Bioequivalence of β-carotene and retinol
2007
Thurnham, D.I.
For many years it was accepted that 6 mg of β-carotene were required to produce 1 mg of vitamin A in the form of retinol. The equivalence was based on the assumptions that two-thirds of dietary β-carotene are not absorbed, while in the metabolism of the remaining third 1 mol of β-carotene is converted to 1 mol retinol. Recently, the bioequivalence was raised to 12 mg β-carotene and 1 mg retinol. The objective of this review was to re-examine the data that were used to support the new equivalence ratio, especially since some of these data were obtained in developing countries where infestation with gut parasites and exposure to other infections is common, yet the influence of inflammation on plasma carotenoid and retinol concentrations is frequently ignored. Bioequivalence studies examined in this review include those done in developing and developed countries, depletion and repletion studies, feeding with vegetable sources of β-carotene or pure supplements, influence of helminths, carotenoid interactions and matrix effects and studies using stable isotopes (SI). SI studies show the bioefficacy of β-carotene conversion to retinol is generally poor even for pure β-carotene unless the dose is small and fed regularly until equilibration is reached. Retinol formation appears to be inversely influenced by previous vitamin A intake, the amount of material given and current vitamin A status. In spite of technical complexities, more SI studies where liver reserves of vitamin A are determined pre and post intervention are needed to evaluate β-carotene bioefficacy of different vegetable sources.
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