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

Vitamin D-Mediated Hypercalcemia: Mechanisms, Diagnosis, and Treatment

Hypercalcemia occurs in up to 4% of the population in association with malignancy, primary hyperparathyroidism, ingestion of excessive calcium and/or vitamin D, ectopic production of 1,25-dihydroxyvitamin D [1,25(OH)₂D], and impaired degradation of 1,25(OH)₂D. The ingestion of excessive amounts of vitamin D₃ (or vitamin D₂) results in hypercalcemia and hypercalciuria due to the formation of supraphysiological amounts of 25-hydroxyvitamin D [25(OH)D] that bind to the vitamin D receptor, albeit with lower affinity than the active form of the vitamin, 1,25(OH)₂D, and the formation of 5,6-trans 25(OH)D, which binds to the vitamin D receptor more tightly than 25(OH)D. In patients with granulomatous disease such as sarcoidosis or tuberculosis and tumors such as lymphomas, hypercalcemia occurs as a result of the activity of ectopic 25(OH)D-1-hydroxylase (CYP27B1) expressed in macrophages or tumor cells and the formation of excessive amounts of 1,25(OH)₂D. Recent work has identified a novel cause of non-PTH-mediated hypercalcemia that occurs when the degradation of 1,25(OH)₂D is impaired as a result of mutations of the 1,25(OH)₂D-24-hydroxylase cytochrome P450 (CYP24A1). Patients with biallelic and, in some instances, monoallelic mutations of the CYP24A1 gene have elevated serum calcium concentrations associated with elevated serum 1,25(OH)₂D, suppressed PTH concentrations, hypercalciuria, nephrocalcinosis, nephrolithiasis, and on occasion, reduced bone density. Of interest, first-time calcium renal stone formers have elevated 1,25(OH)₂D and evidence of impaired 24-hydroxylase-mediated 1,25(OH)₂D degradation. We will describe the biochemical processes associated with the synthesis and degradation of various vitamin D metabolites, the clinical features of the vitamin D-mediated hypercalcemia, their biochemical diagnosis, and treatment.

Clinical Trial of Vitamin D2 vs D3 Supplementation in Critically Ill Pediatric Burn Patients

BACKGROUND

Hypovitaminosis D exists postburn. However, evidence-based guidelines for vitamin D repletion are unknown. This investigation examined differences between D₂ and D₃ supplementation on outcome in children with burn injuries.

METHODS

Fifty patients with total body surface area burn of 55.7% ± 2.6% and full-thickness injury of 40.8% ± 3.8% were enrolled, ranging in age from 0.7-18.4 years. All participants received multivitamin supplementation per standardized clinical protocol. In addition, 100 IU/kg D₂, D₃, or placebo was administered daily during hospitalization using a randomized, double-blinded study design. Assay of total 25-hydroxyvitamin D (D25), 1,25-dihydroxyvitamin D (D1,25), 25-hydroxyvitamin D₂ (25-OH-D₂), 25-hydroxyvitamin D₃ (25-OH-D₃), and parathyroid hormone (PTH) was performed at 4 preplanned time intervals (baseline, midpoint, discharge, and 1 year postburn). Differences in vitamin D status were compared over time and at each specific study interval.

RESULTS

There were no significant differences in serum vitamin D levels between groups, but >10% of patients had low D25 at discharge, and percent deficiency worsened by the 1-year follow up for the placebo (75%), D₂ (56%), and D₃ (25%) groups. There were no statistical differences in PTH or clinical outcomes between treatment groups, although vitamin D supplementation demonstrated nonsignificant but clinically relevant decreases in exogenous insulin requirements, sepsis, and scar formation.

CONCLUSIONS

The high incidence of low serum D25 levels 1 year following serious thermal injury indicates prolonged compromise. Continued treatment with vitamin D₃ beyond the acute phase postburn is recommended to counteract the trajectory of abnormal serum levels and associated morbidity.

FREE AND BIOAVAILABLE 25-HYDROXYVITAMIN D LEVELS IN PATIENTS WITH PRIMARY HYPERPARATHYROIDISM

OBJECTIVE

To evaluate free and bioavailable 25-hydroxyvitamin D (25[OH]D) levels in primary hyperparathyroidism (PHPT) patients.

METHODS

Fifty PHPT patients and 50 healthy age-, gender-, and body mass index (BMI)-matched control subjects were enrolled. Levels of 25(OH)D were determined by a radioimmunoassay and vitamin D-binding protein (DBP) were determined by an enzyme-linked immunosorbent assay. Free and bioavailable 25(OH)D were calculated utilizing equations that use average binding coefficients for DBP and albumin.

RESULTS

There was no significant difference in age and BMI between PHPT patients and controls (P>.05). Levels of 25(OH)D, DBP, and DBP-bound 25(OH)D were lower in PHPT patients compared to controls (P<.01). There was no significant difference in free and bioavailable 25(OH)D levels between PHPT patients and controls (P>.05). Levels of intact parathyroid hormone were inversely correlated with free (r = -0.217; P<.05) and bioavailable 25(OH)D levels (r = -0.296; P<.01).

CONCLUSION

Serum total 25(OH)D levels were lower, while free and bioavailable 25(OH)D remained similar in patients with PHPT compared to controls. We suggest that low 25(OH)D levels might not reflect true vitamin D nutrition status in PHPT patients.

ABBREVIATIONS

25(OH)D = 25-hydroxyvitamin D BMI = body mass index DBP = vitamin D-binding protein iPTH = intact parathyroid hormone PHPT = primary hyperparathyroidism.

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.

Differential Responses to Vitamin D2 and Vitamin D3 Are Associated With Variations in Free 25-Hydroxyvitamin D

25-Hydroxyvitamin D (25D) circulates bound primarily to serum vitamin D binding protein (DBP), with DBP showing higher binding affinity for 25D3 than 25D2. We therefore hypothesized that vitamin D2 (D2) promotes higher serum levels of unbound 25D (free 25D), with different functional responses, relative to vitamin D3 (D3). Week 3 C56BL/6 mice were placed on diets containing either D2 or D3 alone (both 1000 IU/kg). At week 8 and week 16, D2 mice had only 25D2 in circulation (26.6 ± 1.9 and 33.3 ± 4.4 ng/mL), and D3 mice had only 25D3 (28.3 ± 2.0 and 31.7 ± 2.1 ng/mL). At week 8 (44.5 ± 6.4 vs 62.4 ± 11.6 pg/mL, P < .05) and week 16 (78.4 ± 12.6 vs 95.5 ± 11.6), D2 mice had lower serum 1,25-dihydroxyvitamin D relative to D3 mice. By contrast, measured free 25D was significantly higher in D2 mice at week 8 (16.8 ± 0.65 vs 8.4 ± 0.63 pg/mL, P < .001) and week 16 (17.4 ± 0.43 vs 8.4 ± 0.44, P < .001). A two-way ANOVA of bone histomorphometry showed that week 8 D2 mice had significantly higher osteoclast surface/bone surface, eroded surface/bone surface, and mineral apposition rate compared with D3 mice. Osteoblast surface/bone surface was higher in week 8 D2 females but not week 8 D2 males. At week 16, D2 mice had significantly higher bone volume/total volume and trabecular number compared with D3 mice. Differences in bone phenotype were observed despite D2 mice reaching similar serum 25D levels and lower 1,25D levels compared with D3 mice. These data indicate that 25D2 binds less well to DBP than 25D3, with resulting higher levels of free 25D promoting differential effects on bone in mice exposed to D2 alone.

