Supplementary Material for: Elevated Glycerol-3-Phosphate in Patients Undergoing Hemodialysis: Associations with Phosphate and Fibroblast Growth Factor 23

Introduction: Fibroblast growth factor 23 (FGF23) levels are markedly elevated in patients with kidney failure, but the mechanisms are incompletely understood. Recent evidence suggests that kidney-derived glycerol-3-phosphate (G-3-P), a glycolytic byproduct, mediates FGF23 production in response to dietary phosphate loading. However, the role of G-3-P is unknown in patients with kidney failure. Methods: Serum G-3-P levels were quantified by LC/MS in 35 healthy individuals and 650 patients undergoing hemodialysis. Association between serum phosphorus and G-3-P was examined using unadjusted and multivariable linear regression models. We next analyzed the associations of G-3-P and known regulators of FGF23 production with serum FGF23. Results: Median serum G-3-P level in patients undergoing hemodialysis was 220 ng/mL (IQR, 118-325), 2.2-fold higher than that of healthy individuals (98 ng/mL; IQR, 80-129). In patients undergoing hemodialysis, higher serum phosphorus was strongly associated with increased G-3-P in the unadjusted model; this association persisted after multivariate adjustment and when restricted to patients undergoing hemodialysis for over 10 years. In univariate analyses, higher serum phosphorus, calcium, intact PTH, and G-3-P, and active vitamin D use were each significantly associated with higher FGF23. Multivariate analysis identified G-3-P as an independent predictor of FGF23. Further adjustment for transferrin saturation, ferritin, and C-reactive protein did not change these findings. Conclusion: Even in patients with kidney failure, G-3-P may rise in response to phosphate retention and act as a regulator of FGF23 production. Further studies are needed to test these hypotheses and determine whether the apparently non-functioning kidney retains the capacity to produce G-3-P.