Walk down any luxury skincare aisle right now and you'll see two humectants fighting for the same shelf: hyaluronic acid, the established hydrator that's been in nearly every serum since the late 2010s, and polyglutamic acid, the newer arrival that marketing copy loves to describe as holding “four times more moisture.” If you've been trying to figure out which one to buy, the honest news is that the question is slightly wrong — and understanding why will save you money and give you better skin.
Both are humectants. They bind water. But they bind it in different places, in different ways, and the smartest routines use that difference rather than forcing the two to compete. Below, we break down how each one works, what Amazon buyers actually report from using them, and which serums are worth your money — ranked, as always, by what buyers say rather than by anything we tested ourselves.
Hyaluronic Acid: The Deep Hydrator
Hyaluronic acid occurs naturally in your skin, where it cushions and hydrates the connective tissue. As a skincare ingredient, its superpower is water-binding capacity — a single HA molecule can hold a large multiple of its own weight in water. What makes HA genuinely versatile is molecular weight: high-molecular-weight HA sits nearer the surface and plumps the top layers, while low-molecular-weight fragments penetrate further down. A well-formulated HA serum uses several weights at once, hydrating across multiple depths.
Depth is HA's edge. The trade-off buyers report most often is texture: some HA serums feel slightly tacky, and — this is the important one — in a dry environment, HA can draw moisture from the deeper layers of your skin rather than from the air, leaving the surface feeling tight.
That is physics, not a flaw. The fix is to apply HA onto slightly damp skin and seal it with a moisturizer (or, as we'll see, a film-forming layer of PGA). For the full ranking of which HA serums buyers rate highest, see our best luxury hyaluronic acid serums guide.
Polyglutamic Acid: The Surface Sealer
Polyglutamic acid is a peptide-like polymer most often produced by fermenting soybeans — it's the same compound that gives natto its characteristic stickiness. Where HA pulls water inward, PGA's larger molecules stay near the surface and form a light, breathable film. That film does two useful things: it binds a large amount of water right at the surface (this is the basis of the “holds more moisture than HA” claim, which refers specifically to surface water-binding), and it slows transepidermal water loss — the moisture that quietly evaporates off your skin all day.
There's a second, less-advertised benefit that genuinely sets PGA apart: it appears to inhibit hyaluronidase, the enzyme that breaks down the skin's own hyaluronic acid. PGA may protect your own HA. Buyers describe the feel as “silky” and “smoothing” rather than sticky, and many note it makes an excellent makeup base. The most common complaints are minor: a faint chemical smell on application that dissipates, and occasional pilling if layered with the wrong products.