What is a Humectant?
Humectants are ingredients in skincare that increase hydration in the skin. They help make the skin soft, plump and pliable. Some of the most common humectants are Glycerin, Hyaluronic acid and Panthenol. Humectants are the reason your skin doesn't crack when it is too dry or cold. In a way, they function as anti-freeze. In fact, most of the times they are maligned in food and cosmetics because they can be found in Anti-Freeze. I mean, corn cellulose can be found in rat poison, doesn't mean it is poison for you. Moving on.
The Outdated Model for Hydration
Our previous belief for skin hydration is that water that comes from the dermis flows to the upper layers of the skin and retained there because we have lipids that occlude it and humectants that hold it in place. We've been told several times that humectants work because they are super great at forming bonds with water. Newer science begs to disagreee.
This understanding has been recently challenged as results aren't consistent with the theory. In the previous theory, it follows that the best humectant would be the one to hold the most water. In fact, marketers in the skincare industry take advantage of this one by taunting newer ingredients to be 100-1000X more water-binding capacity than Glycerin. Unfortuntely this is not the case. Glycerin, which holds three times less water than sodium pca, is a better humectant as it performs significantly better in clinical trials. Water binding ability is not a predictor of how well a humectant can perform in the skin. Surely there is something else.
The Newer Model for Hydration
Recent thinking suggests something different. NMFs have been found to be able to affect keratin and lamellar lipids. The main function of humectants isn’t that they hold water, it’s that they replace water in your skin. Even when water evaporates, humectants remain because they have lower vapor pressure. This means that are less susceptible to evaporation than water.
They can even do things that water have a hard time doing. When you have a very low amount of water in your skin, it’s dry, flaky and it cracks. This is because Keratin in your skin is in its contracted mode. As your skin gets hydrated, keratin fibres open up to bind water in them. Normally, Keratin opens up around 70-80% humidity in pure water. However, small polar solutes (Humectants) can pry open and stretch out keratin even at lower humidity. This results to your skin being soft and pliable even in dryer conditions.
Picture dry skin like dry noodles - it can easily crack and break. Nobody wants that. Once open, it’s flexible and pliable. Yes please!
But wait there's more..
We also have to rethink humectants based on their effect on proteins and lipids. Barrier lipids can be understood as the cement in between layers of the skin. In 30% relative humidity - so, not a lot of water in the skin - the short layers of the barrier lipids have a tight structure. There’s very little water in between them. This is called the orthorhombic structure - picture a container tightly packed with lollipops. This can be a good thing in terms of permeability because nothing can pass through them, but too much of this structure is not a good thing. It causes the lamellar layer to be brittle and so the skin can easily be broken.As the relative humidity increases, water gets into the lamellar lipids and also gets in between the ceramide heads. This makes the lamellar layers more flowy. This is a double-edged sword as a more flowy lamellar lipids allow our skin to be more flexible and easily hydrated but it also loosens up to allow particles to penetrate the skin. Everything in moderation.
Below is the list of common humectants used in skincare and their corresponding function.
Humectant |
Usage Rate |
Skin Function |
| Glycerin | Up to 100% |
|
| Urea | 5-30% |
|
| Hyaluronic acid | 1-2% |
|
| Lactic acid | 2-30% |
|
| Osmolytes (Betaine, Taurine, Myoinositol) | 2-5% |
|
| Sodium/Potassium Lactate | 2-5% |
|
| Sodium PCA | 1-10% |
|
| Amino Acids | Up to 100% |
|
| Sugars | Up to 100% |
|
|
Ions |
No data |
|
| Sugar Alcohols (Sorbitol, Xylitol, Mannitol) | Depends on the sugar alcohol |
|
| Glycols | Depends on the glycol |
|