Abstract

Assessment of the bioavailability of exogenous ions present in a large amounts in the body, such as magnesium, cannot be performed by the conventional measurement of plasma levels after intravenous and/or oral administration. In the case of magnesium, this is emphasized by the fact that plasma levels are quickly regulated, mainly by the kidney and in storage compartments such as bone, after exogenous administration. Magnesium bioavailability and absorption are studied by indirect methods or by using radioactive or stable isotopes as tracers. Indirect methods are the metabolic balance method and comparison of urinary excretion between a treatment and a placebo period, often after magnesium load. However, the former only measures magnesium absorption and the latter is subject to the fragile balance of magnesium urinary excretion. Isotope studies, in particular with stable isotope probes, have benefited from the developments in mass spectrometry, such as inductively coupled plasma mass spectrometry (ICP-MS). It is possible to follow exogenous magnesium in plasma after oral and intravenous administrations using 25Mg and 26Mg as tracers, and to calculate the absolute bioavailability of magnesium.