title: Direct measurement of dietary fractional absorption using calcium isotopic tracers.
abstract: 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.
mesh_terms: D000293:Adolescent; D002118:Calcium / Q000378:metabolism*; D002127:Calcium Isotopes; D006801:Humans; D007223:Infant; D007231:Infant, Newborn; D007234:Infant, Premature; D007408:Intestinal Absorption; D008297:Male; D013058:Mass Spectrometry / Q000379:methods*; D008954:Models, Biological; D014018:Tissue Distribution
chemical_list: D002127:Calcium Isotopes; D002118:Calcium