1. Mass balance studies were carried out in fasted ileostomy subjects (n = 5) given an oral physiological dose (10 mg) of β-carotene [all-trans: 9-cis, 84:16 (w/w)] dispersed in vegetable oil. Blood and ileal effluent samples were collected and analysed for β-carotene.

2. Results showed that 90% (range 97.0–74.3%) of the total β-carotene was absorbed without measurable perturbation of plasma total β-carotene concentration, or change in the all-trans: 9-cis β-carotene ratio. Peak loss of β-carotene in ileal effluent occurred at 4.9 h (range 2.9–8.4 h) postingestion, and no further loss was detected after 5.4–12.4 h, depending upon the individual. Comparison of the ratio of all trans-β-carotene to 9 cis-β-carotene in the test meal and effluent indicated that isomerization did not occur during passage through the gastrointestinal tract and that both isomers were similarly absorbed. However, the all-trans: 9-cis β-carotene ratio of the plasma did not change. Reasoned assumptions allowed the construction of a mathematical model of plasma β-carotene disposal.

3. It is concluded that physiological doses of isolated all-trans and 9-cis β-carotene are well absorbed without necessarily causing detectable excursions in plasma β-carotene concentrations, or altering the ratio of all-trans to 9-cis β-carotene. Isomerization of β-carotene does not occur during passage through the gastrointestinal tract. Absorbed β-carotene is rapidly cleared from the plasma to an unobservable pool at a rate similar to that of chylomicron triacylglycerol.

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