D and metabolic features (anthropometric measures, insulin resistance, and lipid profile). In a cross sectional study using bio-banked samples, we measured 25(OH)D, DBP and albumin. Bioavailable and free 25(OH)D were calculated using previously validated formula. BMI, body composition (dual X-ray absorptiometry, DXA), insulin resistance (homeostatic model assessment of insulin resistance (HOMA-IR)) and glucose infusion rate (GIR) from hyperinsulinaemic euglycaemic clamp and serum lipids (ELISA) were also measured in a physically and biochemically well-characterised cohort of women with and without PCOS. We studied 90 women with PCOS and 59 controls aged 18-48 years. DBP concentrations were lower in PCOS compared to controls (median [IQR]: 443.40 [314.4] vs 482.4 [156.8] μg/ml, p=0.02). No significant differences were found in bioavailable or free 25(OH)D concentrations between groups. DBP was not associated with BMI, percent body fat or markers of insulin resistance (all p>0.2). High-density lipoprotein (HDL) was the main determinant of DBP in the overall cohort (β=-0.12, p=0.02), after adjusting for covariates including PCOS/control status, age, BMI, total 25(OH)D and HOMA-IR. In PCOS, total and free 25(OH)D were related to markers of insulin resistance and lipids. Only the associations between free 25(OH)D and triglycerides (p=0.02), and HDL (p=0.03) remained significant after adjusting for age and BMI. In conclusion, women with PCOS had lower DBP, but similar bioavailable or free 25(OH)D concentrations compared to controls, independent of BMI and age. DBP was not associated with insulin resistance or BMI in PCOS. Further studies are needed to investigate the pathophysiology and clinical implications of reduced DBP in PCOS.

Vitamin D Status, Cardiometabolic, Liver, and Mental Health Status in Obese Youth Attending a Pediatric Weight Management Center

BACKGROUND

Vitamin D (VitD) deficiency and obesity are reaching epidemic proportions in North America, particularly in those with comorbid conditions such as diabetes or liver disease. The study objective was to determine the prevalence of suboptimal vitD status and interrelationships with anthropometric, cardiometabolic, liver, mental health, and lifestyle (sleep/screen time) parameters in an ambulatory population of children with obesity.

METHODS

Children (2-18 years) attending a pediatric weight management clinic (n = 217) were retrospectively reviewed. Variables studied included anthropometric (weight, height, body mass index, waist circumference), vitD (serum 25-hydroxyvitamin D), cardiometabolic (systolic blood pressure, diastolic blood pressure, glucose, insulin, homeostasis model assessment for insulin resistance, triglyceride, high-density lipoprotein, low-density lipoprotein, total cholesterol), liver enzymes (alanine aminotransferase, gamma-glutamyl transferase), and mental health (number, diagnosis) parameters.

RESULTS

Suboptimal vitD status (25-hydroxyvitamin D <75 nmol/L was present in 76% of children with obesity (12.0 ± 2.9 years). Blood pressure categorized as prehypertension, stage I hypertension, and stage II hypertension was present in 14%, 25%, and 7% of children, respectively. Mental health diagnoses including anxiety, attention-deficit hyperactivity disorder, mood disorders, and learning disabilities/developmental delays occurred in 18%, 17%, 10%, and 15%, of children, respectively. Waist circumferences >100 cm were associated with lower vitD levels (58 ± 18 vs 65 ± 17 nmol/L; P = 0.01). VitD status ≥50 nmol/L was associated with lower insulin (15.8 [11.7-23.1] mU/L vs 21.1 [14.3-34.2] mU/L; P < 0.01) and homeostasis model assessment for insulin resistance (3.5 [2.5-4.9] vs 4.8 [3.1-6.9]; P < 0.01) values and systolic blood pressure percentiles (73.0 ± 25.8 vs 80.6 ± 17.0; P = 0.04).

CONCLUSIONS

Children with obesity had a high prevalence of vitD deficiency, particularly those at risk for hypertension, reduced insulin sensitivity, and central obesity.

Vitamin D metabolites in captivity? Should we measure free or total 25(OH)D to assess vitamin D status?

