Does Hyaluronic Acid actually "rob" your skin of moisture?
So, does Hyaluronic Acid applied topically pull water from your skin and “rob” it of its moisture? You will all be quite surprised that the answer to this is largely NO, with some caveats. Let me explain the science.
Molecules that draw water to themselves, or are hygroscopic, are known as humectants. From a chemistry perspective, humectants share similar properties in that they contain multiple hydroxyl groups (-OH), which can interact with water molecules via hydrogen bonding.
A variety of humectants are native to the skin such as glycerol, hyaluronic acid, urea, etc. In topical preparations, these humectants can help supplement the ones naturally present to increase water concentration to the tissue. Cutaneous hydration is paramount to its health and homeostasis, as many of the enzymatic and biochemical pathways that keep the skin functioning normally are hydrolytic, or water-dependent (the skin itself, and the body, contain roughly 65-70% water…more on this later).
So, the theory goes that if you apply these humectants to the skin in an atmosphere of increased humidity, they will pull the moisture (water) from the environment, and if applied in a dry climate they will pull the moisture (water) from the deeper layers of the skin, “robbing” it of moisture. The former part of this statement is factual, the latter part…not so much (again, with caveats).
Let’s look at glycerol, more commonly known as glycerin, which is by far the most effective and scientifically sound humectant for skin when applied topically (yeah, it kicks hyaluronic acid in the butt and says, “take it, bi*ch!”). When applied topically, glycerin and all those “thirsty” hydroxyl groups, are ready to attract the water molecules to themselves. Yes, it can pull the water from an environment with increased relative humidity, it can hold onto water that is sprayed on the skin before applying it, or it can attract water from the underlying tissue. In fact, the size of the glycerol molecule is quite small (92 g/mol, or 92 Daltons), that it can easily penetrate the skin.
The big question at hand is does this attraction of water from the deeper layers to the upper layers, “rob” our skin of moisture? Guess what, for most of us the answer is a resounding NO! What?! You say this is what you have been told by brands, educators, physicians, and beauty influencers your whole life?! I am in SHOCK that the industry would feed you inaccurate information…that NEVER happens! ;-P
How is this possible you ask? The simple answer…because our body, and our skin, are made up of 70% water. “Robbing” typically means that you are taking something valuable that often cannot be replaced. This is important in this context, and not just semantics. The in-depth answer? Water in the body/skin is not limited in supply. If we use humectants, and they draw deeper water stores to the upper levels of the skin, they are not robbing the water, they are simply redistributing it, and any water drawn out would be replaced via your circulation if there is water to replace it. This is called the concentration gradient (science…it kind of matters). Our skin is quite privileged in terms of water distribution in a total body “drought”. However, if you are clinically dehydrated, can longer sweat, and there is a limited supply of water in your body, then we are in a different situation and you are in dangerous territory. By the way, if this is you, please stop reading this and go to the hospital right away. You have far more to worry about than the ingredients in your skincare.
So, if the water being attracted by the humectant is coming from the deeper layers of the skin, and that water is readily replaced by water from our circulation, what IS the problem with humectants? This is where that concentration gradient can turn on us, especially in dry climates. You see, under normal physiological conditions, the relative humidity of the skin underneath our skin barrier (stratum corneum) is high, around 99%. However, the external environment, depending on where you live, has a relative humidity range between 40-60%. This is an extreme water gradient across the skin’s membrane. Concentration gradient is basically where, to keep things “balanced”, the amount of water inside a membrane, if it is higher than outside the membrane, will deliver water to the outside of the membrane to equalize the situation. This is not an even trade, however. A dry, low humidity environment does not give back. This is essentially what we mean by trans epidermal water loss (TEWL).
Some of you might say that I just contradicted myself. You will say that I just proved humectants can rob the skin of moisture. But I did not. The water supply is still there to replenish. The problem here is twofold. Firstly, I mentioned the “physiologically normal” conditions above. Let’s be honest, for most of us, there is nothing physiologically normal about our skin and its condition. What do I mean?
Let’s talk about how our skin DOES hold on to water under normal, physiological conditions. If your mind is thinking about the skin barrier (stratum corneum), you win the million-dollar prize. The skin barrier is formed when maturing epidermal keratinocytes lose their nucleus to become surface corneocytes. In addition, lamellar bodies inside the keratinocytes release lipids, that will then surround the corneocytes. This is our “waterproof” lipid barrier. Oh, by the way, the processes I just mentioned are largely water-dependent. Under these physiological conditions, water contained in the lower levels of the skin, or any aqueous material, cannot penetrate beyond this lipid-rich structure at the surface of our skin.
However, as mentioned, our skin does not live a physiologically normal life. Our barrier is constantly being compromised, either from the products we use, over-exfoliation, age, or the sun's UV rays that compromise the integrity of the barrier. These factors alone can contribute to water being released from our skin, and even more so in a dry climate due to that concentration gradient increasing water flux. This brings me to the second highlight of the issue…humectants should NEVER be used alone. Why attract all that water to the skin if you aren’t going to “seal it in”? As great as these humectants can be, what truly protects your skin and holds the water in is your lipid barrier. You should always be following up the application of a humectant (if a solo product) with a properly formulated barrier moisturizer, to help reestablish those normal, physiological conditions as much as possible. Some barrier moisturizers have the humectants built-in, and that is perfectly fine as well. At the end of the day, what determines the level of TEWL in our skin is the water concentration gradient between the inside of our skin and the outside environment and the integrity of the skin’s lipid barrier.
