Calcium

Stable isotopes of calcium (⁴²Ca, ⁴⁴Ca, ⁴⁶Ca, and ⁴⁸Ca) and radioisotopes of calcium (⁴⁵Ca and ⁴⁷Ca, with a half-life of 109 h) can be used for tracing calcium uptake, utilization, and excretion in the body. For example, most of our knowledge on the efficiency by which calcium is absorbed in the intestine (bioavailability) comes from studies in which calcium in the diet was labeled with stable or radioactive isotopes. In such studies, the isotope-labeled food is ingested and fecal matter tested for the presence and quantity of unabsorbed isotope. When coupling oral ingestion of food labeled with one calcium isotope with an intravenous injection of a second calcium isotope, this technique can be used as a means to measure calcium absorption within the body by measuring excretion of both tracers in the urine. In a similar fashion, dietary absorption of magnesium and zinc can be studied [184], [188]. Stable and radioactive isotopes are used in biomedical research and clinical practice to study disorders associated with calcium metabolism, in particular in relation to bone health and calcium accumulation in body tissues (vascular calcification, kidney stone formation). Stable isotope tracers have been used successfully to study bone calcium balance during space-flight and in-bed-rest studies. A long-living calcium radioisotope (⁴¹Ca), with a half-life of 9.9×10⁴ years, has been used successfully for labeling of bone calcium to measure bone calcium turnover via urinary excretion of the tracer [189].