A Minute Dose of 14C-β-Carotene Is Absorbed and Converted to Retinoids in Humans

Abstract

Our objective was to quantify the absorption and conversion to retinoids of a 1.01-nmol, 3.7-kBq oral dose of ¹⁴C-β-carotene in 8 healthy adults. The approach was to quantify, using AMS, the elimination of ¹⁴C in feces for up to 16 d after dosing and in urine for up to 30 d after dosing. The levels of total ¹⁴C in undiluted serial plasma samples were measured for up to 166 d after dosing. Also, the levels of ¹⁴C in the retinyl ester (RE), retinol (ROH), and β-carotene fractions that were isolated from undiluted plasma using HPLC were measured. The apparent digestibility of the ¹⁴C was 53 ± 13% (mean ± SD), based on the mass balance data, and was generally consistent with the area under the curve for zero to infinite period of ¹⁴C that was eliminated in the feces collections made up to 7.5 d after dosing. Metabolic fecal elimination, calculated as the slope per day (% ¹⁴C-dose/collection from d 7.5 to the final day), was only 0.05 ± 0.02%. The portion of the ¹⁴C dose eliminated via urine was variable (6.5 ± 5.2%). Participants [except participant 6 (P6)] had a distinct plasma peak of ¹⁴C at 0.25 d post-dose, preceded by a shoulder at ∼0.1 d, and followed by a broad ¹⁴C peak that became indistinguishable from baseline at ∼40 d. Plasma ¹⁴C-RE accounted for most of the absorbed ¹⁴C early after dosing and P1 had the longest delay in the first appearance of ¹⁴C-RE in plasma. The data suggest that plasma RE should be considered in estimating the ROH activity equivalent of ingested β-carotene.