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Lactic acid is a ubiquitous and essential biological molecule, in humans and other mammals, but also in most if not all vertebrate and invertebrate animals, as well as in many micro-organisms. As such the biokinetics, metabolism and distribution of lactic acid have to be considered in the context of its normal biochemistry; exogenous lactic acid will be indistinguishable from endogenous lactic acid and will follow the same biochemical pathways as endogenous lactic acid, at least up to a certain systemic level.

The biochemistry of lactic acid has been reviewed and summarized in Sterenborg, 2007.

Dermal absorption of lactic acid is not a toxicologically relevant pathway, since lactic acid is a natural constituent of the human dermis and epidermis. Lactic acid is frequently used as a humectant in leave-on skin cosmetics, where its main mode of action is through its sequestration in the stratum corneum, where it will aid in attracting water into the SC and holding it there. The cosmetics-oriented skin penetration studies clearly supports the notion that lactic acid does not penetrate the skin, but is sequestered in the SC, and even there only up to ca 16% of the applied amount.

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