Use of Calcium Tracers and Biomarkers to Determine Calcium Kinetics and Bone Turnover

The purpose of this article is to review approaches to measuring calcium metabolism and bone turnover, which can be used to assess bone loss during spaceflight and to evaluate the effectiveness of interventions aimed at reducing bone loss. Isotopic tracers of calcium can be used to determine absorption, excretion, retention, body pool sizes, and turnover rates. Because 99% of the body’s calcium is in the bone, calcium tracer kinetics can provide an accurate assessment of bone turnover rates. Biomarkers can be used to assess qualitative changes in bone formation and bone resorption, but they do not predict bone density or quantitate net calcium retention. The usefulness of biomarkers has been validated against calcium tracer kinetics and bone histomorphometry. Calcium kinetics and biomarkers of bone turnover can assess changes in calcium metabolism and bone turnover long before changes in bone density can be detected.