There is general consensus that serum 25(OH)D is the best biochemical marker for nutritional vitamin D status. Whether free 25(OH)D would be a better marker than total 25(OH)D is so far unclear. Free 25(OH)D can either be calculated based on the measurement of the serum concentrations of total 25(OH)D, vitamin D-binding protein (DBP), albumin, and the affinity between 25(OH)D and its binding proteins in physiological situations. Free 25(OH)D can also be measured directly by equilibrium dialysis, ultrafitration or immunoassays. During the vitamin D workshop held in Boston in March 2016, a debate was organized about the measurements and clinical value of free 25(OH)D, and this debate is summarized in the present manuscript. Overall there is consensus that most cells apart from the renal tubular cells are exposed to free rather than to total 25(OH)D. Therefore free 25(OH)D may be highly relevant for the local production and action of 1,25(OH)₂D. During the debate it became clear that there is a need for standardization of measurements of serum DBP and of direct measurements of free 25(OH)D. There seems to be very limited genetic or racial differences in DBP concentrations or (probably) in the affinity of DBP for its major ligands. Therefore, free 25(OH)D is strongly correlated to total 25(OH)D in most normal populations. Appropriate studies are needed to define the clinical implications of free rather than total 25(OH)D in normal subjects and in disease states. Special attention is needed for such studies in cases of abnormal DBP concentrations or when one could expect changes in its affinity for its ligands.

Impact of 25-hydroxyvitamin D, free and bioavailable fractions of vitamin D, and vitamin D binding protein levels on metabolic syndrome components

INTRODUCTION

Various forms of vitamin D and factors involved in their metabolism can play a role in the etiopathogenesis of metabolic disorders. This paper aims to define the relationship between concentration of the hydroxylated form of vitamin D (25(OH)D), the fraction of free and bioavailable vitamin D, and of vitamin D binding protein (VDBP) levels on the one hand and the prevalence of metabolic syndrome components on the other.

MATERIAL AND METHODS

The studies were conducted on 79 people, including 52 with metabolic syndrome (MetS+) and 27 without it (MetS-). Biochemical measurements (lipid profile, glycemia, 25(OH)D, VDBP, albumin, calcium, parathyroid hormone) were performed, concentration of free and bioavailable vitamin D was mathematically calculated, and anthropometric and blood pressure measurements were taken.

RESULTS

The mean ± SD concentration of 25(OH)D among MetS+ individuals (41.90 ±13.12 nmol/l) was lower (p < 0.0001) than among the MetS- group (66.09 ±18.02 nmol/l). Differences between groups were observed in relation to medians/means of concentrations of free and bioavailable vitamin D (p < 0.0001) but not in the case of VDBP. In the entire study population, 25(OH)D correlated with all metabolic syndrome components, whereas its free and bioavailable fraction correlated with particular components of the syndrome. In the MetS+ group, VDBP concentration negatively correlated with body mass index (p = 0.037) and levels of diastolic pressure (p = 0.022). In the case of the MetS- group, the free fraction of vitamin D negatively correlated with triglyceridemia (p = 0.049).

CONCLUSIONS

The evaluation of various forms of vitamin D and VDBP in different population groups seems to have significant clinical value in evaluating the prevalence of metabolic disorders.

Critically Ill Children Have Low Vitamin D-Binding Protein, Influencing Bioavailability of Vitamin D

RATIONALE

Vitamin D deficiency, often defined by total serum 25-hydroxyvitamin D (25[OH]D) <20 ng/ml, is common in critically ill patients, with associations with increased mortality and morbidity in the intensive care unit. Correction of vitamin D deficiency in critical illness has been recommended, and ongoing clinical trials are investigating the effect of repletion on patient outcome. The biologically active amount of 25(OH)D depends on the concentration and protein isoform of vitamin D-binding protein (VDBP), which is also an acute-phase reactant affected by inflammation and injury.

OBJECTIVES

We performed a secondary analysis of a cohort of critically ill children in which we reported a high rate of vitamin D deficiency, to examine how VDBP level and genotype would impact vitamin D status.

METHODS

We prospectively enrolled 511 children admitted to the pediatric intensive care unit over a 12-month period.

