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Isotope dilution is currently the most accurate technique in humans to determine vitamin A status and bioavailability/ bioconversion of provitamin A carotenoids such as α -carotene. However, limits of MS detection, coupled with extensive isolation procedures, have hindered investigations of physiologically-relevant doses of stable isotopes in large intervention trials. Here, a sensitive liquid chromatography- tandem mass spectrometry (LC/MS/MS) analytical method was developed to study the plasma response from coadministered oral doses of 2 mg [ 13 C 10 ] α -carotene and 1 mg [ 13 C 10 ]retinyl acetate in human subjects over a 2 week period. A reverse phase C 18 column and binary mobile phase solvent system separated α -carotene, retinol, retinyl acetate, retinyl linoleate, retinyl palmitate/retinyl oleate, and retinyl stearate within a 7 min run time. Selected reaction monitoring of analytes was performed under atmospheric pressure chemical ionization in positive mode at m/z 537 ? 321 and m/z 269 ? 93 for respective [ 12 C] α -carotene and [ 12 C] retinoids; m/z 547 ? 330 and m/z 274 ? 98 for [ 13 C 10 ] α -carotene and [ 13 C 5 ] cleavage products; and m/z 279 ? 100 for metabolites of [ 13 C 10 ]retinyl acetate. A single one-phase solvent extraction, with no saponification or purification steps, left retinyl esters intact for determination of intestinally-derived retinol in chylomicrons versus retinol from the liver bound to retinol binding protein. Coadministration of [ 13 C 10 ] retinyl acetate with [ 13 C 10 ] α -carotene not only acts as a reference dose for inter-individual variations in absorption and chylomicron clearance rates, but also allows for simultaneous determination of an individual's vitamin A status. - Oxley, A., P. Berry, G. A. Taylor, J. Cowell, M. J. Hall, J. Hesketh, G. Lietz, and A. V. Boddy. An LC/MS/MS method for stable isotope dilution studies of α -carotene bioavailability, bioconversion, and vitamin A status in humans. Copyright