What it is
Lactobionic acid is a polyhydroxy acid formed from gluconic acid and galactose, classed alongside gluconolactone as a next-generation alternative to alpha-hydroxy acids. Its multiple hydroxyl groups make it strongly water-binding, so it both attracts moisture and provides milder surface exfoliation than smaller AHAs, with the large molecule penetrating more slowly. It also chelates metal ions, which supports formula stability, and is used to help adjust and buffer pH.
Safety & warnings
As a polyhydroxy acid it is generally considered gentler and less irritating than smaller AHAs such as glycolic acid, and is often chosen for sensitive skin. Like other acids, it can increase the skin's sensitivity to the sun, so daytime sun protection is advised; the large molecule and water-binding nature tend to limit stinging.
Origin & sourcing
Synthetic or biotechnological: typically produced by oxidation of lactose (derived from milk), yielding a defined sugar-acid.
History
Lactobionic acid is well known outside cosmetics as a component of organ-preservation solutions, and it was later adopted in skincare as part of the polyhydroxy acid category positioned as a gentler option than traditional alpha-hydroxy acids.
Questions everyone asks
Is lactobionic acid an AHA?
It is a polyhydroxy acid (PHA), a related class often regarded as gentler than traditional alpha-hydroxy acids.
Why is it used in a sunscreen?
It can serve as a humectant, mild exfoliant, chelating agent, and pH buffer; it is not a UV filter.