Oral Acid Load Down-Regulates Fibroblast Growth Factor 23

Increased dietary acid load has a negative impact on health, particularly when renal function is compromised. Fibroblast growth factor 23 (FGF23) is a bone-derived hormone that is elevated during renal failure. The relationship between metabolic acidosis and FGF23 remains unclear. To investigate the effect of dietary acid load on circulating levels of FGF23, rats with normal renal function and with a graded reduction in renal mass (1/2 Nx and 5/6 Nx) received oral NH4Cl for 1 month. Acid intake resulted in a consistent decrease of plasma FGF23 concentrations in all study groups when compared with their non-acidotic control: 239.3 +/- 13.5 vs. 295.0 +/- 15.8 pg/mL (intact), 346.4 +/- 19.7 vs. 522.6 +/- 29.3 pg/mL (1/2 Nx) and 988.0 +/- 125.5 vs. 2549.4 +/- 469.7 pg/mL (5/6 Nx). Acidosis also decreased plasma PTH in all groups, 96.5 +/- 22.3 vs. 107.3 +/- 19.1 pg/mL, 113.1 +/- 17.3 vs. 185.8 +/- 22.2 pg/mL and 504.9 +/- 75.7 vs. 1255.4 +/- 181.1 pg/mL. FGF23 showed a strong positive correlation with PTH (r = 0.877, p < 0.0001) and further studies demonstrated that acidosis did not influence plasma FGF23 concentrations in parathyroidectomized rats, 190.0 +/- 31.6 vs. 215 +/- 25.6 pg/mL. In conclusion, plasma concentrations of FGF23 are consistently decreased in rats with metabolic acidosis secondary to increased acid intake, both in animals with intact renal function and with decreased renal function. The in vivo effect of metabolic acidosis on FGF23 appears to be related to the simultaneous decrease in PTH.