EFFECTS OF SHORT-TERM CALCIUM DEPLETION AND REPLETION ON BIOCHEMICAL MARKERS OF BONE TURNOVER IN YOUNG-ADULT WOMEN

The skeletal responses to calcium depletion and repletion in rodents h ave been well characterized, but those in humans are poorly understood . The present study sought to evaluate the effects of short term dieta ry calcium depletion and repletion on biochemical markers of bone turn over in 15 young Caucasian women (age, 21-30 yr). The study contained 3 phases: 1) 5 days of a regular diet containing more than 800 mg/day calcium to establish baseline Values (baseline phase), 2) 22 days of a restricted diet containing less than 300 mg/day calcium (depletion ph ase), and 3) 7 days of a normal diet containing more than 800 mg/day c alcium (repletion phase). Serum and urine samples were obtained from e ach subject during the baseline phase; on the first, second, and last days of the depletion phase; and on the third and last days of the rep letion phase. Serum levels of calcium, PTH, 1,25-dihydroxyvitamin D-3, osteocalcin, and C-terminal type I procollagen peptide (PICP) and uri nary levels of calcium and deoxypyridinoline were determined. Serum an d urinary calcium levels were significantly reduced, and serum PTH and 1,25-dihydroxyvitamin D, levels were markedly increased during deplet ion. These changes were completely reversed after 1 week of repletion. Depletion also rapidly and significantly increased the urinary deoxyp yridinoline level, indicating increased bone resorption. The increase also returned rapidly to baseline upon repletion. Calcium depletion ha d contrasting effects on bone formation markers; whereas depletion sig nificantly reduced the serum PICP level, it significantly increased se rum osteocalcin level. Past histomorphometric studies in rodents indic ated that the number of mature but inactive osteoblasts was increased during depletion despite an inhibition of bone formation. Thus, it is speculated that although the reduction in serum PICP reflected the dep letion-associated inhibition of bone formation, the increase in serum osteocalcin could represent this depletion-related increase in osteobl ast number. During repletion, serum osteocalcin remained elevated abov e baseline. PICP recovered from its depressed level and increased abov e baseline, a finding consistent with past kistomorphometric findings of increased bone formation during repletion. In summary, this study c onfirms that 1) a short calcium depletion period produces calcium stre ss in young women, which leads to rapid stimulation of bone resorption and inhibition of bone formation; and 2) a subsequent calcium repleti on period could lead to a compensatory increase in bone formation. In conclusion, the skeletal responses to calcium depletion/ repletion in young women may be similar to those in rodents.