This study was conducted to determine whether the markers of bone resorption, pyridinium cross-links of collagen, are sensitive to changes in 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: 1) low nitrogen and low 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/d] diet, and 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 were collected before and during each dietary period and were analyzed for pyridinium cross-links (pyridinoline, deoxypyridinoline), nitrogen and creatinine. The rate of pyrdinium cross-link excretion did not vary with protein intake but was approximately 33% lower (P < 0.01) during periods of high compared with low calcium intake. These data indicate that a short-term increase in calcium intake is accompanied by a reduced rate of bone resorption and that this effect is independent of dietary protein intake.