Studies in calcium metabolism. II. Effects of low calcium and variable protein intake on human calcium metabolism.

Six healthy males consumed diets containing about 0.1 g calcium/day at three levels of protein intake: 0.9, 12, and 24 g nitrogen/day. Daily urinary calcium excretion on the 0.9 nitrogen diet was 51 mg, 99 mg on the 12 g nitrogen diet, and 161 mg on the 24 g nitrogen diet. A calcium supplement of 0.9 g for four subjects on the 12 g nitrogen diet caused an increase in urinary calcium from 68 to 160 mg/day. Varying the calcium and protein intake had no effect on dermal calcium loss or serum calcium. Five subjects had the least negative calcium balance on the 12 g nitrogen diet. Increase in urinary calcium is not likely to result solely from enhancement of intestinal calcium absorption. Our data suggest that increased glomerular filtration with possible inhibition of renal tubular reabsorption of calcium may be an additional mechanism responsible for the calciuretic effect during high-protein intake.