The One Primary Study Behind "Amino-Acid Surfactants Are Milder": Lee, Kawasaki & Maibach (1994)

Duration

24-hour patch application, followed by daily visual grading and TEWL measurement for 4 days after patch removal (total observation window of roughly 5 days from application).

Population

Healthy adult volunteers free of skin disease and with no personal history of atopic dermatitis, recruited at the Department of Dermatology, University of California San Francisco. The abstract does not report age range, sex distribution, skin type, or ethnicity, so these are not published in the available record.

Sample size

15 healthy adult volunteers (per the published PubMed/Contact Dermatitis abstract; no separate dropout/completion count is given, so all 15 are assumed to have completed all arms). Note: a sample size of 20 volunteers appears in secondary/community summaries of a *different, related* SLS-vs-SLG irritation study (indexed separately, e.g. via KoreaMed) - that is not this study, and the two sample sizes should not be merged.

Study design

Controlled, within-subject, site-randomized human 24-hour occlusive patch-test (irritation) study. Multiple test solutions were applied to randomized skin sites on each volunteer's forearm under polypropylene occlusion chambers to minimize anatomical bias; baseline TEWL was recorded before application, and outcomes were re-measured at patch removal and daily for 4 subsequent days. This is a single-study, dermatology irritation model - not a blinded RCT against a marketed product, and the abstract does not report inferential statistics beyond noting some comparisons were 'not statistically different.'

Outcome metric

Visual irritation grading score (scale/criteria not specified in the abstract) and transepidermal water loss (TEWL) measured with an evaporimeter, both recorded at patch removal and once daily for 4 subsequent days.

Funding conflicts

Not disclosed in the available abstract/PubMed record. The paper was published in 1994, predating routine journal-mandated funding/conflict-of-interest disclosure blocks common in later literature, and the full-text article (behind the Wiley paywall) was not accessible in this research to check an acknowledgments section. This is stated plainly as unpublished/unverified rather than assumed to be absent.

Research question

Does sodium lauroyl glutamate (SLG), an amino-acid-derived anionic surfactant, have lower irritant contact dermatitis potential than sodium lauryl sulfate (SLS) when tested alone, and does blending SLG into SLS reduce the irritancy of the SLS component in a mixture, on human skin?

Result effect size

Reported only descriptively in the available abstract, with no numeric means, SDs, or p-values given: (1) 1% SLG produced lower visual scores and lower TEWL than every other test solution except the deionized-water vehicle control. (2) All three SLG/SLS mixtures (M1, M2, M3) showed lower visual scores and TEWL than 1% SLS alone. (3) Increasing the proportion of SLG in a mixture decreased visual scores and TEWL. (4) When mixtures were compared to SLS solutions matched for equivalent SLS content (M1 vs. 0.25% SLS, M2 vs. 0.50% SLS, M3 vs. 0.75% SLS), the mixtures trended lower but the authors explicitly state these differences were 'not statistically different.' No effect-size statistic (e.g. mean difference, Cohen's d) is published in the abstract; full-text tables were not accessible to verify exact figures.

Product equivalence

This study tested only laboratory surfactant solutions - SLG, SLS, and lab-mixed SLG/SLS blends at fixed concentrations in deionized water, applied via occlusive patch - not any finished, marketed cleanser, wash, or shampoo product. It says nothing about pH-adjusted formulations, co-surfactant systems, added emollients/polymers, or real-use (rinse-off, non-occlusive) conditions. The finding that isolated SLG is milder than isolated SLS in this patch model cannot be carried over as evidence that any specific branded 'amino-acid cleanser' is objectively milder than any specific branded 'sulfate cleanser' - finished-product mildness depends on the full formulation, not on one surfactant tested in isolation.

Intervention control

24-hour occlusive patch application of 100 µL test solution per site via polypropylene chamber. Test solutions: sodium lauryl sulfate (SLS) at 1%, 0.75%, 0.50%, and 0.25%; sodium lauroyl glutamate (SLG) at 1%; and three SLG/SLS mixture solutions at 1% total surfactant - M1 (75/25), M2 (50/50), and M3. Vehicle control was deionized water. Discrepancy across sources on the M3 ratio: the verbatim NCBI/PubMed abstract text gives 'M3 (25/50) (SLG/SLS)', while an independently paraphrased extraction of the Wiley publisher abstract page describes M3 as '25/75' SLG/SLS. Both readings are kept here rather than silently reconciled, since the exact M3 ratio could not be confirmed against the full-text tables.

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