Until recent years it has been possible to study calcium absorption in man only by using metabolic balance techniques. These demand a meticulous technique which limits their application to patients confined to a metabolic ward. Early radioactive tracer studies used the beta-emitting isotope 45Ca (Blau, Spencer, Swernov, Greenberg, and Laszlo, 1957) Of the several methods which have been used to assess 47Ca absorption, the determination of plasma radioactivity one or two hours after oral administration of the isotope is the simplest, although most liable to influence by factors other than absorption. Avioli, McDonald, Singer, and Henneman (1965) reported a good correlation between the plasma radioactivity measurement at one hour and absorption of the isotope as determined from faecal radioactivity measurements. On the other hand, Jaworski, Brown, Fedoruk, and Seitz (1963) and Kinney, Tauxe, and Dearing (1965) found little or no correlation between plasma47Ca levels and 47Ca absorption. In this paper the value of plasma 47Ca levels as an indication of 47Ca absorption has been reassessed by comparing results obtained using this method with those obtained in the same subjects by the use of a simple whole-body counter. In addition the validity of the whole-body counting method was assessed bycomparing the results with those obtained by measuring faecal radioactivity.'Present address:
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