Ultimate Humectant Comparison

tl;dr

The study isn't perfect but there are good, important takeaways. Glycerin showed to be a great humectant even in low humidity. Acetamidoethoxyethanol is a humectant you should watch out for. Urea and Ectoine showed decreased skin hydration but there might be an explanation for it. Hyaluronic acid and other film formers did not significantly increase hydration in lower humidity in comparison to other humectants.

Purpose

The primary purpose of this study is to compare and assess various humectants commonly used in skincare. We also aimed to determine whether some humectants dry out the skin in low humidity conditions.

Recap: Humectants

Humectants are ingredients that increase the hydration in the upper layers of the skin (Stratum Corneum). Currently, this vast group of ingredients isn't categorized into groups, which make things a bit confusing. Lumping them up into a single category isn't helpful as their mechanism is key to understanding how to use them properly. Having said this (and also because we can't test all of them at once), we grouped them into categories that can allow us to understand them better:

• Intrinsic Humectants - humectants that are produced by the skin to hydrate itself
• Synthetic Humectants - humectants that are lab-made and purported to increase skin hydration
• Film-formers - humectants that form a film on the surface of the skin to increase hydration

Intrinsic Humectants

• Glycerin (Glycerol)
• Betaine
• Saccharide Isomerate
• Free Amino acids and Urocanic acid - 40%
• Ions 18.5%
• Lactate - 12%
• Pyrrolidone Carboxylic acid (PCA) - 12%
• Peptides, Sugars and Organic acids - 8.5%
• Urea 7.0%
• Ammonia, Uric acid, Glucosamine, Creatine 1.5%
• Citrate, Formate 0.5

Skin hydration is a complex symphony of processes that lead to various molecules for hydration. It constantly produces pre-formed moisture bombs (Filaggrin) that detonate into Natural Moisturizing factors in the skin. The humectants mentioned with percentages above are what compose the NMFs. Other humectants present in the skin are not NMFs (Glycerin, Betaine, etc.) as they have a different mechanism for hydrating the skin.

Some plant extracts are also rich in humectants like amino acids, sugars, ions or peptides AND are usually dissolved in either Glycerin or Propanediol.

Synthetic Humectants

• Glycols (Propanediol, Propylene Glycol, Butylene Glycol, Pentylene Glycol, Hexanediol, Heptylene Glycol etc.)
• Methyl Gluceth
• Aqualxyl (Xylitylglucoside - Anhydroxylitol - Xylitol)
• Ectoin
• Elfamoist (Acetamidoethoxyethanol)

Synthetic humectants are lab-made molecules that can increase skin hydration. Usually, these molecules are tiny, compact, and optimized to contain many hydrogen-binding sites e.g. Ectoin and Methyl Gluceth. In the case of Glycols, they increase hydration by going in between the hydrophilic heads of Ceramides or disrupting their arrangement altogether. They can also replace water to keep skin cells pliable as they have lower vapor pressure, so they evaporate slower than water.

Film-formers

• Polyglutamic acid
• Hyaluronic acid
• Polysaccharides
• Hydrogen-bonding polymers
• Various extracts

Plants, Animals and microorganisms produce various polysaccharides (carbohydrates with linked sugar molecules) for energy storage, structural support or protection. These sugar molecules have hydrogen-binding sites that can bind water. Naturally, when they are linked together to form super long strands of polysaccharides, they form long water-holding strings. Plants like Okra, Fermented soybeans (Polyglutamic acid), Cassia Angustifolia are harvested, and their polysaccharides are purified for use in skincare. Animals like snails secrete mucin, a polysaccharide-rich substance with other hydrating molecules. Marine microorganisms like Pseudomonas secrete polysaccharides to protect themselves during drought.

Hyaluronic acid also belongs to this group of humectants as it is also a long string of repeating molecules (N-acetylglucosamine and Glucorunic acid) capable of holding water. There are different molecular weights of HA available, and they function differently in the skin. Some form a viscoelastic layer on top of the skin, while others penetrate the upper layers of the skin.

A Side Note on Hydrating Plant Extracts

Based on what you learned above, hydrating plant extracts can work as humectants or actives. These ingredients are mostly supplied as solutions (20% usually), with Glycerin, water or glycols as solvents. When you see a plant extract in the ingredient list touted as a hydrating active, you should ask the following questions:

• Is the carrier responsible for the hydrating effect?
• e.g. 1% plant extract in pure Glycerin
• Does the plant extract contain molecules that can replace water?
• search the plant extract composition
• sugars, amino acids, peptides and other solutes
• Is the hydration effect mainly because of polysaccharides?
• search (plant name) polysaccharides
• What does it contain to boost inherent hydration?
• e.g. Asiatic Acid in Centella Asiatica
• Is there enough of that active in the formulation?
• Yes, a molecule in the extract may be hydrating, but the extract is diluted, and only a tiny amount of it is used in the formulation rendering the active insignificant.

Ask yourself these questions anytime you encounter a hydrating plant extract so you can demystify its properties. Nothing is magic; it is usually just the Glycerin, sugars or polysaccharide causing the hydration.

Corneometry

The current gold standard in measuring changes in skin hydration is the Corneometer CM 825. It is based on the capacitance measurement of a dielectric medium. It can detect even the slightest change in hydration level and is specific to water as other solutes have minimal effect during measurement. It is a very precise tool that is very useful in comparing solutions or products in the same experiment. Because of this sensitivity, it is difficult to compare results from another study as many factors can affect it.

