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Although lactic acid doesn't technically exist in the skin, its sodium salt form (sodium lactate) does, and is part of the skin’s ‘natural moisturising factor’.
The natural moisturising factor is a group of naturally occurring humectants, including amino acids, urea, glycerin and hyaluronic acid, which supports the barrier function of the stratum corneum and helps to maintain the skin's elasticity.
Lactic acid in its true form (C3H6O3) is commonly used in topical skincare, and these days it's usually synthetically made in a lab but was traditionally created as a natural by-product of plant and dairy fermentation. Like glycolic acid, lactic acid is an alpha hydroxy acid.
It has a number of benefits for the skin, including its gentle exfoliation properties. “Lactic acid causes the breakdown of the desmosomes (skin binding enzymes) to induce surface cells to be released, stimulating the exfoliation process,” says Rebecca Jones, trainer and ambassador for Noon Aesthetics.
Jones adds, “The deeper layer of skin is stimulated to increase the synthesis of collagen and glycosaminoglycans (the skin’s water carriers).”
Lactic acid has a larger molecule size than some of the other AHAs like glycolic acid, so it is gentler and can't penetrate as far into the skin. This means that it has most of its exfoliating effects on the skin's surface, therefore helping with depigmentation.
Dr Catharine Denning, a cosmetic doctor who runs a regular clinic from The Light Centre in London, says, “Lactic acid breaks the bonds between dead skin cells on the surface of the epithelium, which helps to shift superficial pigmentation (in high strengths and lower pH).”
Lactic acid can also reduce the appearance of fine lines and help stimulate ceramide formation to help strengthen the skin's barrier function.
Denning adds, “Its antioxidant properties help mop up free radicals and heavy metals that may come into contact with the skin and so protects it against DNA damage.”
In addition, there is scientific evidence which shows that lactic acid can boost the skin's vitality. Cigdem Kemal Yilmaz, a chemical engineer, skincare formulator and the founder of Skin Masterclass, explains, “Research shows that treatment with 12% lactic acid resulted in increased epidermal and dermal firmness and thickness, and clinical improvement in skin smoothness and the appearance of fine lines and wrinkles.
“No dermal changes were observed after treatment with 5% lactic acid; however, similar clinical and epidermal changes were noted.”
Yilmaz concludes, “The clinical evidence indicates that the topical application of lactic acid is effective for depigmentation and improving the roughness and mild wrinkling of the skin caused by environmental photodamage.”