Does a rise in plasma erythropoietin after high-altitude exposure affect FGF23 in healthy volunteers on a normal or low-phosphorus diet?

BACKGROUND AND AIMS: Data of experimental rodent models suggest that hypoxia with subsequent increase in erythropoietin stimulates the expression of the phosphaturic hormone fibroblast growth factor 23 (FGF23). METHODS AND RESULTS: To translate the findings of animal studies into human physiology, herein we exposed eight healthy volunteers to high altitude (2656 m above sea level) for four days. The volunteers were randomized on a low-phosphorous diet (n = 4) or a normal phosphorus diet (n = 4). Although high-altitude exposure caused a significant increase in plasma erythropoietin (EPO) (before high-altitude exposure: low phosphorus: median EPO 6.6 mIU/ml [interquartile range (IQR) 6.0; 8.2], normal phosphorus: median EPO 9.0 mIU/ml [IQR 7.9; 11.5]; at day 2: low phosphorus: median EPO 21.3 mIU/ml [IQR 19.5; 23.8], normal phosphorus: median EPO 19.4 mIU/ml [IQR 18.0; 20.8]), there was no consistent increase in plasma c-terminal FGF23 or plasma intact FGF23. We observed only a single, intermittent peak in c-terminal FGF23 levels after 5 h of maximal aerobic exercise. CONCLUSION: These data do not support a substantial effect of moderate hypoxia alone on the expression of FGF23, but they suggest that combined exercise and high-altitude exposure may temporarily induce FGF23 expression.

Strength of Fibroblast Growth Factor 23 as a Cardiovascular Risk Predictor in Chronic Kidney Disease Weaken by ProBNP Adjustment.

BACKGROUND: Various epidemiological studies linked high fibroblast growth factor 23 (FGF23) levels with cardiovascular events in chronic kidney disease (CKD). It remains enigmatic whether high FGF23 exerts adverse cardiovascular effects, or whether it reflects detrimental effects of residual confounders. Earlier studies adjusted for CKD-mineral bone disease (CKD-MBD) regulators of FGF23 rather than for recently discovered non-CKD-MBD regulators, among which iron deficiency and heart failure are of particular importance. Moreover, they used c-terminal FGF23 (cFGF23) assays rather than more specific intact FGF23 (iFGF23) assays. METHODS: The CARE FOR HOMe study analyzed plasma ferritin, iFGF23, cFGF23 and N-terminal proBNP (NT-proBNP) along with conventional risk factors, among 575 CKD G2-G4 patients to determine the interaction between FGF23, its non-CKD-MBD regulators, and incident cardiovascular events in CKD patients. The participants were followed up for 5.1 ± 2.1 years for the occurrence of atherosclerotic events and hospitalization for acute decompensated heart failure. RESULTS: cFGF23 correlated strongly with high iFGF23 (r = 0.607), fairly with high NT-proBNP (r = 0.453) and weakly with low ferritin (r = -0.207); correlation coefficients of iFGF23 with NT-proBNP and ferritin were numerically lower. In Kaplan-Meier analyses, both endpoints were predicted by cFGF23 and iFGF23. In Cox regression models, cFGF23 remained an outcome predictor after adjustment for conventional risk factors and ferritin. This prediction was largely eliminated when further adjusting for NT-proBNP. iFGF23 was less consistently associated with adverse outcome in partly adjusted models, and failed to predict outcome in fully adjusted models. CONCLUSION: In summary, iron deficiency and heart failure affect plasma FGF23. As adjustment for NT-proBNP virtually eliminates the association between plasma FGF23 and predefined outcome, we speculate that high FGF23, rather than exerting detrimental cardiovascular effects, mirrors prevalent heart disease.

External Validation of the Kidney Failure Risk Equation and Re-Calibration with Addition of Ultrasound Parameters.

BACKGROUND AND OBJECTIVES: Progression of CKD toward ESRD is heterogeneous. The Kidney Failure Risk Equation (KFRE) was developed to identify CKD patients at high risk of ESRD. We aimed to externally validate KFRE and to test whether the addition of predefined Duplex ultrasound markers - renal resistive index (RRI) or difference of resistive indices in spleen and kidney (DI-RISK) - improved ESRD prediction. DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS: The prospective Cardiovascular and Renal Outcome in CKD 2-4 Patients-The Fourth Homburg evaluation (CARE FOR HOMe) study recruits CKD stage G2-G4 patients referred to a tertiary referral center for nephrologic care. Four hundred three CARE FOR HOMe participants enrolled between 2008 and 2012 had available RRI measurements at study inclusion; they were subsequently followed for a mean of 4.4±1.6 years. This subcohort was used to validate KFRE and to assess the added value of the ultrasound markers (new models KFRE+RRI and KFRE+DI-RISK). Model performance was assessed by log-likelihood ratio test, c-statistic, integrated discrimination improvement metrics (for study participants without subsequent ESRD [IDI No ESRD] and for patients with ESRD [IDI ESRD]), and calibration plots. If either new model improved on KFRE, we determined to validate it in an independent cohort of 162 CKD patients. RESULTS: KFRE predicted ESRD in CARE FOR HOMe participants with a c-statistic of 0.91 (95% confidence interval, 0.83 to 0.99). Adding RRI improved the KFRE model (P<0.001), and the KFRE+RRI model was well calibrated; however, the c-statistic (0.91 [0.83-1.00]) was similar, and overall sensitivity (IDI No ESRD=0.05 [0.00-0.10]) or overall specificity (IDI ESRD=0.00 [0.00-0.01]) did not improve. Adding DI-RISK did not improve the KRFE model. In the external validation cohort, we confirmed that the KFRE+RRI model did not outperform KFRE. CONCLUSIONS: Routine Duplex examinations among CKD patients did not improve risk prediction for progression to ESRD beyond a validated equation.