Vitamin D Testing-Where Are We and What Is on the Horizon?

Vitamin D testing is part of laboratory practice since more than 30 years but has become a routine parameter only recently, due to a highly increasing amount of research in the field resulting in new clinical applications. Vitamin D actually represents a family of molecules of which 25OH Vitamin D and 1,25(OH)₂ Vitamin D, under their D3 and D2 forms, are the most important to date. Physical detection methods and immunoassays exist for both molecules and are being reviewed and discussed. New developments in the measurement of C3-epi-25OH Vitamin D, 24,25(OH)₂ Vitamin D, and free/bioavailable 25OH Vitamin D are also presented. The future of Vitamin D testing is considered based on the evolution of laboratories and based on the scientific research that is currently performed.

Potential Effect of Substituting Estimated Glomerular Filtration Rate for Estimated Creatinine Clearance for Dosing of Direct Oral Anticoagulants

OBJECTIVES

To determine the potential effect of substituting glomerular filtration rate (GFR) estimates for renal clearance estimated using the Cockcroft-Gault method (CrCL-CG) to calculate direct oral anticoagulant (DOAC) dosing.

DESIGN

Simulation and retrospective data analysis.

SETTING

Community, academic institution, nursing home.

PARTICIPANTS

Noninstitutionalized individuals aged 19 to 80 from the National Health and Nutrition Examination Survey (NHANES) (2011/12) (n = 4,687) and medically stable research participants aged 25 to 105 (n = 208).

MEASUREMENTS

Age, height, weight, sex, race, serum creatinine, CrCL-CG, and GFR (according to the Modification of Diet in Renal Disease and Chronic Kidney Disease Epidemiology Collaboration equations). Outcome measures were dosing errors if GFR were to be substituted for CrCL-CG.

RESULTS

Renal clearance estimates according to all methods were highly correlated (P < .001), although at lower clearances, substitution of GFR estimates for CrCL-CG resulted in failure to recognize needs for dose reductions of rivaroxaban or edoxaban in 28% of NHANES subjects and 47% to 56% of research subjects. At a CrCL-CG of less than 30 mL/min, GFR estimates missed indicated dosage reductions for dabigatran in 18% to 21% of NHANES subjects and 57% to 86% of research subjects. Age and weight contributed to differences between renal clearance estimates (P < .001), but correction of GFR for body surface area (BSA) did not reduce dosing errors. At a CrCL-CG greater than 95 mL/min, edoxaban is not recommended, and GFR esimates misclassified 24% of NHANES and 39% of research subjects. Correction for BSA reduced misclassification to 7% for NHANES and 14% in research subjects.

CONCLUSION

Substitution of GFR estimates for estimated CrCl can lead to failure to recognize indications for reducing DOAC dose and potentially higher bleeding rates than in randomized trials.

Effects of High-Dose Vitamin D2 Versus D3 on Total and Free 25-Hydroxyvitamin D and Markers of Calcium Balance

CONTEXT

Controversy persists over: 1) how best to restore low serum 25-hydroxyvitamin D (25D) levels (vitamin D2 [D2] vs vitamin D3 [D3]); 2) how best to define vitamin D status (total [protein-bound + free] vs free 25D); and 3) how best to assess the bioactivity of free 25D.

OBJECTIVE

To assess: 1) the effects of D2 vs D3 on serum total and free 25D; and 2) whether change in intact PTH (iPTH) is more strongly associated with change in total vs free 25D.

DESIGN

Participants previously enrolled in a D2 vs D3 trial were matched for age, body mass index, and race/ethnicity. Participants received 50 000 IU of D2 or D3 twice weekly for 5 weeks, followed by a 5-week equilibration period. Biochemical assessment was performed at baseline and at 10 weeks.

SETTING AND PARTICIPANTS

Thirty-eight adults (19 D2 and 19 D3) ≥18 years of age with baseline 25D levels <30 ng/mL were recruited from an academic ambulatory osteoporosis clinic.

OUTCOME MEASURES

Serum measures were total 25D, free 25D (directly measured), 1,25-dihydroxyvitamin D, calcium, and iPTH. Urine measure was fasting calcium:creatinine ratio.

RESULTS

Baseline total (22.2 ± 3.3 vs 23.3 ± 7.2 ng/mL; P = .5) and free (5.4 ± 0.8 vs 5.3 ± 1.7 pg/mL; P = .8) 25D levels were similar between D2 and D3 groups. Increases in total (+27.6 vs +12.2 ng/mL; P = .001) and free (+3.6 vs +6.2 pg/mL; P = .02) 25D levels were greater with D3 vs D2. Percentage change in iPTH was significantly associated with change in free (but not total) 25D, without and with adjustment for supplementation regimen, change in 1,25-dihydroxyvitamin D, and change in calcium.

CONCLUSIONS

D3 increased total and free 25D levels to a greater extent than D2. Free 25D may be superior to total 25D as a marker of vitamin D bioactivity.

Vitamin D Metabolism Varies among Women in Different Reproductive States Consuming the Same Intakes of Vitamin D and Related Nutrients

BACKGROUND

The impact of the reproductive state on vitamin D metabolism and requirements is uncertain in part because of a lack of studies with controlled dietary intakes of vitamin D and related nutrients.

OBJECTIVE

We aimed to quantify the impact of the reproductive state on a panel of vitamin D biomarkers among women of childbearing age consuming equivalent amounts of vitamin D and related nutrients.

METHODS

Nested within a feeding study providing 2 doses of choline, healthy pregnant (26-29 wk gestation; n = 26), lactating (5 wk postpartum; n = 28), and control (nonpregnant/nonlactating; n = 21) women consumed a single amount of vitamin D (511 ± 48 IU/d: 311 ± 48 IU/d from diet and 200 IU/d as supplemental cholecalciferol) and related nutrients (1.6 ± 0.4 g Ca/d and 1.9 ± 0.3 g P/d) for 10 wk. Vitamin D biomarkers were measured in blood obtained at baseline and study end, and differences in biomarker response among the reproductive groups were assessed with linear mixed models adjusted for influential covariates (e.g., body mass index, season, race/ethnicity).

