MEDICAL APPLICATIONS OF CALCIUM AND OTHER “NON-TRADITIONAL” STABLE ISOTOPE SYSTEMS

Intra-organismal mass-dependant variations in "non-traditional" stable isotopes (e.g., isotopes of zinc, iron, calcium, etc.) are a promising but little-explored area of biomedical research. Investigations of biological isotopic behavior have been conducted for only a handful of elements, and only the calcium and iron systems have been elaborated in any detail. Here, we report emerging calcium data that point to medical applications.

Analysis of urine from patients in a long-term bed rest study show that the calcium isotope composition of urine changes in response to bone mineral balance. Negative bone mineral balance results in urine calcium that is isotopically light compared to urine calcium from patients with a more positive bone mineral balance. These isotopic changes mainly result from the liberation and removal of isotopically light calcium during bone formation and resorption, respectively. Urinary calcium also is uniformly heavier than dietary calcium, a shift that most likely is caused by renal calcium isotope fractionation.

Calcium isotopic analysis of human urine or blood provides information on short-term changes in bone mineral balance that cannot be obtained with current techniques that employ bone mineral density measurements, calcium isotope tracers or biochemical markers of bone formation and resporption. Hence calcium isotopes may prove to be an important tool in the early diagnosis of metabolic bone diseases like osteoporosis, and in the rapid development of new treatments for these diseases.

The potential utility of calcium and other “non-tradition” stable isotopes does not depend on a uniquely biological mode of isotope fractionation (i.e., “vital effects”). The behavior of calcium isotopes within vertebrate animals is essentially the same as it is in geological systems, and can be understood using the same models. The same is likely to prove true for iron. In fact, the assumption that isotope fractionation that occurs within organisms is mechanistically distinct from abiological isotope fractionation is an impediment to the development of medical applications for isotope chemistry, because it obscures the relevance of geochemical knowledge to medical problems.