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      Marker of vitamin D status in healthy children: Free or total 25-hydroxyvitamin D?

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          Abstract

          Objective

          To assess if serum free 25-hydroxyvitamin D (25OHD) is a better indicator of vitamin D status than total 25OHD in healthy children.

          Methods

          Cross-sectional prospective clinical study was designed. We measured serum free 25OHD concentrations and its correlation with calculated free 25OHD, total 25OHD, intact parathyroid hormone (PTH), and vitamin D binding protein (DBP) in children. The influence of age, sex, ethnicity, body mass index (BMI), season of the year, diet intake, vitamin supplements, time spent outdoors and albumin concentrations on free 25OHD was also analyzed. 241 children aged from 0 days to 14 years, and living in the northern Spain (latitude 43° N), were included.

          Results

          Mean (SD) free 25OHD concentrations were 2.48 (1.39), 5.46 (3.12), 4.12 (1,72), 3.82 (1.43) pg/ml in children aged 0 days, 1 month-2 years, 2–6 years and >6 years, respectively. Correlation between directly measured and calculated free 25OHD was high and significant (r = 0.66) as well as the correlation between serum free and total 25OHD concentrations (r = 0.61). No significant correlation was found between PTH and free 25OHD (r = -0.08). The total 25OHD and PTH concentrations’ correlation was inverse (r = -0.25) and significant. Neither free nor total 25OHD concentrations correlated with DBP concentrations. Among the analyzed variables, free 25OHD values were higher in spring/summer than in autumn/winter in children older than 6 years.

          Conclusions

          : These findings do not support that free 25OHD is a better marker of vitamin D deficiency than total 25OHD in healthy pediatric population.

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          Most cited references26

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          Prevalence of vitamin D deficiency among healthy infants and toddlers.

          To determine the prevalence of vitamin D deficiency and to examine whether 25-hydroxyvitamin D (25OHD) concentration varies as a function of skin pigmentation, season, sun exposure, breastfeeding, and vitamin D supplementation. Cross-sectional sample. Urban primary care clinic. Healthy infants and toddlers (N = 380) who were seen for a routine health visit. Primary outcomes were serum 25OHD and parathyroid hormone levels; secondary measures included data on sun exposure, nutrition, skin pigmentation, and parental health habits. Wrist and knee radiographs were obtained for vitamin D-deficient participants. The prevalence of vitamin D deficiency (< or =20 ng/mL) was 12.1% (44 of 365 participants), and 146 participants (40.0%) had levels below an accepted optimal threshold (< or =30 ng/mL). The prevalence did not vary between infants and toddlers or by skin pigmentation. There was an inverse correlation between serum 25OHD and parathyroid hormone levels (infants: r = -0.27, P < .001; toddlers: r = -0.20, P = .02). In multivariable models, breastfeeding without supplementation among infants and lower milk intake among toddlers were significant predictors of vitamin D deficiency. In vitamin D-deficient participants, 3 participants (7.5%) exhibited rachitic changes on radiographs, whereas 13 (32.5%) had evidence of demineralization. Suboptimal vitamin D status is common among otherwise healthy young children. Predictors of vitamin D status vary in infants vs toddlers, information that is important to consider in the care of these young patients. One-third of vitamin D-deficient participants exhibited demineralization, highlighting the deleterious skeletal effects of this condition.
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            Assessment of vitamin D status - a changing landscape.

            In recent years it has been shown that vitamin D deficiency is associated with an increased incidence as well as the progression of a broad range of diseases including osteoporosis, rickets, cardiovascular disease, autoimmune disease, multiple sclerosis and cancer. Consequently, requests for the assessment of vitamin D status have increased dramatically. Despite significant progress in the analysis of vitamin D metabolites and an expansion of our pathophysiological knowledge of vitamin D, the assessment of vitamin D status remains a challenging and partially unresolved issue. Current guidelines from scientific bodies recommend the measurement of 25-hydroxy vitamin D (25-OHD) in blood as the preferred test. However, growing evidence indicates significant limitations of this test, including analytical aspects and interpretation of results. In addition, the relationships between 25-OHD and various clinical indices, such as bone mineral density and fracture risk, are rather weak and not consistent across races. Recent studies have systematically investigated new markers of vitamin D status including the vitamin D metabolite ratio (VMR) (ratio between 25-OHD and 24,25-dihydroxy vitamin D), bioavailable 25-OHD [25-OHD not bound to vitamin D binding protein (DBP)], and free 25-OHD [circulating 25-OHD bound to neither DBP nor albumin (ALB)]. These parameters may potentially change how we will assess vitamin D status in the future. Although these new biomarkers have expanded our knowledge about vitamin D metabolism, a range of unresolved issues regarding their measurement and the interpretation of results prevent their use in daily practice. It can be expected that some of these issues will be overcome in the near future so that they may be considered for routine use (at least in specialized centers). In addition, genetic studies have revealed several polymorphisms in key proteins of vitamin D metabolism that affect the circulating concentrations of vitamin D metabolites. The affected proteins include DBP, 7-dehydrocholesterol synthase and the vitamin D receptor (VDR). Here we aim to review existing knowledge regarding the biochemistry, physiology and measurement of vitamin D. We will also provide an overview of current and emerging biomarkers for the assessment of vitamin D status, with particular attention methodological aspects and their usefulness in clinical practice.
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              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.
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                Author and article information

