Hyaluronic Acid: What It Is and How It Works

Hyaluronic acid appears in everything from face serums to joint supplements and knee injections. For athletes it is often marketed as a way to keep joints 'lubricated' under heavy training loads. But what exactly is it, how does it work in the body, and can swallowing it in a capsule really reach your knees? Our editors explain the science.

What hyaluronic acid is

Hyaluronic acid, also called hyaluronan or sodium hyaluronate in its salt form, is a long, unbranched polysaccharide made of repeating sugar units: glucuronic acid and N-acetylglucosamine. It belongs to the family of glycosaminoglycans, alongside chondroitin sulphate and keratan sulphate.

What makes it remarkable is its ability to bind water. A single molecule can be extremely large, and in solution it forms a viscous, elastic gel. This property gives synovial fluid its slippery, shock-absorbing quality and keeps the skin plump and hydrated.

The body produces hyaluronic acid continuously through enzymes called hyaluronan synthases, and breaks it down equally quickly with enzymes called hyaluronidases. As Fraser and colleagues described, roughly half of the body's hyaluronan is found in the skin, and its turnover is rapid — a substantial proportion is replaced every day.

Production gradually declines with age, and the molecular weight of hyaluronic acid in joints can fall in osteoarthritis, making synovial fluid thinner and less protective. This is the rationale behind both injections and supplements.

How it works in joints and connective tissue

In a healthy joint, synovial fluid fills the space between cartilage surfaces. High-molecular-weight hyaluronic acid makes the fluid viscous at rest and more fluid during fast movement, so it lubricates during walking and cushions during impact.

Hyaluronic acid also forms part of the cartilage matrix, where it anchors aggrecan molecules that hold water and resist compression. Beyond its mechanical role, it interacts with cell receptors such as CD44, influencing inflammation, cell migration and tissue repair.

• Lubrication: reduces friction between joint surfaces.
• Shock absorption: viscoelastic properties dampen impact.
• Tissue hydration: binds water in cartilage, skin and the eye.
• Cell signalling: influences inflammation and wound healing, with effects depending on molecular size.

Interestingly, fragments of different sizes may have different biological effects: very large molecules tend to be anti-inflammatory, while small fragments generated during tissue injury can act as danger signals. This complexity is one reason results from supplements and injections vary.

Oral supplements: absorption and evidence

A common objection to oral hyaluronic acid is that a large molecule cannot survive digestion. Animal studies using labelled hyaluronan suggest that it is largely broken down by gut bacteria, but that some fragments are absorbed and distributed to tissues including joints and skin. The extent of this in humans is uncertain.

A review by Oe and colleagues summarised several Japanese and other trials in people with knee pain, finding modest improvements in pain scores with oral hyaluronan, typically 200–240 mg per day over several weeks. A pilot study by Kalman and colleagues using a chicken comb extract also reported some benefits.

For skin, Kawada and colleagues reported improved skin moisture after oral supplementation in people with dry skin. These trials are generally small, often industry-funded and measure subjective outcomes, so the results should be seen as encouraging but not conclusive.

There are no convincing studies showing that oral hyaluronic acid improves athletic performance, prevents sports injuries or speeds recovery in healthy athletes.

Injections, creams and other forms

Hyaluronic acid is used in several forms, each with a different purpose and level of regulation in the UK.

Joint injections, sometimes called viscosupplementation, have been studied extensively. Meta-analyses give mixed results, and NICE's 2022 osteoarthritis guideline advises against offering them on the NHS because of limited clinical benefit relative to cost. International guidance from OARSI is similarly cautious.

Topical hyaluronic acid does not penetrate deeply into the skin, but it does attract water to the outer layers, which explains its cosmetic popularity. It has no effect on joints when applied to the skin.

Safety and practical considerations

Oral hyaluronic acid is generally well tolerated. Side effects in trials have been rare and mild, mainly digestive. Products derived from chicken comb may not be suitable for people with poultry or egg allergies, or for vegetarians; fermentation-derived versions are available.

Safety in pregnancy and breastfeeding has not been established, so supplements are best avoided at these times unless advised by a clinician. Injections carry small risks of joint infection, swelling and pain flare-ups and should only be given by trained professionals.

For active people with joint pain, the strongest evidence still supports exercise therapy, strength training, weight management where relevant and appropriate load management. Persistent swelling, locking or instability needs assessment by a physiotherapist or doctor.

If you try an oral supplement, choose a product stating the amount of hyaluronic acid or sodium hyaluronate per serving and give it six to eight weeks before judging the effect. Hyaluronic acid is not prohibited in sport, but buy batch-tested products if you compete under anti-doping rules.

Editorial conclusions

Hyaluronic acid is a natural, water-binding molecule essential for lubricating joints, hydrating skin and maintaining connective tissue.

Oral supplements may offer modest benefits for knee discomfort and skin moisture in some people, but the evidence is limited and there is no proof of performance benefits in athletes.

Joint injections are not recommended by NICE for osteoarthritis, and topical products act on the skin surface only. Exercise-based care remains the foundation for joint health.

You may also want to read our editorial guides on collagen peptides for tendons, on glucosamine and chondroitin and on omega-3 fatty acids for athletes.

References

1. Fraser JR, Laurent TC, Laurent UB. Hyaluronan: its nature, distribution, functions and turnover. J Intern Med. 1997;242(1):27–33.
2. Oe M, Tashiro T, Yoshida H, et al. Oral hyaluronan relieves knee pain: a review. Nutr J. 2016;15:11.
3. Kalman DS, Heimer M, Valdeon A, Schwartz H, Sheldon E. Effect of a natural extract of chicken combs with a high content of hyaluronic acid (Hyal-Joint) on pain relief and quality of life in subjects with knee osteoarthritis: a pilot randomized double-blind placebo-controlled trial. Nutr J. 2008;7:3.
4. Kawada C, Yoshida T, Yoshida H, et al. Ingested hyaluronan moisturizes dry skin. Nutr J. 2014;13:70.
5. Papakonstantinou E, Roth M, Karakiulakis G. Hyaluronic acid: a key molecule in skin aging. Dermatoendocrinol. 2012;4(3):253–258.
6. Bannuru RR, Osani MC, Vaysbrot EE, et al. OARSI guidelines for the non-surgical management of knee, hip, and polyarticular osteoarthritis. Osteoarthritis Cartilage. 2019;27(11):1578–1589.
7. National Institute for Health and Care Excellence. Osteoarthritis in over 16s: diagnosis and management. NICE guideline NG226. London: NICE; 2022.