Effect of the Time of Administration of Calcium Acetate on Phosphorus Binding

Phosphorus binders are given to patients with renal failure to increase gastrointestinal excretion of phosphorus. To determine the relative importance of the binding of dietary as compared with endogenous phosphorus and to determine the optimal dose schedule, we gave either 4.4 g of calcium acetate (25 mmol of calcium) or a placebo to six normal subjects on each of seven different schedules in a randomized sequence. The net gastrointestinal balance of phosphorus and calcium was determined by a one-day lavage technique. After a meal containing approximately 12 mmol of phosphorus, the mean phosphorus absorption (±SE) measured 9.17±0.36 mmol (78 percent) with placebo but decreased to 3.81±0.58 mmol (31 percent) when calcium acetate was given immediately before the meal (representing binding of 5.36±0.77 mmol of phosphorus). Similar binding was observed when calcium acetate was given immediately after the meal and when half the dose was given before and half after the meal. In contrast, when calcium acetate was given two hours after the meal or while the subject was fasting, phosphorus binding was reduced to 2.00±0.52 mmol and 1.81±0.84 mmol, respectively. Calcium absorption from calcium acetate averaged 21±1 percent when the binder was given with a meal; absorption from calcium acetate averaged 40±4 percent when the binder was given while the subject was fasting. We conclude that calcium acetate increases fecal excretion of phosphorus by binding both dietary and endogenous phosphorus, but the binding of dietary phosphorus is quantitatively much more important. For the most efficient phosphorus binding, calcium (and presumably other phosphorus-binding cations) should be given with meals. (N Engl J Med 1989; 320:1110–3.) PATIENTS with chronic renal insufficiency commonly use aluminum or calcium salts to bind phosphorus in the intestine and thereby reduce serum concentrations of phosphorus. 1 , 2 Potentially, such salts may bind either dietary or endogenous phosphorus. If their main effect is on dietary phosphorus, binders would presumably be more effective if ingested with meals. If, however, their main effect is to trap endogenous phosphorus, then the timing of ingestion may be less critical. The most effective schedule of administration is unknown. Experts vary in their recommendations: some advise patients to take phosphorus binders with meals, 1 some suggest that patients take them one . . .