Hyaluronic Acid in skincare use (1 of 2)

The following two posts are technical. Why? There is a propensity for the beauty industry to dazzle us with the supposed benefits of HA. At the outset, I can see the “wowness” of HA dermal fillers but not topical treatments, such as creams and other moisturisers. This all has to do with the metabolism of HA in our skin.

1) Chemical structure

It is a natural glycosaminoglycan (glycoprotein) formed by bonding N-acetyl-D glucosamine with glucuronic acid.

At our physiological pH exists as hyaluronate ion (negative charge) that attracts cations such as Na+ (Sodium ion).  Therefore, when you see Sodium Hyaluronate on an ingredient label, that’s the workable, cosmetic form of Sodium Hyaluronate.

Units of two (disaccharide units) are formed from HA. Such units can be linked together, so that a complete HA molecule can reach 10,000+ disaccharide pairs (molecular mass of c.4 mn Da).

Domain structure

The domain structure of HA allows small molecules such as water, electrolytes, and nutrients to freely diffuse through while large molecules such as proteins will be partially excluded

HA and its derivatives form Low, Medium and High Molecular Weight (LMW, MMW and HMW). You will commonly see reference to these.

Therefore, the chemical structure of HA determines its molecular weight and its physical characteristics.  You can find HA in the skin, joints, umbilical cord, eyes and so on.  HMW hyaluronic acid, in the range of millions of daltons, is present in cartilage, in the vitreous of the eye, and in synovial fluid joints.

2) HA and the skin

If we do not have a common understanding of the skin structure, I cannot then explain the mechanism by which external HA (in fillers, creams etc) may work…

Skin has 3 layers: epidermis, dermis and hypodermis (subcutaneous fat layer).

Epidermis

The epidermis is the top layer of our skin, and its topmost layer is the Stratum Corneum (SC) which is what is visible to us. Attractive skin is therefore, down to the SC.

The SC’s functions include barrier functions, that regulate the loss of water from the epidermis (Trans-Epidermal Water Loss or TEWL). Excessive TEWL causes atopic dermatitis (dry skin) and dry skin causes fine lines/wrinkles.

The mechanism by which the skin is kept hydrated is complicated but includes the presence of Natural Moisturising Factors (NMF) in the SC’s corneocytes. NMFs are humectants – they attract water forming hydrogen bonds with it – as a result its harder for water to leave the skin and it stays hydrated.

Sakai et al (2000) were able to confirm the presence of HA in the SC. HA likely sits in the lamellar lipid matrix: it forms c.0.5% in weight of the NMFs. As a humectant, it attracts water from the environment and creates a voluminous structure.

One gram of HA can hold up to 6 litres of water. Depending on the molecular structure of HA that manufacturers use, its possible that the HA in the formulation (filler/topical) holds hundreds of times its capacity in water.

Dermis

The dermis is the 2nd layer of the skin separated by a basement membrane from the epidermis. The dermis is the layer of significance when we talk about HA, as HA is mainly found in the dermis.

The dermis contains blood vessels, roots of hair, nerves and sweat glands. It also gives skin its strength and elasticity.

Composition of the dermis

The dermis contains horizontal collagen bundles running across it. Its immersed in a gelatinous substance (fundamental substance) that forms part of the ECM (extracellular matrix). Collagen forms three-quarters of the dermis’ weight and is responsible for skin tonicity and elasticity. These collagen bundles are held together by elastin (elastic fibres made of protein) and elastin represents c5% weight of the dermis.

HA in the dermis

HA is a component of the elastoviscous ECM in which collagen fibers, elastin and other structures are immersed. HA here has capacity to attract water in quantities equal to hundreds of times its weight. HA is therefore, a natural hydrating substance, responsible for tonicity of the skin and its reserves of moisture.

HA also facilitates transport of essential nutrients from the blood to skin cells.

3) HA and skin aging

A reader (in her early 50s) wrote she was looking forward to my blogs on HA, as she was consistently recommended HA. Its important to understand what happens to HA as we age. Aging is intrinsic (genetic) and extrinsic, caused by factors such as UV radiation, smoking, alcohol, poor nutrition (including excessive sugar). UV Radiation is the biggest contributor to extrinsic aging, accounting for as much as 80%.

As one ages, the amount of hyaluronic acid in the fluid/gel form in the dermis begins to decline. This is important because fluidity in the dermis means less nutrients are being delivered and essentially less collagen is formed.

Reduced collagen is characteristic of aging skin and that aesthetically that manifests itself as skin having less tone (sagging), rigidity resulting in wrinkles and fine lines. Aged skin is also associated with less water.

4) Skin aging and dermal fillers

Over the past 2 decades, HA dermal fillers have soared in popularity. This is completely understandable as (my interpretation) is that they “sit in empty spaces” in the dermis, attract water and the resulting volume diminishes fine lines and wrinkles.

The massive advantage of HA fillers is that they are cross-linked polymers (see below) and take months for the skin to breakdown. It is not clear that HA introduced into the skin this way participates in the functions of endogenous HA.

However, that doesn’t matter, these fillers are safe (not toxic) and are non-allergenic (unlike collagen).

That concludes Part 1 of 2.

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