Gluconolactone: in the AHA trial, the acid that protected the barrier was this one, not glycolic

The 1997 study marked six 8 × 5 cm test sites on the arms and forearms of eleven healthy people and rotated them randomly: four acids at 8% (glycolic, lactic, tartaric, gluconolactone), vehicle only, and an untreated control. Products went on twice a day for four weeks.

At week 4 the sites were challenged with 5% sodium lauryl sulphate under occlusion for six hours, and transepidermal water loss and erythema were measured at 24 and 48 hours.

The acids parted company.

• Up to week 4 there were no significant differences in water loss or erythema between the four.
• But after the challenge, the gluconolactone and tartaric acid sites showed significantly lower water loss than vehicle and than untreated skin (P < .01) — and significantly lower than glycolic acid (P < .05).
• Erythema (a* value) followed the same pattern.

The authors conclude that AHAs can modulate barrier function, but the effect is not equal for all of them, being more marked in the molecules with antioxidant properties.

This is a small trial of eleven people. It cannot settle the question. What makes it valuable is that four ingredients were compared side by side under identical conditions — for seeing differences between ingredients, that is actually a good design.

The paper did not report pH, even though in an acid pH matters as much as concentration.

A 2004 12-week clinical study compared a gluconolactone-containing regimen (PHA) directly against a glycolic acid-containing one (AHA). Photoaging grading, irritation, silicone replicas of the crow's-feet area, pinch recoil and self-assessment were all taken at baseline, 6 weeks and 12 weeks.

Both regimens showed significant antiaging benefits — on silicone replicas, on clinical grading and on pinch recoil for resiliency.

Only two differences between the regimens reached significance, and both only at week 12:

• Sallowness: AHA 17.1% against PHA 12.4%
• Pinch recoil: AHA 13.5% against PHA 10.2%

Both favoured the AHA. We record that so as not to carry across only the flattering half.

Conversely, the PHA regimen irritated less. Stinging and burning were significantly worse in the AHA group at both 6 and 12 weeks, and rated sensitivity was worse for AHA too.

Two caveats. The abstract gives no participant count — which is why the sample-size field here is empty. And the paper appeared in a journal supplement (73(2 Suppl):14-7) rather than the main issue; supplements can follow a different review process.

The same Polish group ran the same design twice, in sixteen women. A 10% gluconolactone solution went on one side of the face and a 30% solution on the other, across three treatments, measured a week after the last.

2021 (hydration): facial hydration rose significantly after treatment. But there was no significant difference between 10% and 30%.

2023 (pH, water loss, sebum): pH fell at every measurement point after each treatment, and transepidermal water loss was significantly lower around the eyes, on the left forehead and on the right cheek. Sebum changed significantly only on the cheeks. Again, no significant difference between the concentrations.

How should this be read? This site repeats that concentration matters more than the name of an ingredient. These two studies say that tripling the concentration did not move the measurements.

The two are not in conflict. The precise sentence is: concentration matters, but that does not mean higher is better. For some ingredients, past a certain point more adds nothing but irritation. Comparisons like this are how you find that out.

The limits are clear too. Sixteen people, no control group (both sides got gluconolactone), and randomisation is not stated. And the two studies are the same design from the same group — not independent confirmation.

This may be the most practical fact about this ingredient.

Korea fixes by notice the wording that must appear on AHA-containing cosmetics: Annex 1, item 2(b)(9) of the Regulation on Precautions for Use and Labelling of Allergenic Fragrance Ingredients. Above 0.5% the product must carry sunscreen and patch-test lines; above 10%, or below pH 3.5, it must add a line advising consultation with a specialist.

But what the notice names is this: 'alpha hydroxy acid (α-hydroxyacid, AHA)'.

Gluconolactone is a polyhydroxy acid, and it is not on that list. We opened the original text and read all of paragraph (b). The ingredients named there are hydrogen peroxide, benzalkonium salts, zinc stearate, salicylic acid, silver nitrate, IPBC, aluminium salts, arbutin, AHAs, carmine, cochineal extract, formaldehyde, polyethoxylated retinamide, certain parabens and benzophenone-3.

To be exact about what that means: it does not mean 'safer'. Absence from the notice is the fact that the regulator did not require separate wording for this ingredient; it is not a safety assessment result.

There is one practical consequence for a reader, though. PHA products do not carry that warning, so the trick described on the glycolic acid page — reading the concentration band off the label — does not work here.

One last thing, stated plainly. The classification is not fully settled in the literature. The 1997 trial cited above called all four ingredients, gluconolactone included, "AHAs"; the 2004 papers treat PHA and AHA as separate categories and compare them. We have not verified how the regulator classifies it. What we did verify stops here: the notice names "alpha hydroxy acid (AHA)" as its subject, and the name gluconolactone appears nowhere in paragraph (b).