MEASUREMENTS AND MAIN RESULTS

We measured serum VDBP in 479 children. We genotyped single nucleotide polymorphisms rs7041 and rs4588 in the VDBP gene (GC) to determine haplotypes GC1F, GC1S, and GC2 in 178 subjects who consented, then calculated bioavailable 25(OH)D from serum 25(OH)D, VDBP, albumin, and GC haplotype. The median serum VDBP level was 159 μg/ml (interquartile range, 108-221), lower than has been reported in healthy children. Factors predicting lower levels in multivariate analysis included age <1 year, nonwhite race, being previously healthy, 25(OH)D <20 ng/ml and greater illness severity. In the subgroup that was genotyped, GC haplotype had the strongest association with VDBP level; carriage of one additional copy of GC1S was associated with a 37.5% higher level (95% confidence interval, 31.9-44.8; P < 0.001). Bioavailable 25(OH)D was also inversely associated with illness severity (r = -0.24, P < 0.001), and ratio to measured total 25(OH)D was variable and related to haplotype.

CONCLUSIONS

Physiologic deficiency of 25(OH)D in critical illness may be more difficult to diagnose, given that lower VDBP levels increase bioavailability. Treatment studies conducted on the basis of total 25(OH)D level, without consideration of VDBP concentration and genotype, may increase the risk of falsely negative results.

Obese young adults exhibit lower total and lower free serum 25-hydroxycholecalciferol in a randomized vitamin D intervention

OBJECTIVE

Although obesity is a risk factor for vitamin D insufficiency, its impact on vitamin D-binding protein (DBP) concentration, and thereby possibly also on free 25OHD, is less well known. Our aim was to compare total and free serum 25OHD, and DBP concentrations between obese and normal-weight young adults at baseline and their responses to cholecalciferol supplementation.

DESIGN

A 12-week randomized, double-blinded clinical trial.

PATIENTS

Obese subjects N = 18 (BMI = 38, 67% men) with severe childhood-onset obesity and 24 normal-weight subjects (BMI = 23, 46% men), age between 15 and 25 years, were randomized into two groups to receive either placebo or cholecalciferol 50 μg (2000 IU) daily.

MEASUREMENTS

At baseline, 6-week and 12-week blood samples and anthropometric measurements were collected; baseline body composition was assessed by dual-energy X-ray absorptiometry.

RESULTS

At baseline, obese subjects had, compared with normal-weight, lower total and free serum 25OHD (49 vs 62 nmol/l, P = 0·041; 2·8 vs 4·7 pg/ml, P = 0·001), without differences in DBP concentrations (309 vs 346 μg/ml, P = 0·212). Cholecalciferol 50 μg per day increased both total and free 25OHD (ancova P < 0·001 and P = 0·021). The response of total 25OHD to supplementation was inferior in the obese compared with normal-weight subjects (P = 0·027). On the contrary, the change in free 25OHD concentration was similar in groups (P = 0·487).

CONCLUSIONS

Obese young adults exhibit lower total and free 25OHD concentration, which is not directly explained by differences in DBP status. The response of free 25OHD to supplementation did not differ between obese and normal-weight subjects.

Alterations in Hemoglobin and Serum 25-hydroxyvitamin D are Related Before and After Weight Loss Independent of African Admixture

African American (AA) and European American (EA) women often exhibit differences in hemoglobin (Hb) and 25-hydroxyvitamin D [25(OH)D], both of which can be altered by calorie restriction leading to weight loss. Given these known differences, it is of clinical interest to examine the potential for race-specific, adverse responses to weight loss. Sixty-four overweight (BMI 27-29.9 kg/m²), premenopausal women consumed a standardized, very-low calorie diet to reduce BMI < 25 kg/m2. Ancestry informative markers provided estimates of African admixture, an objective mean of expressing race. Blood sampling and anthropometric measures were performed at baseline and upon meeting target BMI. At baseline, in the overweight state, Hb (g/dL) (AA, 11.7 ± 0.9 vs. EA, 12.5 ± 0.8; p < .01) and 25(OH)D (nmol/L) (AA, 35.7 ± 12.9 vs. EA, 57.0 ± 20.0; p < .01) were lower in AAs. After weight loss, Hb decreased (AA, -0.5 ± 0.7 vs. EA, -0.4 ± 0.6; p = .48) to a similar extent among races. Conversely, 25(OH)D increased (AA, 43.4 ± 14.0 vs. EA 68.2 ± 24.3; p < .01) though the magnitude of change (Δ) was not different (AA, +7.8 ± 13.5 vs. EA, +11.2 ± 16.7; p = .37) between races. Multiple linear regression revealed a positive association between ΔHb and Δ25(OH)D (r = .386; p < .01) adjusted for African admixture, Δtestosterone, and Δbody fat%. Path analyses revealed a significant indirect effect of Δbody fat% on ΔHb through Δ25(OH)D, β =-0.023, CI [-0.06, -0.004]. Following 15% weight loss, participants with the largest increase in serum 25(OH)D exhibited the smallest decrease in Hb. Future research should clarify the optimal degree of calorie restriction to stimulate weight loss while mitigating the potential risk of anemia associated with dieting efforts.

