A friend asked me last winter why her new polyglutamic acid serum, marketed as the “next-generation hyaluronic acid,” made her face feel tighter rather than more hydrated. She had been told by an influencer that polyglutamic acid holds five times more water than hyaluronic acid and is therefore five times better at moisturizing the skin. The mathematics of the claim are roughly right. The clinical interpretation is wrong, and the gap between those two is where most cosmetic marketing lives.
This is one of those topics where a real biophysical property of a real ingredient is being marketed in a way that does not survive scrutiny once you understand what the property actually does on skin.
What the studies actually show
Polyglutamic acid is a polypeptide produced by Bacillus subtilis natto, the same bacterium that ferments traditional Japanese natto. It is a long chain of glutamic acid residues, typically 1000 to 5000 kDa in molecular weight when used in cosmetics. Hara and colleagues in the 1990s characterized the biophysical properties for cosmetic use, with the 1995 cosmetic literature establishing that PGA holds approximately 5000 times its weight in water under ideal in vitro conditions, compared to roughly 1000 times for hyaluronic acid.
Park and colleagues in their 2017 work in the Journal of Cosmetic Dermatology (PMID: 28145051) tested a topical PGA serum at 0.5 percent against placebo over 28 days on 30 subjects. The PGA arm showed measurable improvement in skin elasticity, hydration as measured by corneometer, and reduction in transepidermal water loss. The effect sizes were moderate, in the same range as hyaluronic acid serums in comparable studies. The Park study is the most-cited piece of human evidence behind the current marketing push.
Where the marketing diverges from the biology is in the assumption that higher water-holding capacity in solution translates to higher hydration in skin. The 5000-fold versus 1000-fold ratio is measured under laboratory conditions where the molecule is fully dissolved and surrounded by available water. On the surface of human skin, the limiting factor is not the holding capacity of the humectant. It is the available water at the application site and the molecular weight that determines whether the humectant can penetrate or sit on top.
Hyaluronic acid in cosmetics ranges from very low molecular weight (5 to 50 kDa) designed to penetrate the upper epidermis, through mid-molecular weight (50 to 1000 kDa) that sits in the stratum corneum, to very high molecular weight (above 1000 kDa) that forms a film on the surface. Each weight range has a different functional profile.
Polyglutamic acid at 1000 to 5000 kDa is in the “forms a film on the surface” range. It does not penetrate. It sits on top of the stratum corneum and holds water there, which is genuinely useful in low-humidity environments where surface evaporation is the limiting factor. In high-humidity environments the same film property is less useful and may even contribute to a perception of stickiness or tightness as the film hardens.
The contrarian section
The “five times more powerful than hyaluronic acid” framing has the same problem as most cosmetic comparison claims. It compares one biophysical property in isolation, in a non-skin context, and extrapolates to overall product performance. The same logic would tell you that glycerin, which is much smaller and penetrates much better, is inferior to both PGA and HA because it holds less water per molecule. That extrapolation is also wrong.
Humectants work through several mechanisms. They pull water from the environment if humidity is above roughly 70 percent. They pull water from deeper layers of the skin to the surface, which is moisturizing in the short term and potentially drying in the long term if the deeper layers are not being replenished. They reduce TEWL by forming a hygroscopic layer on the stratum corneum.
Glycerin, urea, sodium PCA, and propylene glycol are small enough to penetrate the upper epidermis and act as humectants from within the skin. They are the workhorses of moisturization. Hyaluronic acid at low molecular weight contributes some of this. Polyglutamic acid does not because it is too large.
What polyglutamic acid does well is film formation on the surface. The film reduces TEWL. The hygroscopic property means the film stays hydrated longer than petrolatum or silicone alone. In a dry climate with low humidity, a PGA-containing product can produce a feel of immediate hydration that lasts longer than a glycerin-based equivalent. This is a real effect and the marketing is, in this narrow sense, correct.
The problem with my friend’s tight-feeling experience is that PGA films can dehydrate the skin under them if the ambient humidity drops below the film’s equilibrium humidity. The film acts as a water reservoir but the reservoir can flow either direction. In dry air the film loses water to the air and then pulls water from the skin to replenish itself. The user experiences this as tightness. The product is not failing, it is doing what hygroscopic films do in dry environments.