RESULTS

At study end, pregnant women had higher (P < 0.01) circulating concentrations of 25-hydroxyvitamin D [25(OH)D; 30%], 1,25-dihydroxyvitamin D [1,25(OH)2D; 80%], vitamin D binding protein (67%), and C3 epimer of 25(OH)D3 (100%) than control women. Pregnant women also had higher (P ≤ 0.04) ratios of 25(OH)D to 24,25-dihydroxyvitamin D [24,25(OH)2D; 40%] and 1,25(OH)2D to 25(OH)D (50%) than control women. In contrast, no differences (P ≥ 0.15) in vitamin D biomarkers were detected between the lactating and control groups. Notably, the study vitamin D dose of 511 IU/d achieved vitamin D adequacy in most participants (95%) regardless of their reproductive state.

CONCLUSIONS

The higher concentrations of vitamin D biomarkers among pregnant women than among control women suggest that metabolic adaptations, likely involving the placenta, transpire to enhance vitamin D supply during pregnancy. The study findings also support the adequacy of the current vitamin D RDA of 600 IU for achieving serum 25(OH)D concentrations ≥50 nmol/L among women differing in their reproductive state. This trial was registered at clinicaltrials.gov as NCT01127022.

Free vitamin D does not vary through the follicular phase of the menstrual cycle

The importance of vitamin D (25OHD) in general health and reproductive success has been a focus in the setting of the 25OHD deficiency epidemic. However, there are challenges to understanding 25OHD's effects. The free and bioavailable levels are affected by 25OHD binding protein (DBP) and it is not known how estradiol fluctuations during the menstrual cycle affect these binding parameters. This may impact the most appropriate time to measure 25OHD when determining deficiency. This study characterizes 25OHD throughout the follicular phase of the menstrual cycle. Patients undergoing natural cycle IVF were included. Serum was drawn throughout the follicular phase of the menstrual cycle; 25OHD, DBP, albumin, and estrogen levels were determined for each time point allowing for mathematical calculation of free and bioavailable 25OHD. Early, mid, and late follicular phases were designated by estrogen tertiles among patients. Mean Levels of 25OHD (total, free, bioavailable) and DBP for each tertile were compared with Kruskil-Wallis test for non-parametric groups. Linear regression with GEE was employed due to repeated measures within participants. A total of 33 patients were included with 202 total serum measurements. There was no difference in mean levels of 25OHD (p = 0.77), free 25OHD (p = 0.91), and bioavailable 25OHD (p = 0.76) when measured throughout the follicular phase of the menstrual cycle. Vitamin D metabolism does not fluctuate as estradiol changes in the follicular phase of the menstrual cycle. This data indicates that assessment of 25OHD, in particular when assessed for associations with reproductive outcomes, can be measured reliably at any point during the follicular phase of the menstrual cycle.

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.

Comparison of Two ELISA Methods and Mass Spectrometry for Measurement of Vitamin D-Binding Protein: Implications for the Assessment of Bioavailable Vitamin D Concentrations Across Genotypes

Studies using vitamin D-binding protein (DBP) concentrations to estimate free and bioavailable vitamin D have increased dramatically in recent years. Combinations of two single-nucleotide polymorphisms (SNPs) produce three major DBP isoforms (Gc1f, Gc1s, and Gc2). A recent study showed that DBP concentrations quantified by liquid chromatography-tandem mass spectrometry (LC-MS/MS) did not differ by race, whereas a widely used monoclonal enzyme-linked immunosorbent assay (ELISA) quantified DBP differentially by isoform, yielding significantly lower DBP concentrations in black versus white individuals. We compared measurements of serum DBP using a monoclonal ELISA, a polyclonal ELISA, and LC-MS/MS in 125 participants in the Chronic Renal Insufficiency Cohort (CRIC). Serum free and bioavailable 25OHD were calculated based on DBP concentrations from these three assays in homozygous participants, and race differences were compared. We confirmed that the monoclonal ELISA quantifies DBP differentially by isoform and showed that the polyclonal ELISA is not subject to this bias. Whereas ≤9% of the variability in DBP concentrations quantified using either LC-MS/MS or the polyclonal ELISA was explained by genotype, 85% of the variability in the monoclonal ELISA-based measures was explained by genotype. DBP concentrations measured by the monoclonal ELISA were disproportionately lower than LC-MS/MS-based results for Gc1f homozygotes (median difference -67%; interquartile range [IQR] -71%, -64%), 95% of whom were black. In contrast, the polyclonal ELISA yielded consistently and similarly higher measurements of DBP than LC-MS/MS, irrespective of genotype, with a median percent difference of +50% (IQR +33%, +65%). Contrary to findings using the monoclonal ELISA, DBP concentrations did not differ by race, and free and bioavailable 25OHD were significantly lower in black versus white participants based on both the polyclonal ELISA and LC-MS/MS, consistent with their lower total 25OHD. Future studies of DBP and free or bioavailable vitamin D metabolites should employ DBP assays that are not biased by DBP genotype. © 2016 American Society for Bone and Mineral Research.

Free 25-hydroxyvitamin D is low in obesity, but there are no adverse associations with bone health

BACKGROUND

The mechanism and clinical significance of low circulating 25-hydroxyvitamin D [25(OH)D] in obese people are unknown. Low total 25(OH)D may be due to low vitamin D-binding proteins (DBPs) or faster metabolic clearance. However, obese people have a higher bone mineral density (BMD), which suggests that low 25(OH)D may not be associated with adverse consequences for bone.

OBJECTIVE

We sought to determine whether 1) vitamin D metabolism and 2) its association with bone health differ by body weight.

DESIGN

We conducted a cross-sectional observational study of 223 normal-weight, overweight, and obese men and women aged 25-75 y in South Yorkshire, United Kingdom, in the fall and spring. A subgroup of 106 subjects was also assessed in the winter. We used novel techniques, including an immunoassay for free 25(OH)D, a stable isotope for the 25(OH)D3 half-life, and high-resolution quantitative tomography, to make a detailed assessment of vitamin D physiology and bone health.

RESULTS

Serum total 25(OH)D was lower in obese and overweight subjects than in normal-weight subjects in the fall and spring (geometric means: 45.0 and 40.8 compared with 58.6 nmol/L, respectively; P < 0.001) but not in the winter. Serum 25(OH)D was inversely correlated with body mass index (BMI) in the fall and spring and in the winter. Free 25(OH)D and 1,25-dihydroxyvitamin D [1,25(OH)2D] were lower in obese subjects. DBP, the DBP genotype, and the 25(OH)D3 half-life did not differ between BMI groups. Bone turnover was lower, and bone density was higher, in obese people.

CONCLUSIONS

Total and free 25(OH)D and 1,25(OH)2D are lower at higher BMI, which cannot be explained by lower DBP or the shorter half-life of 25(OH)D3 We speculate that low 25(OH)D in obesity is due to a greater pool of distribution. Lower 25(OH)D may not reflect at-risk skeletal health in obese people, and BMI should be considered when interpreting serum 25(OH)D as a marker of vitamin D status.

