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A complete guide to PHAs (polyhydroxy acids) — why gluconolactone and lactobionic acid are gentler than AHAs, humectant properties, evidence for sensitive and rosacea-prone skin, and how to incorporate them into a routine.
· By MedSpot Editorial · 5 min read
PHAs (polyhydroxy acids) are the third generation of hydroxy acid exfoliants — after AHAs and BHAs — and they're specifically engineered for skin that can't tolerate glycolic or salicylic acid. Here's what makes them different and who genuinely benefits.
Polyhydroxy acids are hydroxy acids with multiple hydroxyl (-OH) groups on the molecule — distinguishing them from AHAs (one OH group, alpha position) and BHAs (one OH group, beta position).
The two primary PHAs in skincare:
Gluconolactone: A lactone (cyclic ester) of gluconic acid — naturally found in honey and wine. The ring structure hydrolyzes in contact with skin to release gluconic acid, which exfoliates.
Lactobionic acid: A sugar acid derived from lactose (milk sugar) — a disaccharide PHA. It is larger and more hydrophilic than gluconolactone, with stronger humectant properties.
Other PHAs: Galactose, maltobionic acid — less commonly used in cosmetics.
Three structural features explain the PHA advantage:
| Exfoliant | MW | Penetration | Irritation |
|---|---|---|---|
| Glycolic acid (AHA) | 76 Da | Deepest | Highest |
| Lactic acid (AHA) | 90 Da | Moderate | Moderate |
| Mandelic acid (AHA) | 152 Da | Shallow | Low |
| Gluconolactone (PHA) | 178 Da | Very shallow | Minimal |
| Lactobionic acid (PHA) | 358 Da | Surface only | Minimal |
Lactobionic acid at 358 Da is almost entirely restricted to the outermost layers of the stratum corneum — it exfoliates by weakening the outermost corneocyte bonds without penetrating to irritation-inducing depths.
Each additional -OH group can form hydrogen bonds with water molecules. Gluconolactone (5 OH groups in its ring-opened form) and lactobionic acid (8 OH groups) are inherently humectant — they attract and hold water while exfoliating. This is fundamentally different from glycolic acid (1 OH group, no humectant effect).
The result: PHAs exfoliate and hydrate simultaneously, with significantly less drying effect than AHAs.
Gluconolactone has demonstrated chelating (metal-binding) activity — it binds iron ions that catalyze free radical generation (Fenton reaction). By sequestering iron, gluconolactone reduces oxidative damage in UV-exposed skin. This antioxidant property is absent in conventional AHAs.
Green & Yu (2003, Journal of Cosmetic Dermatology) — a study specifically comparing PHAs to AHAs in sensitive skin found that gluconolactone and lactobionic acid produced equivalent exfoliation to AHAs with significantly less stinging, burning, and erythema. Subjects with rosacea tolerated PHAs without triggering flares.
This is the evidence that established PHAs as the appropriate exfoliant for rosacea — a skin condition where glycolic acid reliably causes irritation.
Berardesca et al. (1997) — demonstrated that gluconolactone, unlike glycolic acid, does not impair barrier function (TEWL) at equivalent exfoliating concentrations. Glycolic acid at standard concentrations measurably increases TEWL; gluconolactone does not.
Grimes et al. (2004, Cutis) — confirmed that gluconolactone has meaningful antioxidant activity in UV-exposed skin and may reduce UV-induced free radical damage. This is a secondary benefit not shared by conventional AHAs.
Multiple smaller studies confirm PHAs improve skin texture, fine lines, and moisturization comparably to low-dose AHAs — with significantly better tolerability in older skin (which tends to be drier and more sensitive).
| Exfoliant | Best for | Avoid if... |
|---|---|---|
| Glycolic acid (AHA) | Deep wrinkles, oily resilient skin, significant photoaging | Sensitive, rosacea, very dry skin |
| Lactic acid (AHA) | Dry skin, moderate sensitivity, hyperpigmentation | Active rosacea |
| Mandelic acid (AHA) | Acne + pigment, darker skin tones, AHA beginners | Rarely — most well-tolerated AHA |
| Salicylic acid (BHA) | Blackheads, oily/acne-prone, follicular concerns | Aspirin sensitivity; use with care in pregnancy |
| Gluconolactone (PHA) | Rosacea, sensitive, eczema-prone, elderly skin, first exfoliant | Almost no contraindications |
| Lactobionic acid (PHA) | Rosacea, dry sensitive skin, barrier-compromised skin | Almost no contraindications |
PHAs are not a replacement for AHAs for patients who tolerate AHAs — the exfoliation is genuinely shallower and anti-aging evidence is less extensive. PHAs are the correct choice for patients who cannot tolerate AHAs.
For rosacea or sensitive skin transitioning from no exfoliant:
Layering:
Sun protection: PHAs are less photosensitizing than AHAs, but SPF is still advisable given the exfoliation effect.
Lactobionic acid for post-procedure skin:
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