Regulation of C-Terminal and Intact FGF-23 by Dietary Phosphate in Men and Women

UNLABELLED: FGF-23 is a novel regulator of phosphate metabolism. We studied the regulation of FGF-23 by dietary phosphate in 66 men and women using two assays. Dietary phosphate restriction decreased FGF-23 and loading increased FGF-23 significantly. An assay that measured intact FGF-23 showed the effects of dietary phosphate much more clearly than an assay that also measures presumed biologically inactive fragments. Dietary phosphate is a key regulator of circulating FGF-23; choice of assay is critical when studying FGF-23 physiology. INTRODUCTION: Fibroblast growth factor 23 (FGF-23) is a novel phosphaturic factor discovered through genetic studies of patients with renal phosphate wasting disorders. Ablation of the FGF-23 gene in mice reduces renal phosphate excretion and increases serum phosphate, suggesting that FGF-23 is critical for normal phosphate homeostasis. We examined the role of dietary phosphate in the regulation of FGF-23 in humans. MATERIALS AND METHODS: Sixty-six healthy males and females were randomized to either phosphate-depleted or -loaded diets for 5 days, after a 4-day run-in diet. FGF-23 was measured using an "intact" assay that only detects intact FGF-23 peptide and with a "C-terminal" assay that measures both intact FGF-23 peptide and presumed biologically inactive carboxyl terminal fragments. The main outcome was the within group change in FGF-23 with either phosphate depletion or loading. RESULTS: Using the intact FGF-23 assay, mean FGF-23 area under the curve (AUC) decreased by 9 +/- 16% with phosphate depletion (p = 0.0041) and increased by 35 +/- 29% with loading (p < 0.0001). Using the C-terminal FGF-23 assay, mean FGF-23 AUC decreased by 8 +/- 12% with phosphate depletion (p = 0.0003) and increased by 13 +/- 20% with loading (p = 0.0016). Increases in FGF-23 with phosphate loading were greater with the intact assay than with the C-terminal assay (p = 0.0003). Using the intact assay only, FGF-23 was significantly associated with serum phosphate (r = 0.39, p < 0.01), 24-h urinary phosphate (r = 0.47, p < 0.01), fractional excretion of phosphate (r = 0.29, p < 0.01), and 1,25-dihydroxyvitamin D (r = -0.30, p < 0.01). The association between the assays was weak (r = 0.26, p < 0.01). CONCLUSIONS: Dietary phosphate is a key regulator of circulating FGF-23 levels in humans. Additionally, choice of assay is critical when performing physiologic investigations of FGF-23.