Solubility and intestinal transit time limit calcium absorption in rats

Six-week-old male rats were placed on two high calcium regimens: one with calcium carbonate and monobasic calcium phosphate, with calcium content increased via calcium carbonate; and another with calcium phosphate and calcium gluconate, with calcium gluconate the source of increased calcium. Animals fed the gluconate-containing diets absorbed 29% of the ingested calcium over the entire calcium intake range, whereas those fed the calcium carbonate diets absorbed 25% over an intake range of 225 to 450 mg Ca/d, but at calcium intakes above 450 mg Ca/d their absorption reached a plateau at approximately 109 mg/d. Active calcium transport decreased with increased calcium intake in both the calcium carbonate- and calcium gluconate-fed groups. Nonsaturable transport was unchanged as a result of increasing calcium intake and did not differ among the diet groups. Because the absorptive processes were unaffected by the calcium source, events in the lumen must have been responsible for the observed differences. Because phosphate is nearly 18 times more soluble than carbonate, very little calcium of calcium carbonate origin can have been solubilized in the presence of phosphate and this, we conclude, accounts for the limit on calcium absorption observed in diets high in calcium carbonate. Moreover, when intake is expressed as soluble calcium, absorption approaches 50%, the value expected when intestinal transit time (approximately 3 h) is multiplied by 16%/h, the experimental value of nonsaturable absorption.