increased awareness of the role played by the elements in human health and as etiologic factors in diseases (osteoporosis, heart disease, cancer, diabetes, kidney disease) (National Research Council, 1989; U.S. Department of Health and Human Services, 1988) as well as diagnostic and therapeutic adjuncts (obesity, inborn errors of metabolism, heart disease) has created an explosion in the need for stable isotopes at a time when the production capacity is questionable. Although stable isotopes occur naturally, their utility can be greatly enhanced when they are isolated and enriched through processes such as electromagnetic separation, cryogenic distillation, thermal diffusion, or other physical and chemical processes. These enriched stable isotopes are used as target materials in the preparation of radioisotopes with particle accelerators and nuclear reactors and as biological tracers in biomedical research and clinical applications. In addition, they serve as probes for basic physics and chemistry studies. Table 1-1 in the previous chapter lists some of the many fields that use stable or radioactive isotopes. Selected biomedical research topics and potential clinical uses of stable isotopes are given in Table 2-1.

Despite the pioneering role of the United States in this field, the country is rapidly becoming more dependent on sources in the former Soviet Union for many necessary isotopes. Serious debate is needed regarding the desirability of this dependence and the attendant possibility of the pernicious effects on U.S. science and technology of an isotope monopoly controlled by a single financially distressed foreign source.

CURRENT APPLICATIONS IN MEDICINE AND PHYSICAL AND LIFE SCIENCES

In many areas of research, the need for enriched stable isotopes is as vital as the need for pure chemicals (Friedlander and Wagner, 1982). Stable isotopes used primarily for research are usually purchased intermittently in relatively small amounts, from fractions of a gram to a few grams at a time. However, biomedical research often requires kilogram quantities (e.g., oxygen-18). The general approach to biomedical studies that use stable isotopes involves the same tracer methods used for radioactive isotopes. But since there is no radiation to detect, specialized analytical methods for detection of the specific stable isotopes are used. For example, after ingestion or injection of an isotope of zinc, the absorption of zinc by the human body can be determined by measuring the trace amounts that appear in the blood and urine over a period of days or weeks after ingestion or injection. The dilution of the stable isotope tracer gives information on the distribution of zinc in the body, and the rate of excretion gives information regarding how well a particular mineral is absorbed. The simultaneous use of two isotopes in two different foodstuffs can be used to determine how absorption of calcium, for example, might vary as a function of mode of intake.

The major advantages of using enriched stable isotopes for biomedical re-



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