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Introduction: The effects of genetic variation in fibroblast growth factor 23 (FGF23) are unclear. This study explores the associations of single-nucleotide polymorphisms (SNPs) of FGF23 with phosphate and vitamin D metabolism and bone strength in early childhood.Methods: The study is part of the vitamin D intervention in infant (VIDI) trial (2013–2016), in which healthy term infants born to mothers of Northern European origin received vitamin D3 supplementation of 10 or 30 μg/day from 2 weeks to 24 months of age (ClinicalTrials.gov NCT01723852). Intact and C-terminal FGF23 (cFGF23), 25-hydroxyvitamin D (25-OHD), parathyroid hormone, phosphate, and peripheral quantitative computed tomography (pQCT)-derived bone strength parameters were analyzed at 12 and 24 months. The study included 622 VIDI participants with genotyping data on FGF23 SNPs rs7955866, rs11063112, and rs13312770.Results: Rs7955866 minor allele homozygotes had lowest cFGF23 at both time-points (mixed model for repeated measurements, pvariant = 0.009). Minor alleles of rs11063112 were associated with a greater age-related decrease in phosphate concentration (pinteraction = 0.038) from 12 to 24 months. Heterozygotes of rs13312770 had the greatest total bone mineral content (total BMC), cross-sectional area (total CSA), and polar moment of inertia (PMI) at 24 months (ANOVA p = 0.005, 0.037, and 0.036, respectively). Rs13312770 minor alleles were associated with a greater increase of total BMC, but a smaller increase of total CSA and PMI, during follow-up (pinteraction |