Vitamin D status of black and white Americans and changes in vitamin D metabolites after varied doses of vitamin D supplementation

BACKGROUND

Controversy exists over the disparate circulating 25-hydroxyvitamin D [25(OH)D] concentrations between black and white Americans.

OBJECTIVE

We sought to determine whether there are differences in total and directly measured free 25(OH)D concentrations between black and white American adults and how daily supplementation with cholecalciferol changes these concentrations.

DESIGN

Cross-sectional and longitudinal analyses were conducted with the use of data from 2 placebo-controlled, randomized trials at 2 academic medical centers in the United States: CaDDM (Calcium and Vitamin D in Type 2 Diabetes) and DDM2 (Vitamin D for Established Type 2 Diabetes). A total of 208 subjects with pre- or well-controlled diabetes with a mean age of 59.1 y and mean body mass index (BMI; in kg/m(2)) of 31.6 were randomly assigned to receive daily cholecalciferol supplementation at 1 of 2 doses (2000 or 4000 IU) or a matching placebo for 16 wk. We measured serum total 25(OH)D, vitamin D-binding protein (DBP) by 2 different immunoassays (with the use of monoclonal or polyclonal antibodies), parathyroid hormone, and albumin. Free 25(OH)D concentration was directly measured and calculated.

RESULTS

Blacks had lower total 25(OH)D concentrations than whites [adjusted median: 20.3 ng/mL (95% CI: 16.2, 24.5 ng/mL) compared with 26.7 ng/mL (95% CI: 25.2, 28.1 ng/mL), respectively; P = 0.026)], and a higher proportion of blacks had total 25(OH)D concentrations <20 ng/mL (46% compared with 19%, respectively; P < 0.001). Directly measured free 25(OH)D concentrations were lower in blacks than in whites [adjusted median: 4.5 ng/mL (95% CI: 3.7, 5.4 ng/mL) compared with 5.7 ng/mL (95% CI: 5.4, 5.9 ng/mL), respectively; P = 0.044] and were strongly correlated with total 25(OH)D without an effect of race. DBP was lower in blacks when measured by the monoclonal but not the polyclonal antibody immunoassay. Cholecalciferol supplementation increased total and measured free 25(OH)D concentrations proportionally to the dose and without a difference between races.

CONCLUSIONS

The relation between free and total 25(OH)D did not vary systematically by race in this multiracial population with pre- or well-controlled diabetes. The results need to be replicated in additional cohorts before concluding that the clinical assessment of vitamin D status in blacks and whites should follow a single standard. The CaDDM and DDM2 trials were registered at clinicaltrials.gov as NCT00436475 and NCT01736865, respectively.

Vitamin D-binding protein and free vitamin D concentrations in acromegaly

Free 25-hydroxyvitamin D [25(OH)D] is suggested to be important in the determination of vitamin D deficiency, since vitamin D-binding protein (VDBP) may affect total 25(OH)D levels. There are no data about free 25(OH)D concentrations in acromegaly. We aimed to investigate serum VDBP and total and free 25(OH)D levels in patients with acromegaly in comparison with control subjects. We recruited 54 patients with acromegaly and 32 control subjects who were similar according to age, gender, and body mass index. Serum VDBP levels were found to be increased in patients with acromegaly compared to control subjects [90.35 (72.45-111.10) vs. 69.52 (63.89-80.13) mg/l, p = 0.001]. There was statistically no significant difference in serum total 25(OH)D levels between the patients with acromegaly and control subjects [18.63 (13.35-27.73) vs. 22.51 (19.20-28.96) ng/ml, p = 0.05]. Free 25(OH)D levels were significantly decreased in patients with acromegaly compared to control subjects [14.55 (10.45-21.45) vs. 17.75 (15.30-23.75) pg/ml, p = 0.03]. Free 25(OH)D levels correlated positively with total 25(OH)D (p = 0.0001) and HDL cholesterol (p = 0.04) and negatively with fasting blood glucose (p = 0.04). Our findings indicate that VDBP is increased and free 25(OH)D is decreased in acromegaly, while there is no significant alteration in total 25(OH)D.