Vitamin D expenditure is not altered in pregnancy and lactation despite changes in vitamin D metabolite concentrations

Pregnancy and lactation are associated with changes in vitamin D and calcium metabolism but the impact of these changes on vitamin D expenditure is unknown. We measured plasma 25(OH)D₃ half-life with a stable-isotope tracer and investigated relationships with vitamin D metabolites in pregnant, lactating and ‘non-pregnant, non-lactating’ (NPNL) women. Vitamin D metabolites, vitamin D binding protein (DBP), PTH and 25(OH)D₃ half-life were measured in third-trimester pregnant women (n22) and repeated during lactation 12 weeks post-partum (n14) and twice in NPNL women (n23 and n10, respectively) in rural Gambia where calcium intakes are low with little seasonality in UVB-exposure. 25(OH)D₃ half-life was not significantly different between groups (mean(SD): 20.6(6.8), 22.6(7.7), 18.0(4.7) and 17.7(9.5) days in pregnant, lactating and NPNL women, respectively). Plasma 25(OH)D₃, 1,25(OH)₂D, and DBP were higher in pregnancy, and calculated free-25(OH)D₃ and PTH were lower (P < 0.05). In lactation, 25(OH)D₃ and 24,25(OH)₂D₃ were lower compared to pregnant (P < 0.001, P = 0.02) and NPNL women (P = 0.04, P = 0.07). Significant associations were observed between half-life and 25(OH)D₃ (+ve) in pregnancy, and in all groups between 25(OH)D₃ and free-25(OH)D₃ (+ve) and PTH and 25(OH)D₃ (−ve) (P < 0.0001). These data suggest that adaptive changes in pregnancy and lactation occur that prevent pronounced changes in vitamin D expenditure.

Response of Vitamin D Concentration to Vitamin D3 Administration in Older Adults without Sun Exposure: A Randomized Double-Blind Trial

OBJECTIVES

To determine the dose-response relationship between 25-hydroxyvitamin D (25(OH)D) and supplemental vitamin D3 in elderly nursing home residents.

DESIGN

Randomized double-blind investigation.

SETTING

Nursing home.

PARTICIPANTS

Of 81 women (n=51) and men (n=30) (mean age 87.4±8) enrolled, 72 completed the study.

INTERVENTION

Sixteen weeks of oral vitamin D3 at 800, 2,000, or 4,000 IU/d or 50,000 IU/wk.

MEASUREMENTS

The main outcome was 25(OH)D concentrations (tandem mass spectrometry) after 16 weeks. Free 25(OH)D and intact parathyroid hormone (iPTH) were also analyzed. Safety monitoring of calcium and estimated glomerular filtration rate was performed, and adherence and clinical status were measured.

RESULTS

25(OH)D concentrations increased with dose (P<.001) and were higher with 50,000 IU/wk (P<.001) than other doses and with 4,000 IU/d than 800 or 2,000 IU/d, but 800 IU and 2,000 IU/d did not differ. One subject receiving 800 IU/d had concentrations less than 20 ng/mL. All subjects receiving more than 2000 IU/d had concentrations of 20 ng/mL and greater. Free 25(OH)D concentrations rose with total 25(OH) vitamin D. Total and free 25(OH)D were related to calcium concentrations; only free 25(OH)D was related to iPTH.

CONCLUSION

25(OH)D increased linearly with 800 to 4,000 IU/d and 50,000 IU/wk of vitamin D3, without a ceiling effect. Data suggest that some elderly adults will require more than 800 IU/d of vitamin D3 to ensure adequate vitamin D levels. Changes in 25(OH)D with vitamin D3 were related to starting concentrations (greatest with the lowest concentrations and unchanged with 800 and 2,000 IU/d if 20-40 ng/mL). Relationships between serum calcium and iPTH and free 25(OH)D suggest the potential for free 25(OH)D in defining optimal 25(OH)D concentrations.

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.

Characterization of additional vitamin D binding protein variants

The gene (GC) for the vitamin D binding protein (DBP) shows significant genetic variation. Two missense variants, p.D432E and p.T436K, are common polymorphisms and both may influence vitamin D metabolism. However, less common variants, identified biochemically, have been reported previously. This study aimed to identify the underlying mutations by molecular screening and to characterize the mutant proteins by mass spectrometry. Denaturing high performance liquid chromatography (DHPLC) was used for screening genetic variants in GC exons and exon/intron boundaries of genomic DNA samples. Sanger sequencing identified the specific mutations. An immuno-capture coupled mass spectrometry method was used to characterize protein variants in serum samples. Initial molecular screening identified 10 samples (out of 761) containing an alanine deletion at codon 246 in exon 7 (p.A246del, c.737_739delCTG), and 1 sample (out of 97) containing a cysteine to phenylalanine substitution at codon 311 in exon 8 (p.C311F, c.932G>T). The mutant allele proteins and posttranslational modified products were distinguishable from the wild-type proteins by mass spectrum profiling. Loss of a disulfide bond due to loss of cysteine-311 was accompanied by the appearance of a novel mixed disulfide species, consistent with S-cysteinylation of the remaining unpaired cysteine-299 in the mutant protein. We confirm earlier biochemical studies indicating that there are additional deleterious GC mutations, some of which may be low-frequency variants. The major findings of this study indicate that additional mutant proteins are secreted and can be identified in the circulation. By combining molecular screening and mass spectrometric methods, mutant DBP species can be identified and characterized.

Free 25-Hydroxyvitamin D: Impact of Vitamin D Binding Protein Assays on Racial-Genotypic Associations

CONTEXT

Total 25-hydroxyvitamin D (25OHD) is a marker of vitamin D status and is lower in African Americans than in whites. Whether this difference holds for free 25OHOD (f25OHD) is unclear, considering reported genetic-racial differences in vitamin D binding protein (DBP) used to calculate f25OHD.

OBJECTIVES

Our objective was to assess racial-geographic differences in f25OHD and to understand inconsistencies in racial associations with DBP and calculated f25OHD.

DESIGN

This study used a cross-sectional design.

SETTING

The general community in the United States, United Kingdom, and The Gambia were included in this study.

PARTICIPANTS

Men in Osteoporotic Fractures in Men and Medical Research Council studies (N = 1057) were included.

EXPOSURES

Total 25OHD concentration, race, and DBP (GC) genotype exposures were included.

OUTCOME MEASURES

Directly measured f25OHD, DBP assessed by proteomics, monoclonal and polyclonal immunoassays, and calculated f25OHD were the outcome measures.

RESULTS

Total 25OHD correlated strongly with directly measured f25OHD (Spearman r = 0.84). Measured by monoclonal assay, mean DBP in African-ancestry subjects was approximately 50% lower than in whites, whereas DBP measured by polyclonal DBP antibodies or proteomic methods was not lower in African-ancestry. Calculated f25OHD (using polyclonal DBP assays) correlated strongly with directly measured f25OHD (r = 0.80-0.83). Free 25OHD, measured or calculated from polyclonal DBP assays, reflected total 25OHD concentration irrespective of race and was lower in African Americans than in US whites.

CONCLUSIONS

Previously reported racial differences in DBP concentration are likely from monoclonal assay bias, as there was no racial difference in DBP concentration by other methods. This confirms the poor vitamin D status of many African-Americans and the utility of total 25OHD in assessing vitamin D in the general population.

