Vitamin D Deficiency in Adolescent Girls

Fibroblast growth factor 23 (FGF23) is a newly discovered hormone which regulates phosphate and vitamin D levels. In this study, we are looking at what the normal levels of FGF23 are in adolescent girls and how these levels vary with other hormonal measurements. We will also be looking at whether vitamin D supplementation in adolescents who are deficient in vitamin D alters the levels of FGF23 and other factors including insulin resistance.

Fibroblast growth factor 23 (FGF23) is a newly discovered hormone. Its primary function is to regulate phosphate metabolism, which it does both by directly regulating phosphate excretion through the kidney as well as by regulating vitamin D activity. What controls FGF23 levels is still under investigation; potential factors include dietary phosphate intake and vitamin D levels. In this pilot study, we aim to investigate what the normal levels of FGF23 are in healthy girls and how these are related to other measurements of bone and mineral metabolism. Since vitamin D deficiency is very common in healthy girls, we expect a subset of the subjects in this study to be vitamin D deficient. We will then randomize these girls to vitamin D repletion or placebo and follow changes in FGF23 levels. Vitamin D deficiency has also been implicated in insulin resistance which is a precursor to diabetes. We will therefore also look at changes in insulin resistance with vitamin D repletion.

Placebo pill. Note that all subjects in this arm will receive vitamin D repletion at the conclusion of the study.

placebo pill once weekly for 16 weeks. At the end of the study, subjects in this arm will receive vitamin D repletion

Inclusion Criteria: healthy girls aged 9-18 years Exclusion Criteria: significant cardiac, hepatic, oncologic, or psychiatric disease a history of malabsorption, kidney stones, hypoparathyroidism, or growth hormone deficiency pregnancy diabetes mellitus BMI>/= 99th percentile for age and sex fracture within the preceding 3 months hypogonadism (no pubertal development by age 12, absence of menarche by age 14) serum calcium <8 mg/dl or >11 mg/dl radiographic evidence of rickets use of medications know to affect serum phosphate levels including phosphate-binding antacids, sodium etidronate, calcitonin, excessive doses of vitamin D (>1000 units per day), excessive doses of vitamin D (>20,000 units/day), calcitriol, growth hormone, or anti-convulsants. use of hormonal birth control

Burnett-Bowie SM, Henao MP, Dere ME, Lee H, Leder BZ. Effects of hPTH(1-34) infusion on circulating serum phosphate, 1,25-dihydroxyvitamin D, and FGF23 levels in healthy men. J Bone Miner Res. 2009 Oct;24(10):1681-5. doi: 10.1359/jbmr.090406.

Burnett-Bowie SM, Mendoza N, Leder BZ. Effects of gonadal steroid withdrawal on serum phosphate and FGF-23 levels in men. Bone. 2007 Apr;40(4):913-8. doi: 10.1016/j.bone.2006.10.016. Epub 2006 Dec 8.

Burnett SM, Gunawardene SC, Bringhurst FR, Juppner H, Lee H, Finkelstein JS. Regulation of C-terminal and intact FGF-23 by dietary phosphate in men and women. J Bone Miner Res. 2006 Aug;21(8):1187-96. doi: 10.1359/jbmr.060507.

Mitchell DM, Juppner H, Burnett-Bowie SM. FGF23 Is Not Associated With Age-Related Changes in Phosphate, but Enhances Renal Calcium Reabsorption in Girls. J Clin Endocrinol Metab. 2017 Apr 1;102(4):1151-1160. doi: 10.1210/jc.2016-4038.