Hyaluronic acid forms guide: sodium hyaluronate, molecular weights, and what actually penetrates

Hyaluronic acid appears on ingredient lists in at least five distinct molecular forms — each behaving differently on skin. Most product marketing treats "hyaluronic acid" as a single entity. The molecular weight and chemical form determine whether it sits on the surface, penetrates to the epidermis, or does something else entirely. Here is the complete guide to reading HA ingredients correctly.

Sodium hyaluronate vs. hyaluronic acid: the salt distinction

Why labels say "sodium hyaluronate"

Hyaluronic acid (HA) is a polysaccharide with free carboxylic acid groups. At skin-relevant pH (4.5–6.5), these groups are ionized — the molecule carries negative charges along its backbone.

Sodium hyaluronate (NaHA) is the sodium salt of hyaluronic acid — the carboxylic acid groups are neutralized with sodium ions. At physiological pH, sodium hyaluronate dissociates back to the hyaluronate anion and sodium ions — functionally equivalent to HA in the skin environment.

The practical difference is minimal: Sodium hyaluronate is the dominant form in cosmetic formulations because it is more stable in aqueous products than free HA. When you see "sodium hyaluronate" on an ingredient list, you are getting hyaluronic acid's benefits.

Molecular weight note: Sodium hyaluronate can be produced across the full range of molecular weights — the "sodium" prefix tells you nothing about size.

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Molecular weight: the most important variable

How molecular weight determines behavior

| HA Form | Molecular Weight | Skin Behavior | Primary Benefit |
|---|---|---|---|
| High MW HA | 1,000–1,800 kDa | Stays on skin surface | Film-forming; TEWL reduction; surface plumping |
| Medium MW HA | 100–999 kDa | Upper stratum corneum | Surface + shallow hydration |
| Low MW HA | 10–100 kDa | Penetrates into epidermis | Deeper hydration; some anti-inflammatory activity |
| Ultra-low MW HA | < 10 kDa | Epidermis + upper dermis | Deepest penetration; some fibroblast stimulation |
| Oligomeric HA | < 1 kDa | Dermis (limited) | Anti-inflammatory; fibroblast activation signals |

High molecular weight HA (1,000–1,800 kDa): The native form produced by fibroblasts. Too large to penetrate the stratum corneum — sits on the skin surface where it forms a moisture-retaining film and reduces TEWL. Provides the instant "plump" feeling after application; the effect is real but primarily physical and surface-level.

Low molecular weight HA (10–100 kDa): Penetrates into the stratum corneum and upper epidermis. Provides deeper, more sustained hydration. Also has mild anti-inflammatory activity in the epidermis — stimulates keratinocyte production of anti-inflammatory cytokines.

Ultra-low MW / oligomeric HA (< 10 kDa): Penetrates deeper — into the epidermis and potentially the upper papillary dermis. Some evidence for fibroblast stimulation (direct collagen synthesis upregulation). Also has more significant anti-inflammatory activity — CD44 receptor binding on fibroblasts activates signaling pathways. Caution: Very small HA fragments (oligomers < 6 saccharide units) have shown pro-inflammatory activity in some in vitro models — this is a nuance that some researchers flag.

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Hydrolyzed hyaluronic acid

Hydrolyzed HA is enzymatically or chemically cleaved HA — producing a mixture of low and ultra-low MW fragments. The "hydrolyzed" designation means it has been processed to reduce molecular weight.

Penetration: Better than high MW HA; variable depending on the specific fragment distribution.

On ingredient lists: Typically listed as "hydrolyzed hyaluronic acid" or "hyaluronic acid (hydrolyzed)."

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Sodium acetyl hyaluronate

Sodium acetyl hyaluronate is a modified sodium hyaluronate with acetyl groups added to the molecule. The acetylation:
Increases lipophilicity — better affinity for the skin's lipid barrier
Improves stratum corneum penetration vs. standard sodium hyaluronate at the same molecular weight
Provides more sustained hydration than unmodified NaHA (the acetyl groups anchor it within the lipid structure longer)

Some split-face studies show sodium acetyl hyaluronate produces significantly greater skin hydration at 24 hours than standard sodium hyaluronate of the same MW — the penetration and retention advantages translate to measurable performance differences.

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Cross-linked vs. non-cross-linked HA

The filler distinction

Cross-linked HA (dermal fillers): HA chains linked with BDDE (1,4-butanediol diglycidyl ether) or other cross-linkers create a gel matrix that resists degradation by hyaluronidase. This persistence is what makes HA fillers last 6–18 months. Cross-linked HA gels are injected — not topically applied.

Non-cross-linked HA (skincare): All topical HA products use non-cross-linked HA — it is degraded by skin hyaluronidase within 24–48 hours. This is why daily application is required for sustained hydration benefit.

The cross-linking chemistry used in fillers would prevent the HA from being absorbed or providing humectant activity if applied topically — cross-linked HA gels are for injection only.

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The dry-climate humectant paradox (revisited)

As covered in the hyaluronic acid skincare guide, high MW HA in dry environments (< 40% relative humidity) can draw moisture from deeper skin layers to the surface where it evaporates — net dehydrating effect.

Solution: Apply HA to damp skin (within 60 seconds of cleansing or misting) and seal with an occlusive or emollient moisturizer before the surface water evaporates. The film of water from rinsing provides the source for HA to draw from; the occlusive then prevents evaporation.

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Reading an ingredient list: what to look for

A product claiming "multi-molecular weight hyaluronic acid" should ideally list multiple forms distinctly:

Look for combinations like:
Sodium hyaluronate (high MW)
Sodium hyaluronate crosspolymer (modified high MW)
Hydrolyzed hyaluronic acid (low MW)
Sodium acetyl hyaluronate (modified, better penetrating)

Single HA entry: If only one HA form appears, the product is delivering one MW profile — typically high MW (most common) or sodium acetyl hyaluronate (better-penetrating alternative).

Position in list: HA and sodium hyaluronate are typically effective at 0.1–2% — even lower-list appearances can represent meaningful concentrations. Position is not the key concern for HA; form and MW are.

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