AHAs: mandelic and lactic acid

The family

Alpha hydroxy acids carry a hydroxyl group on the carbon adjacent to the carboxyl. Glycolic (76 Da) is the smallest and most aggressive, lactic (90 Da) intermediate, mandelic (152 Da) the largest of the common ones. Size governs penetration rate: smaller molecules move through the stratum corneum faster, which means quicker results and more irritation.

Mandelic acid

Derived from bitter almonds, and roughly twice the molecular weight of glycolic. It penetrates more slowly and more evenly, which makes it markedly better tolerated - the usual choice for sensitive or darker skin, where aggressive exfoliation risks post-inflammatory pigmentation. It also has some antibacterial activity, which is why it appears in blemish-oriented formulas. Vegalab uses it in the Brightening line, paired with tranexamic acid and niacinamide, which is a sensible combination: three different pigmentation mechanisms and a gentle exfoliant rather than a harsh one.

Lactic acid

A natural component of the skin's own moisturising factor as well as an exfoliant, which gives it a dual role: at lower concentrations it behaves more as a humectant, at higher concentrations as a keratolytic. That makes it a good choice where exfoliation and hydration are both wanted, as in the Pro Resurfacing Acid Peel.

What does AHAs: mandelic and lactic acid not do?

They do not work below the epidermis at cosmetic concentrations, so claims about dermal remodelling from a leave-on AHA are overstated. They do not reduce oil production. And they increase photosensitivity - daytime sun protection is required, not advisable.