Effects of vitamin D binding protein phenotypes and vitamin D supplementation on serum total 25(OH)D and directly measured free 25(OH)D

OBJECTIVE

To determine the relationship between serum total 25-hydroxyvitamin D (25(OH)D), directly measured free 25(OH)D and calculated free 25(OH)D with regard to vitamin D-binding protein (DBP) phenotypes, sex, BMI, age and season, and their interrelationship to vitamin D supplementation.

DESIGN, PATIENTS AND INTERVENTIONS

A randomized controlled trial with 20 000 IU of vitamin D3 per week or placebo for 12 months was designed. A total of 472 subjects, 236 in each of the intervention groups, were included in the analyses.

MAIN OUTCOME MEASURES

Baseline serum concentrations and increases in serum total 25(OH)D, directly measured free 25(OH)D, calculated free 25(OH)D and DBP.

RESULTS

Serum total 25(OH)D and DBP concentrations were significantly lower in subjects with the phenotype Gc2/Gc2 compared to phenotypes with the Gc1S allele, and lower in males compared to females. When using directly measured free 25(OH)D, the differences related to DBP phenotypes and sexes were clearly diminished. All calculated free 25(OH)D concentrations were overestimated compared to the directly measured free 25(OH)D. Serum parathyroid hormone showed an inverse correlation with all vitamin D parameters analyzed. The increases after 12 months of vitamin D supplementation were not significantly different for any of the vitamin D parameters regardless of DBP phenotype, sex or age. Supplementation with vitamin D did not affect serum DBP.

CONCLUSION

Direct measurements of free 25(OH)D reduce the differences seen in total 25(OH)D between DBP phenotype groups and sexes, probably caused by differences in DBP concentrations. With conditions affecting serum DBP concentrations, direct measurements of free 25(OH)D should be considered.

PTH and Vitamin D

PTH and Vitamin D are two major regulators of mineral metabolism. They play critical roles in the maintenance of calcium and phosphate homeostasis as well as the development and maintenance of bone health. PTH and Vitamin D form a tightly controlled feedback cycle, PTH being a major stimulator of vitamin D synthesis in the kidney while vitamin D exerts negative feedback on PTH secretion. The major function of PTH and major physiologic regulator is circulating ionized calcium. The effects of PTH on gut, kidney, and bone serve to maintain serum calcium within a tight range. PTH has a reciprocal effect on phosphate metabolism. In contrast, vitamin D has a stimulatory effect on both calcium and phosphate homeostasis, playing a key role in providing adequate mineral for normal bone formation. Both hormones act in concert with the more recently discovered FGF23 and klotho, hormones involved predominantly in phosphate metabolism, which also participate in this closely knit feedback circuit. Of great interest are recent studies demonstrating effects of both PTH and vitamin D on the cardiovascular system. Hyperparathyroidism and vitamin D deficiency have been implicated in a variety of cardiovascular disorders including hypertension, atherosclerosis, vascular calcification, and kidney failure. Both hormones have direct effects on the endothelium, heart, and other vascular structures. How these effects of PTH and vitamin D interface with the regulation of bone formation are the subject of intense investigation.

Free 25-Hydroxyvitamin D Concentrations in Cystic Fibrosis

BACKGROUND

Vitamin D deficiency is common in cystic fibrosis (CF), but there is no previous data on free 25-hydroxyvitamin D (25[OH]D) in CF or in relation to healthy individuals.

METHODS

We assessed total serum 25(OH)D concentration by chemiluminescence and serum free 25(OH)D concentration by both direct measurement (ELISA) and calculation, using serum albumin and vitamin D binding protein (VDBP) levels in 80 subjects (28 healthy adults, 25 clinically stable adults and children with CF and 27 adults experiencing a CF exacerbation).

RESULTS

Serum albumin and VDBP concentrations were lower in CF compared with healthy controls. Total serum 25(OH)D concentrations were positively correlated with both calculated and measured free 25(OH)D (P < 0.001 for both). Calculated and directly measured serum free 25(OH)D levels were positively correlated (P < 0.001).

CONCLUSIONS

Serum levels of directly measured free 25(OH)D positively correlated with total 25(OH)D, suggesting that achieving sufficient total serum 25(OH)D may result in adequate free 25(OH)D levels in CF.

Normalized vitamin D metabolite concentrations are better correlated to pharmacological effects than measured concentrations

Background:

Vitamin D deficiency has been associated with a multitude of diseases, ranging from fractures to cancer. Nearly 99% of vitamin D metabolites are bound to proteins, altering the relationship between concentration and activity.

Methods & results:

Normalized concentrations were calculated and validated using published data regarding the correlation of 25-hydroxyvitamin D with bone mineral density. In addition, healthy and kidney disease subjects were recruited for preliminary investigations. Use of the normalizing equations resulted in statistically significant improvements in the relationship between vitamin D metabolites and several markers of health status.

Conclusion:

Normalized concentrations are similar to clinically reported values and are easier to interpret than free or bioavailable concentrations, since their values match the range of measured total concentrations.

Lay abstract: Vitamin D deficiency has been associated with a multitude of diseases, ranging from fractures to cancer. Most vitamin D metabolites are bound to various body components, altering the relationship between reported concentration and biological effects. To compensate for differences in binding between individuals, normalized concentrations were calculated. Use of the normalizing equations resulted in significant improvements in the relationship between the concentration of vitamin D metabolites and health status. The newly developed normalized concentrations are therefore better indicators of vitamin D status and are easier to interpret than free or bioavailable concentrations, since their values match the range of measured total concentrations.

Free 25(OH)D and Calcium Absorption, PTH, and Markers of Bone Turnover

CONTEXT

It has been proposed that serum free 25-hydroxyvitamin D [25(OH)D] may better reflect vitamin D action than total 25(OH)D. An ELISA for serum free 25(OH)D has recently become available, permitting direct assay.

OBJECTIVE

To determine whether serum free 25(OH)D provides additional information in relation to calcium absorption and other biomarkers of vitamin D action compared to total serum 25(OH)D.

SETTING

Ambulatory research setting in a teaching hospital.

OUTCOME

Serum free 25(OH)D measured in a previously performed study of varied doses of vitamin D3 (placebo and 800, 2000, and 4000 IU) on calcium absorption, PTH, procollagen type 1 N-terminal propeptide, and C-terminal telopeptides of type I collagen. Free 25(OH)D was measured by ELISA. Calcium absorption was measured at baseline and at 10 weeks using stable dual calcium isotopes.

