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 have been well characterized, but those in humans are poorly understood. The present study sought to evaluate the effects of short term dietary calcium depletion and repletion on biochemical markers of bone turnover 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 phase), and 3) 7 days of a normal diet containing more than 800 mg/day calcium (repletion phase). Serum and urine samples were obtained from each 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 repletion phase. Serum levels of calcium, PTH, 1,25-dihydroxyvitamin D-3, osteocalcin, and C-terminal type I procollagen peptide (PICP) and urinary levels of calcium and deoxypyridinoline were determined. Serum and urinary calcium levels were significantly reduced, and serum PTH and 1,25-dihydroxyvitamin D, levels were markedly increased during depletion. These changes were completely reversed after 1 week of repletion. Depletion also rapidly and significantly increased the urinary deoxypyridinoline level, indicating increased bone resorption. The increase also returned rapidly to baseline upon repletion. Calcium depletion had contrasting effects on bone formation markers; whereas depletion significantly reduced the serum PICP level, it significantly increased serum osteocalcin level. Past histomorphometric studies in rodents indicated 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 depletion-associated inhibition of bone formation, the increase in serum osteocalcin could represent this depletion-related increase in osteoblast number. During repletion, serum osteocalcin remained elevated above baseline. PICP recovered from its depressed level and increased above baseline, a finding consistent with past kistomorphometric findings of increased bone formation during repletion. In summary, this study confirms that 1) a short calcium depletion period produces calcium stress in young women, which leads to rapid stimulation of bone resorption and inhibition of bone formation; and 2) a subsequent calcium repletion 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.