Using natural, stable calcium isotopes of human blood to detect and monitor changes in bone mineral balance.

We are exploring variations in the Ca isotope composition of blood and urine as a new tool for early diagnosis and monitoring of changes in bone mineral balance for patients suffering from metabolic bone disease, cancers that originate in or metastasize to bone, and for astronauts who spend time in low gravity environments. Blood samples are often collected instead of, or in addition to, urine in clinical settings, so it is useful to know if variations in the Ca isotope composition of blood carry the same information as variations in urine. We found that the Ca isotope composition of blood shifts in the same direction and to the same magnitude (~2 parts per ten thousand--pptt) as that of urine in response to skeletal unloading during bed rest.