RESULTS

Seventy-one subjects completed this randomized, placebo-controlled trial. Baseline group mean free and total 25(OH)D varied from 4.7 ± 1.8 to 5.4 ± 1.5 pg/mL, and from 23.7 ± 5.9 to 25.9 ± 6.1 ng/mL, respectively. Participants assigned to the 4000-IU dose arm achieved free 25(OH)D levels of 10.4 pg/mL and total 25(OH)D levels of 40.4 ng/mL. Total and free 25(OH)D were highly correlated at baseline and after increasing vitamin D dosing (r = 0.80 and 0.85, respectively). Free 25(OH)D closely reflected changes in total 25(OH)D. PTH was similarly correlated at baseline and follow-up with total and free 25(OH)D. Serum C-terminal telopeptides of type I collagen had a moderate positive correlation with total and free 25(OH)D at follow-up. The serum 1,25-dihydroxyvitamin D change increased significantly with the change in 25(OH)D but not with the change in free 25(OH)D.

CONCLUSION

There was no advantage from measuring free over total 25(OH)D in assessing the response of calcium absorption, PTH, and markers of bone turnover to vitamin D. Free 25(OH)D responded to increasing doses of vitamin D in a similar fashion to total 25(OH)D.

Total 25(OH) vitamin D, free 25(OH) vitamin D and markers of bone turnover in cirrhotics with and without synthetic dysfunction

BACKGROUND & AIMS

Current clinical assays for total 25-hydroxy (OH) vitamin D measure vitamin D bound to vitamin D-binding protein (DBP) and albumin plus unbound ('free') D. We investigated the relationship between total and free 25(OH)D with bone metabolism markers in normal (>3.5 g/dl) vs. low (≤3.5 g/dl) albumin cirrhotics.

METHODS

Eighty-two cirrhotics underwent measurement of free and total 25(OH)D by immunoassay, DBP and markers of bone metabolism [intact parathyroid hormone (iPTH), C-telopeptide (CTX), bone-specific alkaline phosphatase (BSAP), osteocalcin, amino-terminal pro-peptide of type 1-collagen (P1NP)]. Pearson's coefficients assessed relevant associations.

RESULTS

Cirrhotics with low (n = 54) vs. normal (n = 28) albumin had lower total 25(OH)D (12.1 vs. 21.7 ng/ml), free 25(OH)D (6.2vs.8.6 pg/ml) and DBP(91.4 vs. 140.3 μg/ml) [P < 0.01 for each]. iPTH was similar in low and normal albumin groups (33 vs. 28 pg/ml; P = 0.38), although serum CTX(0.46vs.0.28 ng/ml) and BSAP(31.7 vs. 24.8 μg/L) were increased (P < 0.01). An inverse relationship was observed between total 25(OH)D and iPTH in normal (r = -0.47, P = 0.01) but not low albumin cirrhotics (r = 0.07, P = 0.62). Similar associations were seen between free 25(OH)D and iPTH(Normal: r = -0.46, P = 0.01; Low: r = -0.03, P = 0.84). BSAP, osteocalcin and P1NP were elevated above the normal range in all cirrhotics but not consistently associated with total or free 25(OH)D.

CONCLUSIONS

Cirrhotics with low vs. normal albumin have lower levels of DBP, total and free 25(OH)D. The expected relationship between total or free 25(OH)D with iPTH was observed in normal but not in low albumin cirrhotics, demonstrating that total 25(OH)D is not an accurate marker of bioactive vitamin D status in cirrhotics with synthetic dysfunction. Additional investigation into the role of vitamin D supplementation and its impact on bone mineral homoeostasis in this population is needed.

Free 25(OH)D and the Vitamin D Paradox in African Americans

CONTEXT

African Americans have a lower total serum 25-hydroxyvitamin D [25(OH)D] but superior bone health. This has been referred to as a paradox. A recent publication found that free serum 25(OH)D is the same in black and white individuals. However, the study was criticized because an indirect method was used to measure free 25(OH)D. A direct method has recently been developed.

OBJECTIVE

We hypothesized that although total serum 25(OH)D is lower in African Americans, free serum 25(OH)D measured directly would not differ between races.

DESIGN

White and black healthy postmenopausal women were matched for age and body mass index. Serum total 25(OH)D, PTH, 1,25-dihydroxyvitamin D, vitamin D binding protein (VDBP), and bone density were measured. Measurement of free 25(OH)D was carried out using an ELISA.

SETTING

The study was conducted at an ambulatory research unit in a teaching hospital.

OUTCOME

A cross-racial comparison of serum free 25(OH)D was performed.

RESULTS

A propensity match resulted in the selection of a total of 164 women. Total 25(OH)D was lower in black women (19.5 ± 4.7 vs 26.9 ± 6.4 ng/mL), but a direct measurement of free 25(OH)D revealed almost identical values (5.25 ± 1.2 vs 5.25 ± 1.3 ng/mL) between races. VDBP was significantly lower in blacks when using a monoclonal-based ELISA but higher with a polyclonal-based ELISA. Serum PTH, 1,25-dihydroxyvitamin D, and bone density were higher in African Americans.

CONCLUSIONS

Free serum 25(OH)D is the same across races despite the lower total serum 25(OH)D in black women. Results comparing VDBP between races using a monoclonal vs a polyclonal assay were discordant.

Associations of circulating 25(OH)D with cardiometabolic disorders underlying type 2 diabetes mellitus in an Aboriginal Canadian community

AIMS

To investigate the associations of 25-hydroxyvitamin D (25(OH)D) with insulin resistance (IR), beta-cell function and metabolic syndrome (MetS) in a First Nations population.

METHODS

We conducted a cross-sectional analysis using data from the Sandy Lake Health and Diabetes Project (2003-2005). A total of 390 participants (>12 y) were assessed for 25(OH)D, fasting glucose, insulin, lipids, blood pressure, inflammatory markers, anthropometric and lifestyle variables and a 75-g oral glucose tolerance test was administered. IR was calculated using the Matsuda insulin sensitivity index (ISOGTT) and the computational homeostasis model assessment of IR (HOMA2-IR). Beta-cell function was calculated using the insulinogenic index (IGI) divided by HOMA-IR (IGI/IR) and the insulin secretion sensitivity index-2 (ISSI-2). The 2009 harmonized criteria were used to define MetS.

RESULTS

Higher 25(OH)D was associated with a decreased prevalence of dysglycemia (OR = 0.71 95% CI, 0.51-0.97 per SD increase). In addition, there were significant associations of 25(OH)D with measures of insulin action (ISOGTT; beta=0.31; 95% CI, 0.12, 0.49; HOMA2-IR; beta = -29; 95% CI -0.46, -0.11 and beta-cell function (ISSI-2; beta = 0.15; 95% CI, 0.02, 0.28). The prevalence of MetS was 41%. There was a decreased risk (OR=0.73, 95% CI 0.56, 0.94) of MetS per SD increase in baseline 25(OH)D. Finally, there was a significant positive association of 25(OH)D with adiponectin (beta = 0.16; 95% CI = 0.01, 0.31).

CONCLUSIONS

These results support a potential role for vitamin D metabolism in the natural history of T2DM among Aboriginal Canadians, although carefully designed randomized trials will be required to establish causality.

