Use of stable isotopes to study bioconversion and bioefficacy of provitamin A carotenoids.

Provitamin A carotenoids (pVACs) are important contributors to vitamin A status and in reducing vitamin A deficiency in vulnerable populations. However, there are uncertainties about the effectiveness of dietary pVACs to provide sufficient amounts of vitamin A due to large variations in provitamin A bioefficacy, and if provitamin A carotenoids other than β-carotene could be utilized in biofortification programs. Although animal studies have compared the bioefficacy of β-cryptoxanthin and β-carotene in biofortified maize by measuring liver retinol concentrations after dietary exposure, it is unclear whether the higher bioavailability of β-cryptoxanthin can be offset by the lower conversion of β-cryptoxanthin to retinol, and how post-intestinal conversion of β-cryptoxanthin may contribute to the total bioefficacy of β-cryptoxanthin. Here, we present a method that, for the first time, uses stable isotope labeled β-cryptoxanthin and β-carotene to quantify bioconversion and bioefficacy of both pVACs simultaneously in human volunteers. The paper describes how positioning of the [13C] labels around the centric 15,15' double bond on either the β-carotene or β-cryptoxanthin molecule allows quantification of retinoids from both pVACs, and details the procedure for sample preparation and analysis using LC-MS/MS. Finally, we apply and discuss recent approaches to quantify bioconversion and bioefficacy of isotopically labeled β-carotene and β-cryptoxanthin in humans.