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Background: Retinaldehyde (RAL), a natural metabolite of β-carotene and retinol (ROL), is tolerated by human skin after topical application. Purpose: To see if topical application of a large quantity of RAL on human skin is associated with a detectable alteration of constitutive levels of plasma retinoids resulting from metabolism of RAL in the skin. Methods: Plasma retinoids [ROL, all-trans-retinoic acid (all-trans-RA), RAL, retinyl palmitate/oleate, 13-cis-RA and 4-oxo-13-cis-RA] were analyzed by high-pressure liquid chromatography. Determinations were done in 10 healthy male volunteers kept on a vitam-A-poor diet before, during and after daily topical application of 7 mg of RAL to 40% of the body surface for 14 days. Results: The introduction of a restricted vitamin A diet before RAL application resulted in a decrease in the plasma levels of ROL, all-trans-RA and retinyl palmitate/oleate. Topical application of RAL did not induce an alteration of the plasma levels of retinoid metabolites. No RAL was detectable in any of the plasma samples. Conclusion: The skin metabolism of topically applied RAL does not result in detectable alterations of constitutive levels of plasma retinoids in humans.

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