Stable Isotopic Labels Versus Radiotracers

Early attempts to study element absorption in humans were based on chemical balance studies. They are often referred to as cold balance studies as they do not involve the use of isotopic tracers. In such studies, volunteers are fed a highly controlled diet and absorption of the element is assessed by subtracting the amount recovered in feces from the amount present in the food consumed. Element retention can be assessed in a similar maimer by subtracting the amount of element recovered in urine over the observational period. 

Some of the technical difficulties associated with chemical balance studies can be overcome by measuring the increase in element concentration in plasma over several hours after test meal intake instead of analyzing the feces. In such tests, the area under the curve describing changes in element concentration is 

The first ever application of a radiotracer in a biological experiment dates back to 1923 when George de Hevesy used Pb to study plant uptake of lead from solution [5]. His seminal work was honored by the Nobel Prize in Chemistry in 1943 and made him the father of isotope tracing, a tool that is still indispensable in virtually any area of scientific research. The first use of a stable isotope to study mineral metabolism was reported in 1963, when Lowman and Krivit injected stable Fe together with radioactive Fe into a human subject to compare the plasma clearance of the two isotopes [6]. However, it was not until the 1980s that stable isotope techniques were explored systematically to study mineral and trace element metabolism in humans. This was not only due to the increasing recognition of health hazards associated with the use of radioisotopes. Mass spectro-metric techniques had to be refined to measure isotope ratios of the heavier elements at a precision suitable for the exploitation of isotopically enriched elements as tracers. Stable isotopic labels are made up from the same isotopes as the natural element, from which they differ only in terms of composition, that is, in the relative abundances of their isotopes. 

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