To a great extent, individuals are protected from excess intakes of calcium by a
tightly regulated intestinal absorption mechanism through the action of 1,25-
dihydroxyvitamin D, the hormonally active form of vitamin D. When absorbed
calcium is in excess of need, the excess is excreted by the kidney in most
healthy people. Concern for excess calcium intake is directed primarily to those
who are prone to milk alkali syndrome (the simultaneous presence of hypercalcaemia,
metabolic alkalosis and renal insufficiency) and hypercalcaemia.
Although calcium can interact with iron, zinc, magnesium and phosphorus
within the intestine, thereby reducing the absorption of these minerals, available
data do not suggest that these minerals are depleted when humans consume diets
containing calcium above the recommended levels. For example, even though
high intakes of calcium can exert acute effects on iron absorption, there is no
evidence of reduced iron status or iron stores with long-term calcium supplementation.