The use of stable isotopes in studies of mineral metabolism.

In recent years stable isotopes of metallic and semi-metallic elements have become popular as tracers in studies of nutrition and metabolism. There are several recent reviews on the use of stable isotopes of the mineral elements (Janghorbani & Young, 1980; Janghorbani et af . 1985). The orientation of these reviews has tended toward neutron-activation analysis in conjunction with the faecal isotope balance (faecalmonitoring) technique. The present discussion will be orientated toward mass spectrometry and the use of stable isotopes in techniques other than faecal isotope balance. Stable isotopes have been used in studies of nutrition and metabolism since Schoenheimer & Rittenberg (1935) published their first paper on deuterated stearic acid. Since that time, the vast majority of stable-isotope research has utilized the isotopes of hydrogen, carbon, nitrogen and oxygen, with at least 2400 papers using 2H, 13C, "N and '80 being published between 1971 and 1978 (Klein & Klein, 1979). The natural ratios (rare isot0pe:abundant isotope) are usually smaller for H, C, N and 0 than the natural ratios for the metallic and semi-metallic elements. The low natural ratios combined with relatively large sample sizes and the high efficiency of gas-isotope-ratio mass spectrometry (GIRMS) yield analytical precisions (2 x relative SD) varying from l t3 to 1e2% for 2H:1H, 13C:l%, 15N:14N and l%O:l6O (Hachey et al. 1987). In contrast, analytical precision for isotope-ratio analysis of the mineral elements in recent biologicaltracer studies varied from about 0-1 to 10% (from Table 1). Analytical precision is an important factor in the design of an experiment because it determines the number of times a tracer may be diluted before it can no longer be detected. Nevertheless, the less favourable analytical precision available for the mineral elements is adequate for many studies and may be offset by opportunities for multiple-isotope-tracer experiments.