Treatment of Vitamin D Deficiency in Predominantly Hispanic and Black Adolescents: A Randomized Clinical Trial

OBJECTIVES

To compare 3 different treatment regimens for vitamin D deficiency in minority adolescents and to explore factors that impact treatment efficacy.

STUDY DESIGN

We conducted an 8-week, prospective, open-label, randomized clinical trial in an urban, academic, children's hospital. A total of 183 vitamin D-deficient adolescents, mean 25-hydroxyvitamin D or 25(OH)D 13.7 ± 3.9 ng/mL; mean age 16.6 ± 2.2 years, were randomized into 3 vitamin D3 (cholecalciferol) treatment arms: 50,000 IU/wk; 5000 IU/d; and 1000 IU/d. Serum 25(OH)D and vitamin D binding protein (VDBP) levels were measured pre-and posttreatment; 122 (67%) participants completed posttreatment measures. Complete-case and multiple-imputation, intention-to-treat analyses were performed.

RESULTS

Mean change in 25(OH)D level posttreatment was significantly different among the 3 arms, 24.9 ± 15.1 vs 21.0 ± 15.2 vs 6.2 ± 6.5 ng/mL, for 50,000 IU, 5000 IU, and 1000 IU doses, respectively, P < .001. Both high-dose treatments were effective in increasing the 25(OH)D level out of deficiency range (≥ 20 ng/mL) in more than 80% of participants, and 60% remained deficient after low-dose treatment. Only 72%, 56%, and 2% achieved vitamin D sufficiency (>30 ng/mL) with 50,000 IU, 5000 IU, and 1000 IU doses, respectively, P < .001. Obese participants had substantially less mean change in 25(OH)D level after treatment than normal-weight participants, 13.7 ± 10.7 vs 21.9 ± 16.9 ng/mL, P < .001. Mean baseline VDBP level was almost twice as high in Hispanic compared with black participants (P < .001) and did not alter treatment response or change with treatment.

CONCLUSIONS

Adult-sized adolescents require 8 weeks of high-dose cholecalciferol, at least 5000 IU/d, to correct deficiency. Obese adolescents have poorer response to treatment and may need higher doses than nonobese youth. Hispanic and black adolescents have different VDBP levels but similar treatment responses.

TRIAL REGISTRATION

ClinicalTrials.gov: NCT01784029.

Vitamin D and insulin resistance

Vitamin D is a steroid hormone, which in active form binds to the vitamin D receptor. Expression of the vitamin D receptor in diverse cell types (pancreatic islet cells, myocytes, hepatocytes and adipocytes) raises the suspicion that vitamin D may be involved in multiple cellular processes, including the response to insulin. Insulin resistance is a characteristic feature of type 2 DM, and its attenuation may reduce the incidence of type 2 DM and cardiovascular disease. In observational studies, low serum 25-hydroxyvitamin D (25-OHD) concentrations are associated with an increased risk of type 2 DM. It has been suggested that increasing serum 25-OHD concentrations may have beneficial effects on glucose and insulin homeostasis. However, cross-sectional and interventional studies of vitamin D supplementation provide conflicting results and demonstrate no clear beneficial effect of vitamin D on insulin resistance. These studies are complicated by inclusion of different patient cohorts, different 25-OHD assays and different doses and preparations of vitamin D. Any possible association may be confounded by alterations in PTH, 1,25-dihydroxyvitamin D or tissue vitamin D concentrations. We identified 39 studies via MEDLINE and PUBMED. We review the evidence from 10 studies (seven observational and three interventional) examining vitamin D and type 2 DM incidence, and 29 studies (one prospective observational, 12 cross-sectional and 16 interventional trials) examining vitamin D and insulin resistance. Based on this data, it is not possible to state that vitamin D supplementation has any effect on type 2 DM incidence or on insulin resistance. Data from the multiple ongoing randomized controlled trials of vitamin D supplementation due to report over the next few years should help to clarify this area.

