When calcium forms bone, there is an isotopic fractionation compared to the calcium provided by serum. When bone dissolves, it does so as a bulk dissolution; hence, it releases the isotopically light calcium into the soft tissues of the body. This can be detected in serum and urine, and so provide a monitor of bone status that is more sensitive to changes than traditional bone density scans. In addition, unlike measurements of hormonal bone formation and resorption markers, calcium isotopes provide a quantitative value for the net bone gain or loss.
NASA is interested in having rapid indicators of bone status as they try to ameliorate the bone loss that comes in long-term space flight. We are currently validating calcium isotopes as an indicator of bone status in bed rest patients in a NASA facility, and have also examined their use in rhesus and marmoset monkeys.

Image copied without permission from Orthoteers, an orthopedic education website.