Vitamin D and chronic hepatitis C: effects on success rate and prevention of side effects associated with pegylated interferon-α and ribavirin

Chronic hepatitis C (CHC) is one of the most common causes of liver diseases worldwide, affecting 3% of the world population and 3 to 4 million people acquire new infection annually. Despite the recent introduction of novel antiviral drugs for the treatment of CHC, these drugs are expensive and the access to them is not an option for many patients. Hence, the traditional therapy by pegylated interferon-α (Peg-IFN-α) and ribavirin may still have a role in the clinical management of CHC especially in developing countries. However, this standard therapy is associated with several severe extra-hepatic side effects and the most common adverse events are hematological abnormalities and thyroid disorders and they could result in dose reduction and/or termination of therapy. Vitamin D has been shown to be a key regulatory element of the immune system, and its serum concentrations correlate with the severity of liver damage and the development of liver fibrosis/cirrhosis. Furthermore, supplementation with vitamin D with Peg-IFN-α based therapy for the treatment of CHC could be beneficial in increase the response rate to Peg-INF-α based therapy. Vitamin D has also been shown to regulate the thyroid functions and the process of erythropoiesis. This review appraises the data to date researching the role of vitamin D during the treatment of CHC and the potential role of vitamin D in preventing/treating Peg-IFN-α induced thyroiditis and anemia during the course of treatment.

VITAMIN D-BINDING PROTEIN IN HEALTHY PRE- AND POSTMENOPAUSAL WOMEN: RELATIONSHIP WITH ESTRADIOL CONCENTRATIONS

OBJECTIVE

To examine the relationship between endogenous serum estradiol and vitamin D-binding protein (DBP) and total, free, and bioavailable 25-hydroxyvitamin D (25OHD) concentrations in pre- and postmenopausal women.

METHODS

In 165 healthy women (ages, 26 to 75 years) not taking any form of exogenous estrogen, the serum concentrations of estradiol, 25OHD, DBP, parathyroid hormone, and albumin were measured. Free and bioavailable 25OHD (free + albumin-bound) levels were calculated from total 25OHD, DBP, and serum albumin levels.

RESULTS

Premenopausal women had higher serum 25OHD (31.5 ± 7.9 ng/mL), DBP (45.3 ± 6.2 mg/dL), and estradiol (52.8 ± 35.0 pg/mL) levels than postmenopausal women (26.5 ± 4.9 ng/mL, 41.7 ± 5.7 mg/dL, and 12.9 ± 4.9 pg/mL), respectively. In addition, the calculated free and bioavailable 25OHD levels were higher in pre- than postmenopausal women (P<.05). Serum estradiol correlated with DBP (r = 0.22; P<.01) and total 25OHD (r = 0.27; P<.01). In multivariate regression models (with or without serum 25OHD), estradiol was independently associated with DBP (P<.05).

CONCLUSION

Lower estradiol level is one of the factors that contribute to lower DBP levels in older women. Our data indicate that besides well-known factors such as age, gender, and race, serum estradiol concentrations are also a physiologic predictor of DBP concentration.

Effect of feeding 25-hydroxyvitamin D3 with a negative cation-anion difference diet on calcium and vitamin D status of periparturient cows and their calves

Holstein cows (>1 gestation) were fed 1 of 3 diets during the last 13 d of gestation (ranged from 22 to 7 d). The control diet (16 cows) was formulated to provide 18,000 IU/d of vitamin D3 and had a dietary cation-anion difference (DCAD) of 165mEq/kg (DCAD=Na + K - Cl - S). The second diet (DCAD + D) provided the same amount of vitamin D3 but had a DCAD of -139mEq/kg (17 cows). The third diet (DCAD + 25D) had no supplemental vitamin D3 but provided 6mg/d of 25-(OH) vitamin D3 [25-(OH)D3] with a DCAD of -138mEq/kg (20 cows). Diets were fed until parturition and then all cows were fed a common lactation diet that contained vitamin D3. Negative DCAD diets reduced urine pH, with the greatest decrease occurring with the DCAD + D treatment. Urinary Ca excretion was greatest for cows fed DCAD + 25D followed by cows fed DCAD + D. Urinary pH was negatively correlated with urinary excretion of Ca for cows fed DCAD + D. No such correlation was observed with the DCAD + 25D treatment because substantial excretion of urinary Ca occurred at moderate urinary pH values for that treatment. Cows fed DCAD + 25D had greater serum concentrations of 25-(OH)D3 than other treatments from 5 d after supplementation started through 7 d in milk. Concentrations of 1,25-(OH)2D3 in serum were greatest in DCAD + 25D cows starting at 2 d before calving and continued through 7 d in milk. Serum Ca concentrations 5 d before calving were greatest for cows fed DCAD + 25D, but at other time points before and after parturition treatment did not affect serum Ca. Incidence of clinical hypocalcemia was not statistically different between treatments, but cows fed DCAD + 25 had the highest incidence rate (12.5, 0, and 20% for control, DCAD + D, and DCAD + 25D). Calves born from cows fed DCAD + 25D had greater concentrations of 25-(OH)D3 in serum at birth than calves from other treatments (before colostrum consumption), but concentrations were similar by 3 d of age. Concentrations of 25-(OH)D3 in colostrum and transition milk were increased by feeding DCAD + 25D, but by 28 d in milk treatment effects no longer existed. Overall, feeding 25-OH vitamin D with a negative DCAD diet increased vitamin D status of the cow and her newborn calf but had minimal effects on calcium status and did not have positive effects on the incidence of hypocalcemia.

Nutritional vitamin D supplementation in dialysis: a randomized trial

BACKGROUND AND OBJECTIVES

Vitamin D (25-hydroxyvitamin D; 25[OH]D) deficiency is common in patients initiating long-term hemodialysis, but the safety and efficacy of nutritional vitamin D supplementation in this population remain uncertain.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

This randomized, placebo-controlled, parallel-group multicenter trial compared two doses of ergocalciferol with placebo between October 2009 and March 2013. Hemodialysis patients (n=105) with 25(OH)D levels ≤32 ng/ml from 32 centers in the Northeast United States were randomly assigned to oral ergocalciferol, 50,000 IU weekly (n=36) or monthly (n=33), or placebo (n=36) for a 12-week treatment period. The primary endpoint was the achievement of vitamin D sufficiency (25[OH]D >32 ng/ml) at the end of the 12-week treatment period. Survival was assessed through 1 year.

RESULTS

Baseline characteristics were similar across all arms, with overall mean±SD 25(OH)D levels of 21.9±6.9 ng/ml. At 12 weeks, vitamin D sufficiency (25[OH]D >32 ng/ml) was achieved in 91% (weekly), 66% (monthly), and 35% (placebo) (P<0.001). Mean 25(OH)D was significantly higher in both the weekly (49.8±2.3 ng/ml; P<0.001) and monthly (38.3±2.4 ng/ml; P=0.001) arms compared with placebo (27.4±2.3 ng/ml). Calcium, phosphate, parathyroid hormone levels, and active vitamin D treatment did not differ between groups. All-cause and cause-specific hospitalizations and adverse events were similar between groups during the intervention period. Lower all-cause mortality among ergocalciferol-treated participants was not statistically significant (hazard ratio, 0.28; 95% confidence interval, 0.07 to 1.19).

