Iron deficiency, elevated erythropoietin, fibroblast growth factor 23, and mortality in the general population of the Netherlands: A cohort study

Emerging data in chronic kidney disease (CKD) patients suggest that iron deficiency and higher circulating levels of erythropoietin (EPO) stimulate the expression and concomitant cleavage of the osteocyte-derived, phosphate-regulating hormone fibroblast growth factor 23 (FGF23), a risk factor for premature mortality. To date, clinical implications of iron deficiency and high EPO levels in the general population, and the potential downstream role of FGF23, are unclear. Therefore, we aimed to determine the associations between iron deficiency and higher EPO levels with mortality, and the potential mediating role of FGF23, in a cohort of community-dwelling subjects. We analyzed 6,544 community-dwelling subjects (age 53 ± 12 years; 50% males) who participated in the Prevention of Renal and Vascular End-Stage Disease (PREVEND) study-a prospective population-based cohort study, of which we used the second survey (2001-2003)-and follow-up was performed for a median of 8 years. We measured circulating parameters of iron status, EPO levels, and plasma total FGF23 levels. Our primary outcome was all-cause mortality. In multivariable linear regression analyses, ferritin (ß = -0.43), transferrin saturation (TSAT) (ß = -0.17), hepcidin (ß = -0.36), soluble transferrin receptor (sTfR; ß = 0.33), and EPO (ß = 0.28) were associated with FGF23 level, independent of potential confounders. During median (interquartile range [IQR]) follow-up of 8.2 (7.7-8.8) years, 379 (6%) subjects died. In multivariable Cox regression analyses, lower levels of TSAT (hazard ratio [HR] per 1 standard deviation [SD], 0.84; 95% confidence interval [CI], 0.75-0.95; P = 0.004) and higher levels of sTfR (HR, 1.15; 95% CI 1.03-1.28; P = 0.01), EPO (HR, 1.17; 95% CI 1.05-1.29; P = 0.004), and FGF23 (HR, 1.20; 95% CI 1.10-1.32; P < 0.001) were each significantly associated with an increased risk of death, independent of potential confounders. Adjustment for FGF23 levels markedly attenuated the associations of TSAT (HR, 0.89; 95% CI 0.78-1.01; P = 0.06), sTfR (HR, 1.08; 95% CI 0.96-1.20; P = 0.19), and EPO (HR, 1.10; 95% CI 0.99-1.22; P = 0.08) with mortality. FGF23 remained associated with mortality (HR, 1.15; 95% CI 1.04-1.27; P = 0.008) after adjustment for TSAT, sTfR, and EPO levels. Mediation analysis indicated that FGF23 explained 31% of the association between TSAT and mortality; similarly, FGF23 explained 32% of the association between sTfR and mortality and 48% of the association between EPO