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A complete guide to polyhydroxy acids (PHAs) — how gluconolactone and lactobionic acid differ structurally from AHAs, why their larger molecular weight and multiple hydroxyl groups produce slower penetration and a built-in humectant effect, the evidence for PHAs in sensitive and rosacea-prone skin, their compatibility with retinoids, and when PHA is preferable to AHA.
· By MedSpot Editorial · 5 min read
Polyhydroxy acids (PHAs) are the third generation of chemical exfoliants — after AHAs and BHAs — designed to provide the surface renewal benefits of acid exfoliation with a significantly gentler delivery profile. They are not simply weaker AHAs; their structural differences produce distinct properties that make them the preferred exfoliant for specific skin types and conditions. Here is the complete guide.
AHAs (glycolic, lactic, mandelic) have a single hydroxyl (-OH) group adjacent to the carboxylic acid group. This relatively simple structure allows the molecule to penetrate the stratum corneum at a rate governed primarily by molecular weight.
PHAs have multiple hydroxyl groups on the same molecule — hence "poly" (many) hydroxy acids:
Why multiple hydroxyls change the behavior:
Larger effective molecular size: Multiple hydroxyl groups increase both molecular weight and the molecule's interaction with water (hydrogen bonding). Lactobionic acid at 358 Da is more than 4× the size of glycolic acid — penetration rate is dramatically slower.
Hygroscopic (water-attracting) properties: The multiple -OH groups bind water molecules through hydrogen bonding — PHAs are intrinsically humectant. Unlike AHAs, which are purely exfoliants, PHAs simultaneously exfoliate and attract moisture to the treated skin.
Chelating activity: PHAs chelate (bind) metal ions, including iron, which catalyzes the Fenton reaction generating hydroxyl radicals. By chelating iron, PHAs may reduce UV-stimulated ROS-driven oxidative stress — a modest antioxidant-like secondary benefit not shared by simple AHAs.
PHAs work by the same corneodesmosome-digestion mechanism as AHAs — low pH accelerates serine protease activity → loosens corneocyte cohesion → accelerated desquamation. The slower penetration means:
The primary clinical rationale for PHAs over AHAs is their superiority in sensitive, reactive, or rosacea-prone skin:
Bernstein et al. (2004, Cutis): Head-to-head comparison of gluconolactone 14% vs. glycolic acid 10% in patients with sensitive facial skin. Gluconolactone produced equivalent improvement in skin texture and hydration with significantly less stinging, erythema, and irritation — the defining study establishing PHAs as the sensitive-skin-appropriate acid exfoliant.
PHA in rosacea: The anti-inflammatory properties of gluconolactone (partially mediated by its iron-chelating antioxidant activity) and the absence of the acute stinging response make PHAs more tolerable in rosacea-prone skin than glycolic or lactic acid. Multiple dermatologist-authored case series document PHAs as a viable exfoliant option for mild rosacea where AHAs would trigger flares.
A critical practical advantage: PHAs can be used on the same night as retinoids in many protocols — something generally avoided with AHAs and BHAs (which produce cumulative barrier disruption when combined with retinoids).
The rationale:
Caveat: This compatibility applies to low-to-moderate concentrations; high-concentration PHAs (>10%) combined with prescription tretinoin still carry cumulative irritation risk and should follow a cycling protocol.
PHAs produce the same surface renewal mechanism as AHAs — accelerated corneocyte turnover → smoother surface texture; mild collagen stimulation in the papillary dermis from the wound-healing signaling of accelerated turnover.
Green et al. (2009, Journal of Cosmetic Dermatology): PHA-containing cream significantly improved skin texture, tone, and hydration scores vs. vehicle in a 12-week controlled study — with no significant irritation in a mixed skin-type population that included rosacea and sensitive skin subjects.
PHAs' iron-chelating activity reduces free-radical cascade driven by UV-exposed iron-porphyrin complexes in the skin. While PHAs do not replace SPF, their antioxidant secondary activity is a genuine (if minor) complementary benefit that AHAs lack.
After in-office procedures (laser, chemical peels, microneedling), the recovering skin barrier is too compromised for standard AHAs during the healing phase. PHAs — with their slower penetration, humectant properties, and lower irritation profile — are used by some clinicians as the first exfoliant reintroduced post-procedure, typically starting 4–6 weeks after treatment.
Ideal candidates:
When AHAs are preferable to PHAs:
Leave-on products (4–10% gluconolactone or lactobionic acid):
In-office PHA peels (20–40%): Less common than AHA peels but used in sensitive skin protocols; minimal downtime; appropriate for rosacea-prone or sensitized skin that cannot tolerate standard peels.
Key products: NeoStrata is the dominant PHA brand (gluconolactone was patented and developed by NeoStrata's founders, Van Scott and Yu, who pioneered the entire AHA/PHA field). ISDIN, First Aid Beauty, and Paula's Choice also offer PHA-containing formulations.
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