What Hyaluronic Acid Is: Chemistry and Source
Hyaluronic acid skin science begins with the molecule itself. Hyaluronic acid (also referred to as hyaluronan) is a glycosaminoglycan: a long-chain polysaccharide made up of repeating disaccharide units of N-acetylglucosamine and glucuronic acid. It is a natural component of the extracellular matrix found throughout the body, with the highest concentrations in connective tissue, the vitreous humour of the eye and, most relevantly for aesthetics, the dermis.
In younger skin, dermal hyaluronic acid contributes substantially to tissue hydration and volume. It is estimated that the skin contains approximately half of the total body hyaluronic acid content. As the skin ages, endogenous hyaluronic acid is degraded by enzymes called hyaluronidases, and its synthesis by fibroblasts declines. This reduction contributes to the loss of skin volume, elasticity and moisture retention that characterises intrinsic ageing. Research published in the Journal of Investigative Dermatology has described age-related changes in hyaluronic acid distribution within the dermis.
Commercial hyaluronic acid used in aesthetics products is produced primarily through bacterial fermentation, specifically using Streptococcus equi subspecies, under controlled manufacturing conditions. This process yields high-purity hyaluronic acid without animal-derived materials, an important consideration for both manufacturer quality assurance and patient information. Some older manufacturing processes used rooster combs as a source; this approach is now largely obsolete in licensed medical-grade products.
High Versus Low Molecular Weight: What It Means for the Skin
Molecular weight is expressed in daltons or kilodaltons (kDa) and reflects the length of the hyaluronic acid chain. Understanding high vs low molecular weight hyaluronic acid is essential for interpreting both product claims and clinical evidence.
High molecular weight hyaluronic acid, generally defined as chains above 1,000 kDa, is too large to penetrate the skin barrier when applied topically. It sits on the surface of the stratum corneum, forming a film that physically reduces transepidermal water loss (TEWL). This is a well-characterised mechanism with consistent support in published dermatological literature. The effect is real and clinically relevant, but it is a surface action rather than a deep tissue effect.
Low molecular weight hyaluronic acid (below approximately 50 kDa, with some researchers classifying oligomeric fragments below 10 kDa as a further subcategory) shows greater penetration potential into the upper epidermis. In vitro studies have demonstrated that smaller fragments can interact with cell surface receptors including CD44 and RHAMM, triggering cellular signalling responses. Whether these interactions translate to clinically meaningful outcomes in intact human skin following topical application remains an area of ongoing research. A review published in the International Journal of Molecular Sciences (accessible via PubMed Central) has summarised the current evidence and its limitations.
In injectable contexts, molecular weight plays a different role. Products such as Profhilo combine high and low molecular weight hyaluronic acid using a thermal bonding process. The manufacturer Summary of Product Characteristics describes how the high molecular weight component contributes to longer tissue residence time, while the low molecular weight fraction is thought to support cellular receptor engagement. Unlike topical products, injectables deposit hyaluronic acid directly into the dermis or subdermis, bypassing the epidermal barrier.
How Hyaluronic Acid Binds Water and Supports the Skin Barrier
One of hyaluronic acid's most widely cited properties is its capacity to retain water. A single molecule can bind up to 1,000 times its own weight in water, according to biochemistry reviews indexed on PubMed Central. This derives from the molecule's highly charged, hydrophilic structure. In the dermis, this water-binding capacity supports skin turgor and firmness. When dermal hyaluronic acid declines with age, the tissue becomes less resilient and hydration is harder to maintain, contributing to fine lines and loss of plumpness.
At the epidermal level, the skin barrier depends on lipid layers, tight junction proteins and the natural moisturising factor (NMF) system. Hyaluronic acid contributes to keratinocyte function and barrier integrity; its disruption has been associated with impaired barrier function in research published in the Journal of Investigative Dermatology. Topical high molecular weight hyaluronic acid supports this system by reducing surface water loss. This is distinct from any claim that topical application restores dermal volume or stimulates collagen, which the current evidence does not support.
What the Clinical Evidence Does and Does Not Support
The hyaluronic acid evidence base is broad but uneven in quality, and it is important for practitioners to read it at the level of application route, product formulation and outcome measure rather than as a single category.
For injectable hyaluronic acid dermal fillers, the evidence is relatively robust. Multiple randomised controlled trials have evaluated safety, duration and patient-reported outcomes, and systematic reviews of this literature are available via PubMed Central. The MHRA has licensed hyaluronic acid fillers as Class III medical devices (now transitioning to UK Conformity Assessed status post-Brexit), which requires a defined standard of clinical evidence prior to market approval.
