Background: We previously reported that dietary sphingomyelin (SM) concentrate from bovine milk improves epidermal functions. SM is a known precursor of ceramide (Cer) in the stratum corneum (SC). Neither the uptake nor distribution of orally administered SM nor its effects on epidermal functions have been demonstrated. Objective: We evaluated the effects of dietary SM on epidermal functions, and the distribution and fate of its radiolabeled metabolites in mice orally administered [4,5-3H-sphinganyl] sphingomyelin (3H-SM). Methods: Bovine milk SM (98% purity) was administered orally to 13-week-old hairless mice at 142mg/kg per day for eight weeks. Their SC hydration, transepidermal water loss (TEWL), and SC Cer content were measured. 3H-SM was then administered orally to 10-week-old hairless mice. Its distribution and metabolites in the skin were evaluated with whole-body autoradiography, liquid scintillation counting, and thin-layer chromatography. Results: SC hydration in the SM-administered mice was higher than that in control mice, whereas their TEWL and Cer contents did not differ. Radioactivity was distributed extensively in the bodies of the experimental mice and decreased gradually with time. In contrast, the radioactivity in the SC remained constant after its administration, and radiolabeled SM and Cer were detected in the skin. This suggests that dietary SM is transferred to the skin and then converted to Cer in the SC. Conclusions: Orally administered SM is incorporated into skin SM and converted to SC Cer, which is involved in the water-holding capacity of the SC.
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