SHORT-TERM CHANGES IN CALCIUM BUT NOT PROTEIN-INTAKE ALTER THE RATE OF BONE-RESORPTION IN HEALTHY-SUBJECTS AS ASSESSED BY URINARY PYRIDINIUM CROSS-LINK EXCRETION

This study was conducted to determine whether the markers of bone reso rption, pyridinium cross-links of collagen, are sensitive to changes i n dietary protein and calcium intake. Fifteen young healthy subjects ( 7 males and 8 females) participated in three 5-d diet periods. Dietary intake during each dietary period consisted of: I) low nitrogen and l ow calcium [0.49 +/- 0.11 g protein/ (kg . d), 429 +/- 190 mg calcium/ d]; 2) low nitrogen and high calcium [0.44 +/- 0.08 g protein/(kg . d) , 1643 +/- 171 mg calcium/d]; and 3) a high nitrogen and high calcium [2.71 +/- 0.75 g protein/(kg . d), 1589 +/- 633 mg calcium/dl diet, an d this was compared with subjects' baseline dietary intake [0.99 +/- 0 .51 g protein/(kg . d), 589 +/- 152 mg calcium/d]. The order of these diets was randomly assigned. Twenty-four-hour and 3-h urine samples we re collected before and during each dietary period and were analyzed f or pyridinium cross-links (pyridinoline, deoxypyridinoline), nitrogen and creatinine. The rate of pyridinium crosslink excretion did not var y with protein intake but was similar to 33% lower (P < 0.01) during p eriods of high compared with low calcium intake. These data indicate t hat a short-term increase in calcium intake is accompanied by a reduce d rate of bone resorption and that this effect is independent of dieta ry protein intake.