                Contributors
                Role: ConceptualizationRole: Data curationRole: Formal analysisRole: InvestigationRole: MethodologyRole: Writing – original draft
                Role: ConceptualizationRole: Formal analysisRole: Funding acquisitionRole: InvestigationRole: MethodologyRole: SupervisionRole: ValidationRole: Writing – original draftRole: Writing – review & editing
                Role: Data curationRole: Investigation
                Role: Data curationRole: Investigation
                Role: Data curationRole: Formal analysisRole: Investigation
                Role: Data curationRole: Formal analysisRole: InvestigationRole: Writing – original draft
                Role: ConceptualizationRole: Formal analysisRole: Funding acquisitionRole: InvestigationRole: MethodologyRole: Project administrationRole: ResourcesRole: SoftwareRole: SupervisionRole: ValidationRole: VisualizationRole: Writing – original draftRole: Writing – review & editing
                Role: Editor
                Journal
                PLoS One
                PLoS ONE
                plos
                plosone
                PLoS ONE
                Public Library of Science (San Francisco, CA USA )
                1932-6203
                23 August 2018
                2018
                : 13
                : 8
                : e0202237
                Affiliations
                [1 ] Department of Pediatrics, Hospital Universitario Central de Asturias (HUCA), Health Service of the Principality of Asturias, SESPA, Oviedo, Spain
                [2 ] Department of Pediatrics Complejo Asistencial Universitario León, (CAULE), Health Service of Castilla-León, SACYL, León, Spain
                [3 ] Department of Biochemistry, Hospital Universitario Central de Asturias, Health Service of the Principality of Asturias, Oviedo, Spain
                Charles P. Darby Children's Research Institute, UNITED STATES
                Author notes

                Competing Interests: The authors have declared that no competing interests exist.

                ‡ VM and AGA also contributed equally to this work. NA and EMM also contributed equally to this work.

                Author information
                http://orcid.org/0000-0001-6770-8263
                http://orcid.org/0000-0002-1206-3346
                Article
                PONE-D-18-18824
                10.1371/journal.pone.0202237
                6107142
                30138329
                9f4d0ac3-6ba2-4ed3-89cb-fac54bda32c6
                © 2018 Mantecón et al

                This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

                History
                : 24 June 2018
                : 30 July 2018
                Page count
                Figures: 4, Tables: 3, Pages: 12
                Funding
                Funded by: Foundation “Ernesto Sánchez-Villares”
                Funded by: Foundation “Nutrición y Crecimiento
                Funded by: GRUPIN 14-020, grant from de Hospital Universitario Central de Asturias.
                Funded by: funder-id http://dx.doi.org/10.13039/501100004587, Instituto de Salud Carlos III;
                Award ID: PI15/02122
                Award Recipient :
                This research study was supported by PI15/02122 (Plan Estatal I+D+I 2013-2016) grant from the Instituto de Salud Carlos III (Spain) (FS) and fondos FEDER; by GRUPIN 14-020, grant from Hospital Universitario Central de Asturias and by grant from the Foundation “Ernesto Sánchez-Villares”, and the Foundation “Nutrición y Crecimiento”. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
                Categories
                Research Article
                Physical sciences
                Chemistry
                Chemical compounds
                Organic compounds
                Vitamins
                Vitamin D
                Physical sciences
                Chemistry
                Organic chemistry
                Organic compounds
                Vitamins
                Vitamin D
                Biology and Life Sciences
                Biochemistry
                Hormones
                Parathyroid Hormone
                Biology and Life Sciences
                Biochemistry
                Proteins
                Albumins
                Medicine and Health Sciences
                Pediatrics
                Child Health
                Medicine and Health Sciences
                Public and Occupational Health
                Child Health
                Medicine and Health Sciences
                Pediatrics
                Biology and Life Sciences
                Physiology
                Physiological Parameters
                Body Weight
                Body Mass Index
                Medicine and Health Sciences
                Physiology
                Physiological Parameters
                Body Weight
                Body Mass Index
                Biology and Life Sciences
                Developmental Biology
                Neonates
                People and Places
                Population Groupings
                Age Groups
                Children
                Infants
                People and Places
                Population Groupings
                Families
                Children
                Infants
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                All relevant data are within the paper.

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