PHA guide: polyhydroxy acids — the gentlest exfoliant and who should use them
A complete guide to PHAs (polyhydroxy acids) — how gluconolactone and lactobionic acid differ from AHAs, why they're gentler for sensitive and eczema-prone skin, and how to use them effectively.
· By MedSpot Editorial · 5 min read
Polyhydroxy acids occupy the gentlest tier of the chemical exfoliant hierarchy. They're the right choice for a specific group of patients — and often oversold to everyone else. Here's an accurate guide.
What PHAs are
Polyhydroxy acids are a subgroup of hydroxy acids with multiple hydroxyl groups (-OH) on their molecular structure, unlike AHAs which have only one. This structural difference has significant practical consequences.
The main PHAs:
Gluconolactone: The most widely used PHA. A lactone (cyclic ester) that converts to gluconic acid on skin. Gluconic acid is a naturally occurring component of human skin (part of the natural moisturizing factor, NMF). MW ~178 Da — significantly larger than glycolic (76 Da) or lactic acid (90 Da).
Lactobionic acid: A disaccharide acid derived from lactose. MW ~358 Da — the largest and slowest-penetrating common PHA. Has strong humectant properties in addition to exfoliation.
Maltobionic acid: Similar to lactobionic; less commonly used. Good humectant profile.
Why PHAs are gentler than AHAs
1. Larger molecular size → slower penetration
PHAs penetrate the stratum corneum more slowly than glycolic or lactic acid. This reduced penetration rate delivers the exfoliation effect without the acute spike in acid exposure that causes stinging, redness, and irritation with smaller AHA molecules.
2. Multiple hydroxyl groups = additional moisture binding
The extra -OH groups in PHAs bind and retain water in the stratum corneum — PHAs function simultaneously as humectants and exfoliants. Glycolic acid exfoliates; gluconolactone exfoliates AND attracts moisture.
3. Natural compatibility with skin components
Gluconolactone is a physiological compound already present in skin. Lactobionic acid is structurally similar to hyaluronic acid's sugar backbone. This structural familiarity may contribute to the lower irritation profile.
4. Antioxidant properties
PHAs — particularly gluconolactone — have documented antioxidant activity, chelating metal ions that catalyze free radical generation. This property is absent from AHAs.
Evidence for PHAs
Exfoliation efficacy: PHAs effectively exfoliate the stratum corneum, improving surface texture, tone, and brightness. The effect is equivalent to AHAs per unit of irritation — but slower and gentler, requiring either higher concentrations or longer use periods for the same outcome.
Sensitive skin and eczema: A key study by Green et al. (1996, Cutis) demonstrated that gluconolactone 8% was as effective as glycolic acid 8% for treating photodamage, but with significantly better tolerability in sensitive skin. Studies in eczema patients show PHAs can be used during remission periods when AHAs would trigger flares.
Rosacea: PHAs are the only exfoliant category with evidence of tolerability in rosacea patients — AHAs and BHAs can trigger rosacea flares; PHAs' gentler penetration profile makes them usable in milder rosacea-prone skin.
Photoprotection: Gluconolactone has been shown to provide mild UV-protective effects as a free radical scavenger — a bonus not shared by AHAs.
PHAs vs. AHAs: when to choose which
| AHA (glycolic/lactic) | PHA (gluconolactone/lactobionic) | |
|---|---|---|
| Exfoliation speed | Fast | Slow-moderate |
| Irritation | Moderate-high | Very low |
| Sensitive skin tolerance | Limited | Strong |
| Rosacea tolerance | Poor (can trigger flares) | Moderate (generally tolerated) |
| Eczema compatibility | Limited | Yes (in remission) |
| Humectant effect | Lactic acid only | Yes (all PHAs) |
| Antioxidant benefit | No | Yes |
| Evidence for anti-aging | Strong (glycolic) | Moderate |
| pH required | 3–4 | 3–5 (slightly more forgiving) |
| Results speed | Faster | Slower |
Choose AHA when: You have normal to oily skin with no sensitivity issues and want maximum exfoliation efficacy on a faster timeline.
Choose PHA when: You have sensitive, rosacea-prone, eczema-adjacent, or barrier-compromised skin; you're new to chemical exfoliation; or you've had irritation reactions to AHAs.
Concentrations
Effective range: 5–15% gluconolactone or lactobionic acid.
Because PHAs penetrate more slowly, higher concentrations than AHAs may be needed for equivalent effect:
- 5–8%: Gentle intro; good for very sensitive skin
- 10–12%: Standard effective range for general use
- 15%: Stronger; still well-tolerated by most sensitive skin
pH: PHAs require low pH for exfoliation activity, but the effective pH window is slightly broader than AHAs — products at pH 3–5 can be active, compared to the strict ≤3.5 needed for glycolic acid.
How to use PHAs
Application: After cleansing; before serum and moisturizer. Leave on — PHAs are leave-on exfoliants.
Frequency:
- Week 1–2: Daily (PHAs are gentle enough for daily use from the start for most people)
- Ongoing: Daily or twice daily for some people with very tolerant skin
SPF: Required — PHAs increase UV sensitivity just as AHAs do. Daily broad-spectrum SPF.
Compatible with: Virtually all other skincare ingredients. PHAs don't have the incompatibility concerns of stronger acids — they can be layered on the same night as niacinamide, hyaluronic acid, and (once adapted) even retinoids with less irritation concern than AHAs.
PHAs in combination products
Many products combine PHAs with AHAs or BHAs — the PHA provides gentle surface exfoliation and humectancy while the AHA or BHA provides deeper efficacy. Common combinations:
- Lactic acid + gluconolactone: Moderate AHA efficacy buffered by PHA humectancy
- Salicylic acid + lactobionic: Follicular BHA penetration + gentle surface PHA
Realistic expectations
PHAs produce genuine exfoliation, improved skin tone, and surface texture benefits — but on a slower timeline than AHAs. Expect:
- Weeks 2–4: Improved surface brightness and reduced dullness
- Weeks 8–12: Improved texture and even tone
- Long-term: Mild improvement in fine lines and photoaging comparable to low-concentration AHA
They are not high-potency treatments for significant photoaging, deep acne scarring, or stubborn hyperpigmentation — for those concerns, AHAs or professional peels are more appropriate.
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