Bone Mineral Content as a Driver of Energy Expenditure in Prepubertal and Early Pubertal Boys

OBJECTIVE

To examine the associations of bone and bone-secreted factors with measures of energy metabolism in prepubertal and early pubertal boys.

STUDY DESIGN

Participants in this cross-sectional, observational study included 37 (69% black, 31% white) boys, aged 7-12 years (Tanner stage <III). Dual-energy X-ray absorptiometry was used to measure bone mineral content (BMC) and percent body fat. Indirect calorimetry was used to assess resting energy expenditure (REE). Fasting blood levels of osteocalcin (OCN), fibroblast growth factor 23 (FGF23), insulin, glucose, precursor product of type I collagen (N-terminal pro-peptide) and type I collagen, and C-terminal cross-linked telopeptide were measured. Pearson correlations were performed to evaluate relationships among BMC, OCN, FGF23, fasting insulin and glucose, and REE. Multiple linear regression models were used to test associations between OCN and BMC (independent variables) with fasting insulin and glucose and with REE, adjusting for bone turnover markers and further adjusted for percent body fat.

RESULTS

BMC was correlated with REE and insulin. OCN was correlated with glucose in blacks only (r = 0.45, P < .05). FGF23 was not correlated with any markers of energy metabolism. BMC was associated with insulin level in blacks (β = 0.95, P = .001), which was attenuated by percent body fat (β = 0.47, P = .081). BMC was associated with REE in whites (β = 0.496.7, P < .05) and blacks (β = 619.5, P < .0001); but accounting for percent body fat attenuated the association in whites (β = 413.2, P = .078).

CONCLUSION

Our findings suggest that BMC is a determinant of fasting insulin and REE, and that the contribution of body fat appears to be race-specific. Endocrine effects of FGF23 and OCN on energy metabolism were not apparent.

TRIAL REGISTRATION

Registered with ClinicalTrials.gov: NCT02040740, NCT02040727, and NCT01410643.

Vitamin D-binding protein as a biomarker of active disease in acute intermittent porphyria

Acute intermittent porphyria (AIP) is an autosomal dominant metabolic disorder caused by a deficiency of hepatic porphobilinogen deaminase (PBGD). The disease is characterized by life threatening acute neurovisceral attacks. The aim of this study was to identify metabolites secreted by the hepatocytes that reflect differential metabolic status in the liver and that may predict response to the acute attack treatment. Plasma vitamin D binding protein (VDBP) from a mouse model of AIP displayed an abnormal migration in 2D-electrophoresis that is efficiently recovered upon gene therapy leading to liver specific over-expression of the PBGD protein. The change in VDBP mobility results from a differential isoelectric point suggesting a post-translational modification that takes place preferably in the liver. Liquid chromatography-mass spectrometry (LC-MS) analysis of human samples before and after glycosidase treatment revealed glycosylated plasma VDBP specifically in patients with recurrent attacks of AIP. Glycosylated VDBP recovered normal values in three severely afflicted AIP patients submitted to therapeutic liver transplantation. Our findings suggest that post-translational modification of VDBP might be considered as a promising biomarker to study and monitor the liver metabolic status in patients with AIP.

SIGNIFICANCE

We describe an increased glycosylation of VDBP in porphyric livers. Normal glycosylation was recovered upon liver gene therapy in a mouse model of porphyria or after liver transplantation in severely afflicted patients with AIP. Moreover, quantification of glycosylated VDBP by our ELISA immunoassay or LC-MS protocol in patients undergoing PBGD-gene therapy (www.aipgene.org) may be used as a marker indicating improvement or normalization of the patient's hepatic metabolism. This article is part of a Special Issue entitled: HUPO 2014.

Associations between vascular health indices and serum total, free and bioavailable 25-hydroxyvitamin D in adolescents

Objective

The role of vitamin D in cardiovascular health remains debated as results have been inconsistent. Previous studies have not considered the bioavailability of 25-hydroxy vitamin D [25(OH)D]. Objectives of our study were to investigate the association between serum concentrations of total, free and bioavailable 25(OH)D and independent predictors of cardiovascular risk such as flow mediated dilatation (FMD) and augmentation index (AIx).

Design

This cross-sectional study included 47 post-menarchal, adolescent females [31 African American (AA) and 16 European American (EA)].