CONCLUSIONS

Oral ergocalciferol can increase 25(OH)D levels in incident hemodialysis patients without significant alterations in blood calcium, phosphate, or parathyroid hormone during a 12-week period.

High prevalence of vitamin D insufficiency/deficiency in Dutch multi-ethnic obese children

Vitamin D insufficiency/deficiency is common among non-white children; however, little is known about the prevalence of vitamin D insufficiency/deficiency in non-white obese children living in the Netherlands. Therefore, a retrospective analysis was performed on data from multi-ethnic Dutch children and adolescents 6–18 years who visited the obesity outpatient clinic in 2012–2013. We performed anthropometric measurements, oral glucose tolerance test, and measured 25(OH)D and lipid levels. Vitamin D insufficiency was defined as 25(OH)D levels 37.5- <50 nmol/L and vitamin D deficiency as 25(OH)D <37.5 nmol/L. In total, data from 387 children were obtained (mean age 11.6 years, 41.1 % boys, 10.3 % Dutch native, 25.6 % Turkish, 24.5 % Moroccan, 7.5 % African Surinamese, and 7.0 % West African). The median 25(OH)D level was 34 (range 12–105) nmol/L. In total, 17.8 % were vitamin D sufficient, 24.5 % with vitamin D insufficiency, and 57.6 % with vitamin D deficiency. Obese ethnic children showed the highest (87.5 %) and normal weight white children showed the lowest (20.0 %) prevalence of vitamin D insufficiency/deficiency . Conclusion: Vitamin D insufficiency and deficiency is extremely prevalent in treatment-seeking obese ethnic children. However, there was no evidence of an effect of vitamin D status on various components of the metabolic syndrome in our cohort.

Can vitamin D status be assessed by serum 25OHD in children?

BACKGROUND

To examine the relationship of serum 25-hydroxyvitamin D (25OHD) concentrations with serum parathyroid hormone (PTH) levels, body mass index (BMI), and environmental factors in a population of Caucasian children living at latitude 43°N.

METHODS

Cross-sectional study on 288 children aged 1 month to 13 years who presented to a pediatric emergency unit during a 21-month period.

RESULTS

Mean (SD) serum 25OHD concentrations were 40.6 (17.6), 30.9 (12.0), and 26.4 (9.9) ng/ml (1 ng/ml = 2.5 nmol/l), in children aged 0-1, 2-5, and ≥ 6 years, respectively. Serum PTH levels were 26.6 (13.6), 24.3 (11.9), and 32.7 (12.1) pg/ml in the same groups. Infants had 25OHD concentrations significantly higher. PTH levels were significantly higher in children aged ≥ 6 years. There was no significant correlation between serum 25OHD and PTH concentrations. Totals of 15.6 % and 2.1 % of children had 25OHD values less than 20 and 10 ng/ml, respectively, but none had elevated serum PTH or clinical manifestations related with vitamin D deficiency. Age (inverse correlation) and season (higher values in summer), but not BMI, sex, and time spent outdoors, influenced serum 25OHD concentrations.

CONCLUSIONS

Our results raise doubt on the assumption of only a serum 25OHD threshold as indicative of vitamin D deficiency in children.

Vitamin D supplementation: when and how?

The multiple effects of vitamin D on skeletal and extra-skeletal tissues increased the attention of scientists and public to the possible relationship between hypovitaminosis D and a variety of acute and chronic diseases. However, several points are still largely debated. In particular, the definition of optimal vitamin D status [as assessed by the circulating levels of 25-hydroxyvitamin D (25(OH)D)] remains controversial, and experts still disagree about several related outcomes: how to estimate the prevalence of vitamin D deficiency, when to start treatment, how to reach optimal 25(OH)D levels, which type of vitamin is preferable for supplementation, which dosing strategy is the better option. In this context, a matter of major debate is represented by the measurement of circulating level of 25(OH)D, whose determination is affected by the lack of standardization and by several technical problems. It has been recently hypothesized that free and bio-available, rather than total 25(OH)D, mostly determine its biological action. However, further evaluation of directly measured free 25(OH)D levels is needed, in order to establish its role in research and clinical practice. Finally, it is not yet defined if a threshold of optimal vitamin D status for reducing the risk of extra-skeletal diseases exists. Actually, it is plausible that the desired 25(OH)D level may vary widely, depending on the health outcome in question. However, this topic is uncertain, partly due to the lack of randomized controlled trials assessing the effect of vitamin D supplementation on extra-skeletal end-points.

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.

Anti-Inflammatory and Antimicrobial Actions of Vitamin D in Combating TB/HIV

TUBERCULOSIS (TB) DISEASE ACTIVATION IS NOW BELIEVED TO ARISE DUE TO A LACK OF INFLAMMATORY HOMEOSTATIC CONTROL AT EITHER END OF THE SPECTRUM OF INFLAMMATION: either due to immunosuppression (decreased antimicrobial activity) or due to immune activation (excess/aberrant inflammation). Vitamin D metabolites can increase antimicrobial activity in innate immune cells, which, in the context of HIV-1 coinfection, have insufficient T cell-mediated help to combat Mycobacterium tuberculosis (MTB) infection. Moreover, maintaining vitamin D sufficiency prior to MTB infection enhances the innate antimicrobial response to T cell-mediated interferon-γ. Conversely, vitamin D can act to inhibit expression and secretion of a broad range of inflammatory mediators and matrix degrading enzymes driving immunopathology during active TB and antiretroviral- (ARV-) mediated immune reconstitution inflammatory syndrome (IRIS). Adjunct vitamin D therapy during treatment of active TB may therefore reduce lung pathology and TB morbidity, accelerate resolution of cavitation and thereby decrease the chance of transmission, improve lung function following therapy, prevent relapse, and prevent IRIS in those initiating ARVs. Future clinical trials of vitamin D for TB prevention and treatment must be designed to detect the most appropriate primary endpoint, which in some cases should be anti-inflammatory and not antimicrobial.

Vitamin d binding protein and bone health

Vitamin D binding protein (DBP) is the major carrier protein of 25-hydroxyvitamin D (25(OH) D) in the circulation, where it may serve roles in maintaining stable levels during times of decreased 25(OH) availability and in regulating delivery of 25(OH) D to target tissues. Several genetic polymorphisms of DBP have been described that lead to phenotypic changes in the protein that may affect affinity, activity, and concentration. These polymorphisms have been linked with alterations in bone density in several populations. One of the mechanisms by which DBP may alter bone health involves regulating vitamin D bioavailability. DBP-bound vitamin is thought to be relatively unavailable to target tissues, and thus alterations in DBP levels or affinity could lead to changes in vitamin D bioactivity. As a result, functional vitamin D status may differ greatly between individuals with similar total 25(OH) D levels. Additionally, DBP may have independent roles on macrophage and osteoclast activation. This review will summarize recent findings about DBP with respect to measures of bone density and health.