Alpha-hydroxy acids are effective agents for the treatment of skin xerosis and it is known that, following treatment with lotions containing D,L-lactic acid, the stratum corneum prevents xerosis more effectively. To date, the relative efficacy of the different isomers of lactic acid has not been evaluated and the mode of action of lactic acid in improving stratum corneum resilience is not known. The objective of the present studies was to determine the effects of lactic acid isomers on keratinocyte ceramide biosynthesis, stratum corneum barrier function and the resistance of the stratum corneum to the appearance of skin xerosis. In vitro, lactic acid enhanced the production of ceramides by keratinocytes. L-Lactic acid was more effective than the D isomer (300% increase vs 100% increase). Carbon label from lactic acid was incorporated into all keratinocyte lipid species and a greater incorporation of label into ceramides was achieved with L-lactate than with D-lactate. In vivo, lactic acid increased the levels of stratum corneum ceramides. Whereas, lotions containing L-lactic acid resulted in the greatest increase (48% increase) followed by D,L-lactic acid (25% increase), D-lactic acid had no effect on stratum corneum ceramide levels. The increases in stratum corneum ceramide levels following lactic acid treatment also led to improvements in stratum corneum barrier function, measured by transepidermal water loss following a challenge to the skin with SLS and in the regression phase of a moisturization efficacy study. Significant improvements in barrier function and resistance to the appearance of skin xerosis were observed following L-lactic acid and D,L-lactic acid, but not following D-lactic acid treatment. From these results we believe that lactic acid, particularly the L isomer, stimulates ceramide biosynthesis leading to increased stratum corneum ceramide levels which results in superior lipid barrier and a more effective resistance against xerosis.

Alpha-hydroxy acids are effective agents for the treatment of skin xerosis and it is known that, following treatment with lotions containing D,L-lactic acid, the stratum corneum prevents xerosis more effectively. To date, the relative efficacy of the different isomers of lactic acid has not been evaluated and the mode of action of lactic acid in improving stratum corneum resilience is not known. The objective of the present studies was to determine the effects of lactic acid isomers on keratinocyte ceramide biosynthesis, stratum corneum barrier function and the resistance of the stratum corneum to the appearance of skin xerosis. In vitro, lactic acid enhanced the production of ceramides by keratinocytes. L-Lactic acid was more effective than the D isomer (300% increase vs 100% increase). Carbon label from lactic acid was incorporated into all keratinocyte lipid species and a greater incorporation of label into ceramides was achieved with L-lactate than with D-lactate. In vivo, lactic acid increased the levels of stratum corneum ceramides. Whereas, lotions containing L-lactic acid resulted in the greatest increase (48% increase) followed by D,L-lactic acid (25% increase), D-lactic acid had no effect on stratum corneum ceramide levels. The increases in stratum corneum ceramide levels following lactic acid treatment also led to improvements in stratum corneum barrier function, measured by transepidermal water loss following a challenge to the skin with SLS and in the regression phase of a moisturization efficacy study. Significant improvements in barrier function and resistance to the appearance of skin xerosis were observed following L-lactic acid and D,L-lactic acid, but not following D-lactic acid treatment. From these results we believe that lactic acid, particularly the L isomer, stimulates ceramide biosynthesis leading to increased stratum corneum ceramide levels which results in superior lipid barrier and a more effective resistance against xerosis.

Synopsis Ceramide 1 is the main repository of stratum corneum linoleic acid, and changes in the levels of ceramide 1 linoleate are associated with cutaneous abnormalities: essential fatty acid deficiency, atopic dermatitis and acne. We have previously reported seasonal variation in stratum corneum lipids where lipids, particularly ceramides, are very much reduced in winter which probably influences the appearance of skin xerosis in this season. However, investigations into the seasonal variation in ceramide 1 fatty acid levels and the effects of topical essential fatty acids on the composition of these lipid species have not been conducted in healthy subjects. We determined the composition of stratum corneum ceramide 1 esterified fatty acids in leg skin from Caucasian women in winter and summer and also investigated the effects of topical triglycerides rich in linoleic acid on stratum corneum ceramide 1 fatty acid levels in winter. A dramatic seasonal variation was observed. Whereas decreased levels of linoleic acid and some saturated fatty acids (C-15 and C-24) were observed in the winter months, increased levels of oleic acid were found. Nevertheless, it was possible to normalize the levels of stratum corneum ceramide 1 linoleate following topical application of formulations containing linoleic acid-rich triglycerides for 4 weeks. The reduction in ceramide 1 linoleate may lead to impaired stratum corneum function and increased susceptibility to skin xerosis in winter. However, by using formulations containing linoleic acid esters it is possible to correct these changes and possibly improve barrier resilience.