For injectable skin boosters and bio-remodelling products, the evidence base is growing but less uniform. Profhilo has published clinical studies in its product literature, including assessments of skin laxity improvement, but the volume of large randomised trials remains limited compared to the conventional filler literature. Practitioners should treat manufacturer-cited studies as a starting point and seek independent peer-reviewed publications.
For topical hyaluronic acid, the evidence supports barrier reinforcement and surface hydration at high molecular weight, and there is early-stage data on cellular signalling effects at low molecular weight. Claims that topical products provide deep tissue hydration or structural rejuvenation are not currently supported by the quality of evidence that would be required to substantiate them under the Advertising Standards Authority's guidance on cosmetic claims.
How to Read Concentration and Molecular Weight When Selecting Products
When evaluating injectable hyaluronic acid products, practitioners should look for four key data points: concentration (expressed in mg/ml), molecular weight profile (whether high, low, or combined), degree of cross-linking (relevant to fillers but absent in bio-remodelling products), and whether the product holds the appropriate regulatory approval for use in the UK.
Concentration alone is not a proxy for clinical effect. A skin booster at 18 mg/ml and a structural filler at 20 mg/ml may have the same approximate concentration but entirely different rheological properties, tissue behaviours and clinical indications, because the cross-linking architecture differs. The manufacturer SPC is the correct document for this information; marketing materials should not be the primary source for clinical decision-making.
For topical products, the relevant parameter is whether the molecular weight is declared and what the supporting evidence says about that weight range in the specific vehicle and concentration used. Products that cite generic hyaluronic acid benefits without specifying molecular weight make it impossible to evaluate whether the claimed mechanism is plausible.
Practitioners selecting injectable hyaluronic acid products should also verify that each product carries the appropriate UKCA or CE marking under the Medical Devices Regulations 2002, as updated. The MHRA is the relevant regulatory authority. Topical cosmetic products fall under a separate framework (the Cosmetic Products Enforcement Regulations 2013) and are subject to lower pre-market evidence requirements. Understanding which framework applies to a given product helps set appropriate expectations about the clinical substantiation behind any claims made.
Frequently Asked Questions
What is the difference between high and low molecular weight hyaluronic acid?
High molecular weight hyaluronic acid (above 1,000 kDa) sits on the surface of the skin and forms a film that reduces transepidermal water loss. Low molecular weight hyaluronic acid (generally below 50 kDa) penetrates more readily into the upper layers of the epidermis, where it can interact directly with skin cells. The two forms have complementary actions, and many formulated products combine both. This distinction is explained in peer-reviewed literature published in journals including the Journal of Drugs in Dermatology.
Does hyaluronic acid penetrate the skin when applied topically?
This depends on molecular weight. High molecular weight hyaluronic acid does not penetrate the skin barrier; it acts at the surface. Low and very low molecular weight fragments show greater potential for deeper penetration, though the degree varies with formulation vehicle and skin condition. The claim that topical hyaluronic acid deeply hydrates the dermis is not robustly supported by current evidence; the primary benefit of surface application is barrier reinforcement and reduced water loss.
How does hyaluronic acid support the skin barrier?
Hyaluronic acid is a natural component of the extracellular matrix in the dermis and epidermis. When applied topically as a high molecular weight film, it reduces transepidermal water loss by forming a physical barrier on the skin surface. In the dermis, endogenous hyaluronic acid retains water within the tissue matrix, contributing to skin volume and turgor. Research published in PMC (PubMed Central) has described this dual role in barrier and matrix function.
What concentration of hyaluronic acid should I look for in a skin booster?
Injectable hyaluronic acid skin boosters vary in concentration, typically ranging from 1 mg/ml to 32 mg/ml depending on the product and indication. Higher concentrations do not automatically produce better outcomes; the relevant factors are molecular weight profile, whether cross-linking is present, and the intended tissue target. Practitioners should review the manufacturer Summary of Product Characteristics for each product and consider the clinical evidence rather than selecting on concentration alone.
Is the evidence for hyaluronic acid in aesthetics strong?
The evidence base varies by application route and outcome measure. For injectable hyaluronic acid fillers, there are multiple randomised controlled trials and long-term follow-up studies supporting safety and efficacy, summarised in published systematic reviews including those indexed on PubMed Central. For topical products, the evidence for barrier support and surface hydration is reasonable, but claims of deep tissue remodelling are not well supported. Practitioners should evaluate evidence at the product and route level rather than treating hyaluronic acid as a homogeneous category.