Phosphate-binding capacity of ferrihydrite versus calcium acetate in rats

Calcium salts, such as calcium carbonate and calcium acetate, are the princ ipal compounds used as phosphate binders in patients with chronic renal fai lure. The dose required is three to six times the normal requirement for ca lcium. Use of these large doses of calcium salts in the diet can result in hypercalcemia. Other compounds have been investigated as phosphate binders with varying degrees of success. Synthetic ferrihydrite (5Fe(2)O(3). 9H(2)O ) has a high adsorptive capacity for phosphate and may be an effective phos phate binder. The phosphate-binding capacity of ferrihydrite was compared w ith that of calcium acetate in 250-g male Sprague Dawley rats. After an ove rnight fast, rats (n = 5 per group) were gavaged with an American Institute of Nutrition (AIN) 76 formula containing one third the daily phosphorus in take labeled with phosphorus-32 (P-32). Either two levels of calcium acetat e, representing three (1/2 X) or six (1 X) times the usual calcium intake f or one third of the day, or equivalent amounts of ferrihydrite were added t o the diet. An additional group received two times (2 X) the larger dose, a nd a sixth control group received no binder in the diet. Phosphorus absorpt ion curves were determined from P-32 appearance in the serum. The 1/2 X dos e of ferrihydrite reduced P-32 by approximately one half, and the 2 X dose nearly completely suppressed P-32 absorption, similar to the 1 X dose of ca lcium acetate. The advantage of using a ferrihydrite binder would be to avo id the hypercalcemia resulting from the use of high-dose calcium salts. An added advantage may result from the small amounts of iron absorbed in these chronically iron-deficient patients. (C) 1999 by the National Kidney Found ation, Inc.