Different effects of three high-dose oral calcium salts on acid-base metabolism, plasma electrolytes and urine parameters of rats.

The oral calcium (Ca) load test has been applied to estimate the enteral absorbability of Ca salts in humans; provided that the deep bone compartments are filled up, excess Ca should be excreted in the urine. Using this "overflow model" three Ca salts were tested in rats at increasing oral doses of 0 to 14 mmol/kg body weight: CaCO3 and two other compounds containing chloride at a Ca:Cl ratio of 1:2 (CaCl2) and 1:1 (Ca-aspartate-hydrochloride). The carbonate was poorly absorbed and hence did not significantly affect acid-base metabolism nor urine pH. Both chloride-containing salts increased Ca excretion to a significantly higher degree in a dose-dependent manner; in contrast to the organic compound, the CaCl2 induced metabolic acidosis at 14 mmol/kg body weight. At decreasing base excess and urinary pH, renal excretion of Ca and of magnesium (Mg) increased, indicating that acid-base alterations must be considered when evaluating the oral load test. All Ca salts induced moderate hypomagnesemia pointing to decreased enteral absorbability of food-borne Mg in rats. Studies on volunteers reported in the literature suggest, however, that this effect is not relevant for humans.