What Is Glycerin, What Does It Do? How Does It Bind Water to the Skin?

Molecularly it is very simple: a short three-carbon backbone with three hydroxyl groups on it. These groups form hydrogen bonds with water, which makes glycerin one of the most common humectants in cosmetics. At room temperature it is a colourless, syrupy liquid. It is also no stranger to the skin: it occurs naturally in the upper layer of the skin, coming mainly from the blood and the sebaceous glands.

Why is glycerin in so many products?

Its prevalence is shown in numbers as well. In US cosmetic records dated 2014, it was reported in 15.654 products, making it the third most frequent ingredient after water and fragrance. There are several reasons:

• It does more than one job. Besides being a humectant, it is used as a hair care ingredient, a skin protectant and a viscosity reducer.
• It is easy to produce. It comes out as a by-product of fat hydrolysis, and is also synthesised from starting materials such as propylene oxide. Vegetable, animal and synthetic glycerin are chemically identical. So "vegetable glycerin" describes the source, not the effect.
• It is very well tolerated. In a comprehensive safety assessment it was found to be neither irritating nor sensitising. A moisturiser containing 65,9% glycerin caused no allergy in people. In 420 patients with dermatitis, 50% glycerin in a closed patch test caused no irritation; only one person showed a reaction. Allergy reports have remained at the level of isolated cases.
• Its concentration is flexible. It goes into formulas from shampoo to hand cream, from eye products to lipstick.

How does it bind water to the skin?

Glycerin does not work only as a magnet for moisture sitting on the surface. Its effect is described at several levels:

1. Reservoir effect. Osmotically it passes into the upper layer and the underlying epidermis, carrying water with it. In this way the skin's water-holding capacity increases.
2. Transport channel. The aquaporin-3 channel in the cell membrane carries both water and glycerin. In mice lacking this channel, glycerin in the skin dropped, the skin dried, its elasticity decreased, and barrier recovery was delayed. When glycerin was given, these defects were corrected; a humid environment or occlusive application did not correct the dryness. The level of this channel decreases with age and long-term sun exposure, which is considered one of the possible explanations for dryness in older and sun-exposed skin.
3. Barrier lipids. In the lab, it delayed the transition of skin lipids from liquid to solid, which may keep the upper layer more flexible.
4. Flaking. In isolated upper layers it facilitated the breakdown of the structures that bind cells together, which may contribute to reduced flaking.

Water-attracting power alone does not determine the result. In a study on subjects, diglycerol and triglycerol attracted more water than glycerin in the lab, but in a dry skin model only glycerin removed the redness. Water retention in the lab and moisturising of the skin are not the same thing.

Size matters too. Being a small molecule of about 92 daltons, it can pass into the upper layer, while large-molecule hyaluronic acid mostly stays on the surface. For this reason the two ingredients are usually used side by side rather than in place of each other.

Does glycerin dry out the skin in dry air?

The common claim goes like this: if the air is very dry, a humectant draws water not from outside but from the deeper layers of the skin, so the skin dries more. One part of this logic is correct: humectants can move water from the dermis to the epidermis, and in humid air they also take water from the environment. But direct data showing that glycerin increases dryness in human skin in dry air is weak.

The clearest measurement we have comes from a lab study dated 1990. At 6% relative humidity, pure glycerin kept its mass, while at 92% it absorbed water. In the model, glycerin added to skin lipids did not absorb water in dry air, nor did it slow water loss. Still, it prevented the lipids from solidifying. The researchers attributed its benefit in dry air to this lipid arrangement rather than to a humectant effect. Because this is a single model study, it is not treated as a firm conclusion.

The suggestion drawn from this is not a measured protocol but a starting point derived from logic: in a heated or very dry environment, do not use a light glycerin serum alone; add a cream containing oil or an occlusive layer on top. For choosing, see the how to choose a moisturiser article.

Concentration and texture: What do the percentages mean?

There is no binding "correct concentration" for glycerin, and labels are not required to state concentration. The values below were gathered from sources of different nature; the last column shows how much you can rely on each.

The texture side varies greatly with the formula. Oily esters and silicones are often used to reduce the sticky feel, so the same concentration can feel different in two creams. If you have pure glycerin, there is no solid reason to apply it straight to the face; contact with the eye also causes strong stinging and watering.

What does the evidence say on dry and atopic skin?

Reviews report that glycerin-containing products improve upper-layer water and the mechanical properties of the skin in dry skin with barrier impairment. Clinical comparisons are more mixed:

• 197 atopic patients for 30 days: A 20% glycerin cream was compared with its glycerin-free base and with a cream containing 4% urea and 4% salt. Dryness assessments were similar across the groups; the glycerin cream stung less than the urea and salty creams.
• 109 atopic patients for 30 days: Glycerin did not differ from the base cream in water loss. Urea cream reduced water loss more, and dermatologists rated dryness better in the urea group.

So glycerin may be a more comfortable alternative to urea in terms of stinging, but it is not superior in every measurement. In these studies one must also consider that the base cream is itself a moisturiser.

Names confused on labels, and safety notes

• Glyceryl Stearate: Similar name, different job. It is an emulsifier that holds oil and water together.
• Butylene Glycol, Propylene Glycol: Other humectant polyols; they are not glycerin.
• Purity: For impurities in the raw material such as diethylene glycol and ethylene glycol, pharmacopoeia standards set limits: a single impurity may not exceed 0,1%, and the total may not exceed 1%. Glycerin that is a by-product of biodiesel made from the Jatropha plant has been the subject of warnings because it may carry toxic substances.
• New product: The problem usually comes from the rest of the formula rather than from the glycerin. If your skin is sensitive, test the new cream on the inner arm or elbow crease, twice a day, for 7-10 days.