Abstract
BACKGROUND: Increased serum phosphate is associated with cardiovascular disease. Compared with whites, blacks have significantly higher serum phosphate and increased risk of hyperphosphataemia in health and chronic kidney disease (CKD). While population-based studies suggest that diminished urinary phosphorus excretion in blacks may explain these differences, few physiological studies explored the potential mechanisms. The aim of this study was to examine racial differences in postprandial urinary mineral ion excretion in health and in CKD. METHODS: Twenty-eight healthy (18 white and 10 black) and 19 CKD (9 white and 10 black) subjects consumed a standardized meal; after which, blood and urine samples were collected for 4 h for measurement of phosphate, calcium, parathyroid hormone (PTH) and fibroblast growth factor 23 (FGF23). RESULTS: Although serum phosphate did not differ by race, blacks had ∼30% lower postprandial fractional excretion of phosphate than whites in health (P < 0.001) and CKD (P = 0.02). Similarly, blacks had ∼35% lower fractional excretion of calcium in health (P = 0.02) and CKD (P = 0.3). Moreover, the postprandial response in serum calcium among CKD subjects differed by race (P = 0.03), with serum calcium significantly decreasing in whites but not blacks. CONCLUSIONS: Blacks had lower fractional excretion of phosphate than whites despite similar levels of PTH and FGF23 in health and in CKD, suggesting racial variability in renal sensitivity to phosphaturic hormones. Furthermore, blacks defend postprandial serum calcium more effectively than whites in CKD. Further studies are needed to define the mechanisms underlying these observations and evaluate whether racial differences in mineral ion handling may contribute to disparities in CKD outcomes.