Methods

AIx was standardized to a heart rate of 75 beats/min (AIx75). Free and bioavailable 25(OH)D concentrations were calculated from standard formulas.

Results and Conclusions

Mean age of the participants was 15.8±1.4 years and mean body mass index was 23.1±4.0 kg/m². Serum total 25(OH)D was not associated with FMD, but was positively associated with AIx75 in the adjusted model (rho = 0.4, P = 0.03). AIx75 was positively associated with bioavailable 25(OH)D (rho = 0.4, P = 0.004) and free 25(OH)D (rho = 0.4, P = 0.009) and the associations persisted after adjusting for covariates. In race-specific analyses, total, free and bioavailable 25(OH)D were strongly positively associated with AIx75 in AA (rho = 0.5, 0.4, 0.4, respectively), which persisted even after adjusting for covariates. Whereas in EA there was an inverse association between total 25(OH)D and AIx75 in EA (rho = −0.6), which attenuated after adjusting for covariates.

Conclusion

Circulating total, free and bioavailable 25(OH)D were associated with arterial stiffness in adolescent girls, and these associations were race dependent. Notwithstanding, the implications of associations between vascular function indices and 25(OH)D remains unclear.

Vitamin D Binding Protein Impact on 25-Hydroxyvitamin D Levels under Different Physiologic and Pathologic Conditions

There is a high prevalence of vitamin D deficiency worldwide, but how to define vitamin D deficiency is controversial. Currently, the plasma concentration of total 25-hydroxyvitamin D [25(OH)D] is considered an indicator of vitamin D status. The free hormone hypothesis states that protein-bound hormones are inactive while unbound hormones are free to exert biological activity. The majority of circulating 25(OH)D and 1,25(OH)2D is tightly bound to vitamin D binding protein (DBP), 10-15% is bound to albumin, and less than 1% of circulating vitamin D exists in an unbound form. While DBP is relatively stable in most healthy populations, a recent study showed that there are gene polymorphisms associated with race and ethnicity that could alter DBP levels and binding affinity. Furthermore, in some clinical situations, total vitamin D levels are altered and knowing whether DBP is also altered may have treatment implications. The aim of this review is to assess DBP concentration in different physiological and pathophysiological conditions. We suggest that DBP should be considered in the interpretation of 25(OH)D levels.

Lipid accumulation product and 25-OH-vitamin D deficiency in type 2 diabetes

BACKGROUND

Emerging data suggest a link between vitamin D (25(OH)D) deficiency, type 2 diabetes (T2D), and visceral adiposity. The lipid accumulation product (LAP), strictly correlated with abdominal fat depots, is proposed as marker of dysfunctional adiposity.

AIM

To verify the association between 25(OH)D levels and LAP in T2D.

METHODS

Body mass index (BMI), waist circumference (WC), glucose, HbA1c, lipids, and 25(OH)D were assessed in 420 T2D outpatients and in 150 non-diabetic obese with similar anthropometric characteristics. LAP was computed as the product of sex-specific enlarged WC and triglycerides (TG).

RESULTS

In T2D patients, 63.0% showed 25(OH)D deficiency (<20 ng/ml) vs. 71.3% in the obese control group. Overweight males showed a higher prevalence of 25(OH)D deficiency (60.3%) than women (48.8%, p < 0.001), while in obese patients this prevalence was not significant. In both genders, 25(OH)D was not significantly associated with HbA1c and fasting glucose. Age-adjusted 25(OH)D levels were inversely correlated with BMI (p < 0.001), WC (p < 0.001), and LAP (p < 0.001) in both genders. Metabolic syndrome presented an odds ratio (OR) for 25(OH)D deficiency of 1.6 (1.1-2.5, p = 0.048) in females and 1.7 (1.2-2.7, p = 0.016) in males, while the highest quartile of LAP showed an OR of 2.1 (1.2-3.6, p = 0.019) in females and 3.2 (1.6-6.5, p = 0.02) in males. A similar trend was observed in the obese control group.

CONCLUSIONS

In the presence of excess weight, subjects with and without T2D frequently feature low 25(OH)D levels. Subjects with higher LAP exhibit a high risk of 25(OH)D deficiency, suggesting that dysfunctional adiposity is a worsening factor for vitamin D hypovitaminosis.