How PHAs Differ Mechanistically
How PHAs Differ Mechanistically — polyhydroxy acids, such as gluconolactone and lactobionic acid, that exfoliate much like AHAs but carry larger molecules with multiple hydroxyl groups. Their bigger size means they penetrate the skin more slowly and superficially, which tends to make them gentler and less likely to irritate sensitive skin. The added hydroxyl groups also give them humectant and antioxidant-like qualities that AHAs largely lack.
Key points
- PHAs loosen surface cell adhesion to exfoliate, similar to AHAs but at a slower pace.
- Their larger molecular size limits penetration depth, generally reducing stinging and irritation.
- Multiple hydroxyl groups let them attract water, giving a humectant, hydrating effect.
- Some PHAs like lactobionic acid also show antioxidant and metal-chelating properties.
The same reaction, run more slowly
| Feature | PHA | AHA |
|---|---|---|
| Molecule | Bulky — gluconolactone, lactobionic acid | Compact — glycolic, lactic acid |
| Rate of entry | Slow diffusion through the lipid matrix | Fast, especially at low pH |
| Depth reached | Largely the uppermost stratum corneum | Deeper within the stratum corneum |
| Route to shedding | Acid-driven loss of corneodesmosomal cohesion | The same route, by the same chemistry |
| Spare hydroxyl groups | Several, free to bind water where they sit | One or none, little water-binding |
| Secondary chemistry | Antioxidant and metal-chelating behaviour reported | Not a documented feature |
Why size changes the timing rather than the chemistry
A PHA is still a hydroxy acid, and it still exfoliates by donating protons that weaken corneodesmosomal attachments between corneocytes. What differs is how quickly it gets to them. Gluconolactone and lactobionic acid are several times heavier than glycolic acid and carry many polar hydroxyl groups, which makes diffusion through the lipid matrix between cells slow and shallow.
That slowness is the whole basis of the tolerability difference. Stinging on application tracks how fast acid reaches free nerve endings in the epidermis; a molecule that seeps in over hours rather than minutes never produces a sharp concentration front. The exfoliating result is not milder in kind, only spread over a longer period and confined to a thinner slice of tissue.
What the extra hydroxyl groups do
Those hydroxyl groups are not just ballast. Each is a site that hydrogen-bonds water, so a PHA left in the upper stratum corneum behaves as a humectant while it exfoliates — an unusual combination, since most exfoliating acids leave the surface drier before it recovers. Gluconolactone, the most studied example, also hydrolyses to gluconic acid on the skin, which is part of why it acts gradually.
Gluconolactone has additionally been reported to scavenge free radicals and to chelate metal ions, which would slow iron-catalysed oxidation at the surface. That work is largely laboratory rather than clinical, so it is best described as supported chemistry with limited human outcome data — a plausible bonus, not a substitute for a dedicated antioxidant.
What “gentler” honestly means here
Gentler here means slower and shallower, not chemically different. A PHA at a high enough concentration, in a low enough pH, applied often enough, will irritate skin as readily as an AHA — the same protons acting on the same junctions. The safety margin comes from kinetics, and kinetics can be overridden by dose and frequency.
Two other honest limits. The evidence base for PHAs is considerably smaller than for glycolic and lactic acid, so comparisons rest on fewer and smaller human studies. And reduced photosensitivity relative to AHAs is a reasonable inference from shallower action rather than a settled finding — daily sunscreen remains sensible. For how the shared exfoliating chemistry works in detail, see How AHAs Exfoliate.
Frequently asked
Are PHAs better than AHAs for sensitive skin?
PHAs are often better tolerated because their larger molecules penetrate more slowly, making them a gentler option for sensitive or reactive skin.
Do PHAs cause sun sensitivity like AHAs?
PHAs are generally considered to increase photosensitivity less than AHAs, but daily sunscreen is still recommended when using any exfoliating acid.
How long does a PHA take to show what an AHA shows sooner?
Expect a slower curve rather than a smaller one. Smoothness that an AHA might deliver in a fortnight often takes four to eight weeks with a PHA, because the same reaction proceeds at a lower rate.
Can I use a PHA every day because it is milder?
Often yes, and daily use is one way slower kinetics are put to work. Tolerance still varies, so the signals to watch are tightness, flaking and stinging rather than the label's claim of gentleness.
Is gluconolactone the same as gluconic acid?
They are two forms of the same chemistry. Gluconolactone is the cyclic ester, which hydrolyses to gluconic acid in the presence of water, so a formula usually contains both in equilibrium.
Do PHAs repair the skin barrier?
They do not rebuild it, but they disturb it less than a fast-acting acid and add water-binding at the surface, which can make skin feel more comfortable. Repair itself depends on lipids and time.
Related topics
This is a foundational entry in the SYNC Skin Encyclopedia and is expanded over time. Educational information only — not medical advice.