The other piece of the marketing that does not survive scrutiny is the claim that PGA forms a “smart film” that holds water during the day and releases it to the skin overnight. There is no published mechanism for this. The film is a passive equilibrium with the surrounding air. It does not know what time of day it is.
What I would tell my past self
If I were choosing a humectant strategy for someone with dehydrated skin, I would not start with PGA. I would start with the basics: glycerin at 5 to 10 percent in a well-formulated moisturizer, combined with mid-molecular-weight hyaluronic acid for surface hydration and a low-molecular-weight HA for epidermal penetration. This combination is in CeraVe Hydrating Cleanser, Vichy Mineral 89, La Roche-Posay Toleriane Sensitive Fluide, and many other products. The chemistry has been worked out for decades.
I would add PGA if I lived in a dry climate and wanted an additional film-forming humectant on top of the basics. I would not expect it to replace anything else. The product would be a layered routine where PGA adds to glycerin and HA rather than substituting for them.
I would also have been more careful about reading the ingredient list of PGA serums. Many products use PGA at 0.1 percent or less while marketing it as the hero ingredient. The Park 2017 study used 0.5 percent. Below the tested concentration the effect sizes are unknown. A product with 0.05 percent PGA and 5 percent glycerin is mostly a glycerin serum that uses PGA as a marketing claim.
The other thing I would tell my past self is that humectant claims are usually true in vitro and partial in vivo. The water-holding capacity of a molecule in a beaker is a real number, and it is not the same number as the moisturization effect on skin. Skin is a much more complicated solvent than a beaker. The film, the penetration, the climate, the underlying barrier function, the rest of the routine all matter. Picking a single number from the marketing and treating it as the answer is not how the chemistry works.
I do not think polyglutamic acid is a scam. I think it is a useful ingredient that does one specific thing well, oversold as a generalist that replaces hyaluronic acid. The biology is interesting and the marketing has run ahead of the biology, as it usually does.
Frequently asked
Is polyglutamic acid better than hyaluronic acid for dehydrated skin?
Different, not better. PGA forms a hygroscopic film on the surface that reduces TEWL and can deliver longer-lasting surface hydration in dry climates. HA, especially at multiple molecular weights, distributes water both on the surface and into the upper epidermis. A well-formulated moisturizer often contains both, plus glycerin, plus possibly urea or sodium PCA. The right comparison is not PGA versus HA but rather a complete humectant strategy versus a single-ingredient claim.
Why does my PGA serum feel sticky?
PGA forms a film as the water in the serum evaporates. The film is hygroscopic but it is also a polypeptide that retains some surface tackiness. Layering an occlusive moisturizer over the PGA reduces the stickiness by smoothing the film and reducing direct skin contact. If the stickiness persists, the product may have a higher PGA concentration or a less-refined molecular weight distribution. Different brands feel different even at similar percentages.
Can I use PGA and HA in the same routine?
Yes, and many formulations already combine them. The typical layering is HA serum first, on damp skin, followed by PGA serum, followed by a moisturizer that locks both films in. PGA is not displaced by HA or vice versa. They occupy slightly different layers and work at different mechanisms.
Is PGA safe in pregnancy?
Polyglutamic acid is a non-irritating polypeptide produced by food-grade bacterial fermentation. There are no known pregnancy-specific concerns. As with any new product, patch testing first is reasonable for sensitive skin. If you are pregnant and looking for a fragrance-free, low-irritation humectant, PGA is not on any list of concerns.
Will PGA work in a humid climate?
PGA works less impressively in humid climates because the surface evaporation it prevents is already slow in high-humidity air. In Singapore or Miami in July, the limiting factor on skin hydration is not surface evaporation, it is sebum, exfoliation, and barrier integrity. PGA still forms a film but the marginal benefit over a simple HA-and-glycerin moisturizer is small. In Phoenix or Calgary in February, the marginal benefit is larger.
Sources: Park et al. J Cosmet Dermatol 2017, PMID: 28145051. Hara 1995 cosmetic PGA literature. Stern et al. 2007 hyaluronic acid molecular weight and penetration. Rawlings and Harding 2004 review of moisturization mechanisms.