Direct measurement of dietary fractional absorption using calcium isotopic tracers.

Fractional dietary Ca absorption, 'a', is measured by determining the ratio of two stable isotopic tracers, one of them orally (44Ca + 0.2-0.5 mg/kg) and the other intravenously (42Ca / 0.02-0.1 mg/kg). Thermal ionization mass spectometry (TIMS) is used to measure the perturbation of natural abundance isotope ratios (delta % excess). Typical sensitivity of the TIMS permits detection of a 2.5 delta % excess change from the natural Ca isotope ratio with relative standard deviations of about 0.5%. At sufficiently long times absorption becomes constant so that 'a' is determined by a product of constants and a measured ratio.