This study examined the effects of daily oral magnesium (Mg) supplementation on bone turnover in 12 young (27-36 yr old) healthy men. Twelve healthy men of matching age, height, and weight were recruited as the control group. The study group received orally 15 mmol Mg (Magnosolv powder, Asta Medica) daily in the early afternoon with 2-h fasting before and after Mg intake. Fasting blood and second void urine samples were collected in the early morning on days 0, 1, 5, 10, 20, and 30, respectively. Total and ionized Mg2+ and calcium (Ca2+), and intact PTH (iPTH) levels were determined in blood samples. Serum biochemical markers of bone formation (i.e. C-terminus of type I procollagen peptide and osteocalcin) and resorption (i.e. type I collagen telopeptide) and urinary Mg level adjusted for creatinine were measured. In these young males, 30 consecutive days of oral Mg supplementation had no significant effect on total circulating Mg level, but caused a significant reduction in the serum ionized Mg+ level after 5 days of intake. The Mg supplementation also significantly reduced the serum iPTH level, which did not appear to be related to changes in serum Ca2+ because the Mg intake had no significant effect on serum levels of either total or ionized Ca2+. There was a strong positive correlation between serum iPTH and ionized Mg2+ (r = 0.699; P < 0.001), supporting the contention that decreased serum iPTH may be associated with the reduction in serum ionized Mg2+. Mg supplementation also reduced levels of both serum bone formation and resorption biochemical markers after 1-5 days, consistent with the premise that Mg supplementation may have a suppressive effect on bone turnover rate. Covariance analyses revealed that serum bone formation markers correlated negatively with ionized Mg2+ (r = -0.274 for type I procollagen peptide and -0.315 for osteocalcin), but not with iPTH or ionized Ca2+. Thus, the suppressive effect on bone formation may be mediated by the reduction in serum ionized Mg2+ level (and not iPTH or ionized Ca2+). In summary, this study has demonstrated for the first time that oral Mg supplementation in normal young adults caused reductions in serum levels of iPTH, ionized Mg2+, and biochemical markers of bone turnover. In conclusion, oral Mg supplementation may suppress bone turnover in young adults. Because increased bone turnover has been implicated as a significant etiological factor for bone loss, these findings raise the interesting possibility that oral Mg supplementation may have beneficial effects in reducing bone loss associated with high bone turnover, such as age-related osteoporosis.