3 Trace Element Utilization in Humans
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Studied with Enriched Stable Isotopes and Thermal Ionization Mass Spectrometry JUDITH R. TURNLUND U.S. Department of Agriculture, Western Regional Research Center, Berkeley, CA 94710
Stable isotopes have recently proved to be valuable for determining absorption of zinc, copper, and iron in humans. Thermal ionization mass spectrometry, with i t s high degree of precision, has been successfully used for analysis of stable isotopes in samples from bioavailability studies. While precision of this analytical approach i s excellent, analysis i s time consuming. Isotopic ratios of iron can now be determined using an automated thermal ionization mass spectrometer, which markedly reduces analytical time. Two methods can be used to determine the amount of an isotopic spike in a sample with mass spectrometry: (1) by using the ratio of an enriched isotope to a natural isotope and the total mineral content of the sample, determined independently and (2) by using isotope dilution and determining two isotopic ratios. Several aspects of experimental design are critical when using stable isotopes to study mineral bioavailability. These include the level of isotope required to achieve an adequate enrichment in the tissue to be sampled, the effect of this level on mineral metabolism or absorption, complete intestinal transit time in studies using fecal monitoring, and sample homogeneity. Adaptation to type of diet or level of a nutrient, individual v a r i a b i l i t y , and nutritional status must also be considered. Mineral absorption has been determined in several studies, comparing zinc, copper, and iron absorption from several types of diets and in different population groups. Results of these studies suggest a high level of phytate in the diet inhibits zinc, but not copper, absorption; pregnant women tend to absorb slightly, but not significantly more zinc and significantly more copper than nonpregnant women; and elderly men absorb less zinc, but not less copper or iron, than young men. This chapter not subject to U.S. copyright. Published 1984, American Chemical Society Turnlund and Johnson; Stable Isotopes in Nutrition ACS Symposium Series; American Chemical Society: Washington, DC, 1984.
STABLE ISOTOPES IN NUTRITION
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Several approaches have /been used to determine absorption of trace elements in humans. The most frequently used method has been balance studies, in which the amount of a mineral ingested is compared with the amount eliminated in the feces. However, absorption calculated from total mineral eliminated in fecal collections generally differs greatly from true absorption, since some of the mineral eliminated in the feces is of endogenous origin (1). A number of other d i f f i c u l t i e s with metabolic balance studies, such as variation in intestinal transit time and inadequate analytical precision, limit their usefulness and often result in conflicting results (2). Use of Isotopic Tracers Many of the disadvantages of the balance approach to determining bioavailability can be eliminated or minimized by using radioactive tracers to measure absorption and u t i l i z a t i o n of trace elements. This approach has been used by a number of investigators to study zinc and iron absorption (3-7). Recently methods have been developed using enriched stable isotopes of trace elements to study absorption and u t i l i z a t i o n . A number of these studies, using a variety of methods and minerals, are described in this symposium. Using enriched stable isotopes provides many of the advantages of radioisotopic tracers without the exposure to radioactivity. Minerals contained in a specific meal or consumed on a specific day can be labeled and differentiated from minerals consumed at other times and from other endogenous minerals (8). Use of either stable or radioactive isotopes eliminates the problems of variablity of fecal flow, prolonged transit time, and excretion of endogenous minerals which confound results of absorption studies using the balance approach. The stable isotopes and several of the radioactive isotopes which have been used in human and animal studies are shown in Table 1. The natural abundance of stable isotopes and the half-lives of some of the most suitable radioisotopes are also included in Table I. Both zinc and iron have several stable isotopes and radiosotopes well suited for use as tracers. Magnesium and copper have relatively abundant amounts of their stable isotopes. Therefore, higher levels of these isotopes are required to achieve adequate sample enrichment. Nevertheless, stable isotopes of these minerals are particularly attractive alternatives to radioisotopes, since the radioactive isotopes have very short half-lives. The half l i f e of Mg is only 21 hours. Half-lives of **Cu and C u are 12.9 and 61.9 hours respectively. Stable isotopes techniques cannot be used for manganese, since i t has only one stable isotope. 28
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Turnlund and Johnson; Stable Isotopes in Nutrition ACS Symposium Series; American Chemical Society: Washington, DC, 1984.
3.
Table I.
Stable Isotope
Mineral
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Trace Elements in Humans
TURNLUND
Stable Isotopes
Radio Isotope
Zinc
64 66 67 68 70
Abundance % 48.89 27.81 4.11 18.57 0.62
Iron
54 56 57 58
5.82 91.66 2.19 0.33
59 55
45.1 days (γ & β") 2.6 years (X-Ray)
Magnesium
24 25 26
78.70 10.13 11.17
28
21 hours (γ & β")
Copper
63 65
69.09 30.90
64 67
12.9 hours
