Effect of solubility on the gastrointestinal absorption of aluminum from various aluminum compounds in the rat

The present study was carried out to determine whether the insoluble property of most aluminum compounds was the reason for the limited absorption of this element from the gastrointestinal tract. Aluminum compounds of varying degrees of solubility were studied. At pH 3, more than 25% of the aluminum from all of the compounds studied (with the exception of sucralfate [13%]) was in solution. At pH 6, the solubility of aluminum in Al(OH)₃ and sucralfate was less than 1%; it was 15% in AlCl₃ and 33% in aluminum lactate. Aluminum solubility did not change with a change in pH for the citrate compounds, which varied between 38%, with sucrafate plus citric acid, and 91%, with aluminum citrate. The fraction of the administered dose of aluminum absorbed as estimated by urinary excretion after gastric gavage was 0.015% for sucralfate and Al(OH)₃, 0.037% for AlCl₃ and aluminum lactate, and greater than 0.80% for all aluminum compounds administered with citrate. A similar relationship was found between the solubility of the aluminum compounds and absorption, as determined by calculated absorption from the changes in plasma aluminum levels. Solubility alone, however, could not totally explain the effect of citrate on aluminum absorption. The solubility of aluminum in aluminum lactate and sucralfate plus citric acid were the same at pH 6. Absorption of aluminum from aluminum lactate, however, was only 1.6% as much as that found for sucralfate plus citric acid. Additional evidence against solubility alone being responsible for the marked enhancement of aluminum absorption from the various citrate-aluminum mixtures is the fact that aluminum solubility in these mixtures varied greatly (38% to 91%) but did not correlate with the amount of aluminum absorbed. It is concluded that although solubility is a critical factor in aluminum absorption, citrate must have an additional effect on aluminum absorption, either by facilitating transcellular absorption or by opening the tight junctions and enhancing the paracellular movement of aluminum.