If you are still following, bear with me. This is where I get into the good stuff. Hyaluronic Acid (HA). I mean that is what I came here to talk about, right? Let me first tell you that HA is a bit of a different animal compared to what I described above, at least from the perspective of us applying it topically. As you all know, HA is another humectant that our skin naturally produces. It is also known as a glycosaminoglycan (GAG), and it’s the most ubiquitous humectant found in our skin that gives it buoyancy, plumpness, and cushion, among many other benefits that I won’t go into now (Stay focused, Daniel!). What makes topical HA different than the other humectants when applied topically? One simple word – SATURATION!
Most people (surprisingly) don’t realize that 100% PURE Hyaluronic Acid is a solid powder. This is never applied to your skin NEAT. It is always hydrated in water first, then mixed into a formula that eventually goes on the skin. Because HA is such an effective hygroscopic agent, usage levels in skincare are quite low. Most of the time HA is found at less than 1% in a topical product, with 0.2-0.5% being the average. Anything above 2% and the formula would be quite thick, especially if you are using high molecular weight (which you should, by the way). Those brands that claim they have 20%, 50%, 90%, HA in their products…THEY ARE LIARS! HA powder reconstituted in water, at these levels, would be as hard as a ROCK!
So, HA in pure form has not had any of its water-binding sites activated yet. However, when you mix it in water, those sites become hydrated/saturated, and then that water-saturated HA gets applied to the skin. Are you listening? When you apply HA to your skin in any cosmetic formula, it has ALREADY been saturated with water molecules, meaning it is no longer attracting water to itself! It is not “thirsty” anymore. Let me break down some chemistry and math to help you understand:
HA is a sugar polymer, and a pretty large polymer at that, with smaller disaccharide units on its structure. Native HA that your skin releases are high molecular weight polymers. This means many polymers linked together in a large chain. Within these chains are water binding sites, just like any humectant. Each water-binding site on the HA polymer can hold about 15 water molecules. Let’s look at the amount of water binding sites on a large, high molecular weight polymer of HA used in skincare. I will use 2,000 kDa, or 2M Daltons as an example. I have done the math for you, and an HA polymer of this size would have 5,114 units that can bind to water molecules. If each unit can bind to 15 water molecules, the 2 Million Dalton HA polymer can attach to 76,710 water molecules before saturation.
So, how many H2O molecules are inside water? Let’s look at a formula of pure HA saturated in water. Let’s use 1% HA and make a 100-gram sample. In this case, 1% = 1 gram. If we have 1 gram of HA, we have 99 grams of water. How many H2O molecules are in 1 gram of water? A LOT! 1 gram of water contains 3.3455 x 10^22 water molecules. I know, I know. I will do the math for you again. The latter part of that equation is 10 to the 22nd power. Are you ready for it? That number is 10 SEXTILLION! I will take that in dollars, thank you. Now times that by 3.3455…and you have more water molecules than you can even imagine. If that formula has 99 grams of water to its 1 gram of HA….I don’t think I need to do more math to get you to see my point.
The HA you apply to your skin is FULLY SATURATED. There is some theory that even when fully saturated, HA can bind to more water, due to the general chemistry principles we know of about water, and how each water molecule can bind to 4 others, without needing any other influence. This is based on theory and has not been proven in the literature for HA. Even so, given my breakdown above, there are still more than enough water molecules in that 99 grams of water for this to still leave water molecules behind, and the HA no longer “seeking” it from somewhere else (i.e., your lower skin levels).
Next, you will tell me “But I FEEL dry after I apply my HA!”. Let me explain! Again, if you are using HA in your skincare, it should always be high molecular weight. When you apply that HA to your skin and the excess water evaporates, it will leave the polymer film on the skin which can make the skin feel dry/tight, but it is STILL acting as a water-loss barrier, keeping the underlying skin happy. I addition, HA can spontaneously cross-link, creating a stronger mesh-like network on the skin, which will also lead to that temporary tightening effect. High Weight HA is less of a humectant than it is an occlusive, believe it or not. That is its real value. It will hold on to the saturated water molecules and create a barrier that helps prevent TEWL. High Weight HA is too big of a polymer to enter the skin.
However, HA, just like all in this class of humectants, should never be used alone. In fact, HA is the worst offender in this context. It is NOT an elegant molecule in a formula by itself. Have you ever looked at all these popular HA products on the market? They are loaded with other materials that give the skin some comfort (glycerin, possibly lipids, etc.) A pure, 1% HA serum, with nothing else but water in it, will not feel great on the skin if used alone, especially if it’s HIGH WEIGHT HA, which it should be. Why bring all these water molecules to the skin if you are not going to seal them in?
What about Low Molecular Weight HA? While yes, this does have the ability to penetrate better into the skin and can maintain a bit more humectant properties compared to High Weight, the same rules apply as far as water is able to be replenished. The problem with Low Molecular Weight HA is that it is INFLAMMATORY. I can take up a whole article discussing this, so I won’t get into it here. Just know this is IRREFUTABLE. LMW-HA can increase TEWL because the inflammatory cascade it triggers can weaken the skin barrier, allowing water to evaporate. LMW-HA is bad news, no matter how you look at it.
So, to sum things up, HA used topically is fully saturated with water. High Molecular Weight HA, if used alone, will leave a large polymeric film on the surface that can FEEL tight/dry, but is not robbing or pulling moisture from your skin. Still, seal it up! Low Weight HA…yeah, throw it in the garbage. And all other humectants…use in a responsible way, with a balanced portfolio of products that restore barrier function and hold that water in. Make sense?
