Dual isotope test for assessing β-carotene cleavage to vitamin A in humans

Summary. Background: The ability of β-carotene to deliver bioactive retinoids to tissues is highly variable. A clearer understanding of the environmental and genetic factors that modulate the vitamin A potential of β-carotene is needed. Aim of study: Assess the vitamin A value of orally administered β-carotene relative to a co-administered reference dose of preformed vitamin A. Methods: Equimolar doses (30 μmol) of hexadeuterated D 6 β-carotene and D 6 retinyl acetate were orally co-administered in an emulsified formulation to a male subject. The plasma concentration time courses of D 6 retinol (derived from D 6 retinyl acetate) and bioderived D 3 retinol (from D 6 β-carotene) were determined for 554 h postdosing using gas chromatography/mass spectrometry. Intact D 6 β-carotene plasma concentrations were determined by high-pressure liquid chromatography. The ratio of the two forms of vitamin A, D 6 retinol/D 3 retinol, at any single time point is postulated to reflect the quantity of vitamin A derived from β-carotene relative to preformed vitamin A. Additionally, a minute amount of 14 C β-carotene (50 nCi; 0.27 μg) was included in the oral dose and cumulative 24-h stool and urine samples were collected for two weeks to follow absorption and excretion of the b-carotene. The 14 C nuclide was detected using accelerator mass spectrometry (AMS). Results During the absorption/distribution phase (3–11 h) the D 6 /D 3 ratio of the two retinols was not stable and ranged between a value of 3 and 16. Between 11 and 98 h postdosing the ratio was relatively stable with a mean value of 8.5 (95 % CI: 7.5, 8.7). These data suggest that in this subject and under these conditions, 8.5 moles of β-carotene would provide a vitamin A quantity equivalent to 1 mole of preformed vitamin A. On a mass basis, 15.9 μg of β-carotene was equivalent to 1 μg of retinol. The total administered β-carotene was found to be 55 % absorbed by AMS analysis of cumulative stool. Conclusion: The co-administration of D 6 β-carotene and D 6 retinyl acetate provides a technique for assessing individual ability to process β-carotene to vitamin A. The results indicate that a single time point taken between 11–98 h after dose administration may provide a reliable value for the relative ratio of the two forms of vitamin A. However, results from more subjects are needed to assess the general utility of this method.