Radioisotopes, metal absorption studies

Stable isotopes of minerals are safe and convenient to use for bioavailability studies in humans Chelation of metals to form volatile complexes makes it possible to use ordinary EI/MS instrumentation for measurement of isotopic enrichment Stable isotopes can be complementary to radioisotopes in some of experiments Observations of fecal isotopic excretion are inherent to bioavailability and absorption studies using stable isotopes and are unlikely to be made in studies employing low level radioiaotopes ... 

Studies in human adults using radioisotopes or stable isotopes of Zn showed that Zn absorption is higher from human milk (41 percent) than from cow (28 percent) or soy formulas (14 percent) [77]. In any case, it seems that there is no need to fortify cow-based formulas with Zn, since cow milk contains much more Zn than breast milk [77]. However, Zn bioavailavility from breast milk is clearly higher than from formulas, which indicates a different composition in Zn species. Experiments in speciation of Zn in formula milk whey carried out by SEC showed that Zn was mostly found in the LMW (< 10 kDa) fraction, as a complex of citrate and only a small fraction of the metal was bound to whey proteins (435 kDa) [15, 18, 21]. Although such species are detected in human milk, important differences have been found between the distribution patterns of Zn in breast milk and in cow-based formula milks (see Fig. 17.11 and compare with Fig. 17.4). The levels of Zn in formulas are higher than in human milk and in colostrums [18]. 

In the last five years stable isotopes of many metals have proven to be useful in studying mineral absorption and metabolism in human subjects There is growing reluctance to use radioisotopes for such experiments and they are contraindicated in some populations Stable isotopes are beginning to fill this gap ... 

Industry has applied radiotracers in a very large variety of ways. More than half of the SOO largest manufacturing concerns in the United States use radioisotopes in the production of metals, chemicals, plastics, pharmaceuticals, paper, rubber, clay and glass products, food, tobacco, textiles, and many other products. Radioisotopes are used to study mixing efficiency, effect of chamber geometry, residence time in reactors, flow rates and patterns in columns and towers for fractionation, absorption, racemization, etc. Some of die many uses are listed in Table 9.6 and a few are described below to reflect the scope and value of the industrial applications of radioisotopes. Quite often the radionuclide used is not isotopic with the system studied. 

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