298 JOURNAL OF COSMETIC SCIENCE reported to be approximately 5 mg water/100 mg of dry SC. Secondary water, held loosely by molecular bonds to the SC components, was measured using DSC techniques after subtracting the primary component. Normal skin had a 20-30% higher level of secondary water than xerotic and psoriatic skin, due to its higher secondary water- holding capacity. Takenouchi eta/. showed that normal skin had substantially increased levels of amino acids, i.e., hygroscopic NMF, and significantly higher water-holding capacity than scaly skin. Secondary bound water was associated with primary bound water through hydrogen-bonding interactions and exhibited rapid hydration and dehy- dration with environmental changes. Previous investigations in our laboratory have demonstrated that the water-holding capacity of normal skin, measured with the sorp- tion-desorption technique of Tagami, was significantly reduced following fresh water bathing (38,39). Based on the current study, we suggest that water soaking removes some water-soluble amino acids, i.e., NMF, from skin and thereby reduces the amount of secondary bound water. The decreased rate of moisture accumulation observed after soaking might be due to a decrease in the NMF-dependent bound water that gives rise to the higher capaci- tance reading prior to the soak. We further speculate that, in the presence of a normal SC barrier (i.e., normal TEWL, no damage) and the absence of eccrine sweating, the rate of moisture accumulation provides a dynamic, functional assessment of the loosely bound water ascribed to the hygroscopic NMF. In contrast to the single-point measurements of baseline hydration, the MAT technique differentiated the effects caused by soaking and NMF application (Tables IV, V). Therefore, MAT is preferred over single-point deter- minations. The results of the water soaking experiment suggest that SC recovery to normal is relatively slow. The findings raise questions about the effects on the skin of cumulative repeated wet-dry-wet exposures, such as those encountered in the diapering environment or in multiple hand washing situations (for health care workers, parents, child care providers). Additional studies are required to examine the kinetics of the depletion/ restoration and to confirm the removal of NMF by direct quantitation of the amino acids. Finally, investigations of the mechanisms by which the epidermis responds to common environmental effects (e.g., bathing) are warranted in order to develop appro- priate skin care practices and products. REFERENCES (1) I. H. Blank, Factors which influence the water content of the stratum corneum,J. Invest. Dermato/., 18, 433-440 (1952). (2) A. V. Rawlings, I. R. Scott, C. R. Harding, and P. A. Bowser, Stratum corneum moisturization at the molecular level, J. Invest. DermatoL, 103, 731-740 (1994). (3) M. Gloor, J. Bettinger, and W. Behring, Modification of stratum corneum quality by glycerin- containing external ointments, Hautarzt., 49, 6-9 (1998). (4) J. W. Fluhr, M. Gloor, L. Lehmann, S. Lazzerini, F. Distante, and E. Berardesca, Glycerol accelerates recovery of barrier function in vivo, Acta Derm. VenereoL, 79, 418-421 (1999). (5) R. Ghadially, L. Halkier-Sorensen, and P.M. Elias, Effects of petrolatum on stratum corneum struc- ture and function, J. Am. Acad. Dermatol., 26, 387-396 (1992). (6) I. Willis, The effects of prolonged water exposure on human skin, J. Invest. Dermatol., 60, 71 (1973).

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