Metabolic balance studies have a long and important history in medical research [1]. On the most fundamental level balance studies are used to quantitate whether the amount of a substance (mass) has been added, retained or lost from the body. Over a fixed period of time, input and output are measured, the latter is subtracted from the former, and if there is no internal production or degradation, the results indicate either a positive (gain of mass) or a negative (loss of mass) balance. The demands of the balance technique require that the organism be in a ‘steady state’; during these measurements, the ionic and hormonal milieu must be constant or, if not, at least accounted for by this now more complex model. During growth, one would expect a positive mass balance of most ions, including calcium, and with osteoporosis a negative calcium balance. However, while balance studies have value, they are not designed nor are they sufficient to understand how an ion, such as calcium, is redistributed within the body. For that more complex understanding, which can be termed kinetic modeling, one would need to know not only if the patient was in positive, neutral or negative mass balance but if an ion was moving from one body compartment to another. During growth, one would expect not only a positive mass balance for calcium but movement of this ion from the intestine into the blood and then into bone. This complex ionic choreography is controlled by a number of ions in addition to calcium and several hormones and growth factors.

With respect to calcium, as far as we know, humans do not have a mechanism to measure mass nor balance; our bodies sense only blood-ionized calcium concentration ([Ca2+]), a measure of mass/volume of a liquid phase. …

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