Improved, simplified methods to determine vitamin A status are needed to accurately assess both individuals and populations at risk of either vitamin A deficiency or toxicity. The vitamin A statuses of women and children in lower socioeconomic groups in both the underdeveloped world and the United States are compromised. Multiple reasons for this compromised status include low dietary intakes, increased demands during pregnancy and lactation and recurrent infections. The 13C-retinol isotope dilution (13C-RID) test, first developed in the rodent model using 13C4-retinyl acetate and gas chromatography-combustion-isotope ratio mass spectrometry (GCCIRMS), is sensitive and accurate. The major goals of this project are to validate the 13C-RID test for use in human population groups to assess total body reserves of vitamin A and determine vitamin A requirements of young adults. The success of this project will provide a sensitive tool for investigators to evaluate the vitamin A status of individuals and population groups using minimal sample sizes. The test has direct application to determining the outcome of intervention strategies aimed at improving vitamin A status and in determining vitamin A needs of humans and animals throughout their life cycles. The proposed studies will for the first time: (1) validate the 13C-RID test to assess a sub-toxic vitamin A status in non-human primates; (2) validate the 13C-RID test to assess sub-clinical vitamin A deficiency in the weanling pig; (3) refine the 13C-RID test for use in humans by looking at different dose levels and blood sampling times; and (4) investigate the use of the 13C-RID test in a human intervention study designed to look at vitamin A requirements in young adults.