Procedure

The humectants chosen to be studied were assigned one of three groups: skin-identical humectants, synthetic humectants, and film formers. The three groups were studied on three separate days following the same procedure for each study. Groups 1 and 2 consisted of 9 humectants, and group 3 consisted of 20. A solution was made for each of the humectants in which a predetermined percentage of humectant was dissolved in water. Most of the humectants tested are in a 5% concentration except for the film-formers where 5% would make them too thick to dissolve.

On the day of the study, the participants (n=5) did not apply anything to their forearms at least 5 hours before. The participants arrived early at the study location in order to acclimate to the conditions of the room for at least 30 minutes prior to baseline readings. The humectant solutions were applied to the inside of the forearms. 20 locations were marked on the forearms for application - 5 rows on each arm with 2 locations per row.

The instrument that we used to measure skin hydration was a Corneometer, which is an extremely sensitive piece of equipment used to measure the level of hydration. It works by measuring the capacitance of the stratum corneum, which is the standard method of assessing skin surface hydration. The measurement depth of this instrument (10-20 μm of the stratum corneum) is calibrated so that there is no influence from deeper skin layers (e.g., from the blood vessels) in order to obtain a highly accurate measurement of moisture in the skin.
Baseline measurements were taken for each participant to assess the initial hydration level of their skin. This measurement was taken immediately before application and taken to be T=0 values.
The humectants were then applied using a micropipette calibrated to 25 μL. The application and spreading of the solution within the marked location were made by one person to ensure consistency between readings. For the first two groups, each humectant was applied twice, and one location was occluded using petrolatum. It was found that the Corneometer readings were very inconsistent due to the occlusive, and this step was taken out for the last study.
Readings were done at 30 minutes, 1 hour and 2 hours after application, following the Corneometer standard operating procedure and following the methodology of previous studies. The change in hydration was obtained by subtracting the test measurements from the negative control.

Results

In the first group of humectants, Glycerin performed the best, followed by Betaine then Lactic acid. The average change for Glycerin was about 19 au. Urea and Sodium PCA surprised us with decreased hydration compared to the others. Urea also caused slight itchiness and tingling to some of the subjects.

In Group two, Acetamidoethoxyethanol (a relatively unknown humectant) performed better than Glycerin, which is surprising for us. Propanediol also performed well, considering that the relative humidity dropped to 24% compared to 46% in the first group. Ectoin was drying on the skin and left a white powdery residue.

Group three results were the most surprising. Glycerin still performed the best out of all the humectants tested in this group. Followed by Algae extract in Glycerin and Tremella extract in Propanediol. Interestingly, all the Hyaluronic acids and other film formers caused a decrease in hydration. Pentylene glycol, which is touted to be the most hydrating, also caused a decrease in hydration. 1,2-Hexanediol was the worst among the group.

Discussion

Before we proceed with the discussion, there are 4 things that we need to keep in mind when interpreting these results. These would be discussed further in the limitations, but it is important to mention them before interpreting the results.

1. The sample size
2. Reproducibility of the Corneometer study
3. The humidity level
4. The scale

Sample Size

We only had 5 subjects in the study which means that the study will not have enough power to detect minute differences between humectants. However, it still had enough power to show us drastic differences between the best and the worst.

Reproducibility of the Corneometry

The Corneometer CM 825 is the gold standard for skin hydration measurement. It is an extremely sensitive device that detects changes in your skin hydration level. Because of this, it is hard to reproduce studies exactly. What this means is that you can't compare results from two different studies. In our case, we have three different groups that cannot be compared with each other unless they are in the same group. Even with the addition of Glycerin as a standard for all three groups, the results of group 1 cannot be perfectly compared to group 2 and 3.

Humidity Level

The humidity level for Group 1 is 46%, 24% for Group 2 and 21% for Group 3. We knew going in that humidity would definitely have an impact on the results, but we didn't have a way to control it properly; hence we decided to still proceed with the study. As you see, it did affect the results, but it is actually interesting to see how humectants performed in low humidity (where we need hydration serums the most).

The Scale

Groups 1 and 2 have a similar Y-axis scaling, while Group 3 has a smaller scale. This is because the results for Group three are much less than the other groups, and this was done to visualize the results properly. In any case, we discourage comparing results between groups anyway. To begin the discussion, it is not surprising that Glycerin performed the best in Groups 1 and 3. It is still the gold standard of hydration. Interestingly, many people gloss over Glycerin when they view the ingredient list, but I consider it an active rather than a humectant. Aside from its ability to replace water, it also serves as the substrate for Aquaporin 3 transport to trigger Ceramide and other Barrier Lipid production. Currently it is present at 5% in Wave, 5% in C.R.E.A.M. and 10% in Tabula Rasa, but given these results, we'll definitely be revisiting those soon.

Betaine followed suit after Glycerin; I didn't expect it to be second to Glycerin because it was up against other well-known humectants. Lactic acid is another surprising humectant as we thought its effects are more long-term as it can be used as a building block for ceramides. Group one also showed some disappointing results from Sodium PCA and Urea. Some studies show that Urea's hydrating effects are only seen in water in oil emulsions and not on aqueous solutions or oil-in-water emulsions. One plausible theory is that Urea is a strong barrier loosener, and it improves hydration only when occluded. Because of this, we also tested for hydration levels in occluded sites + various humectants. However, we did not get any meaningful results as the Corneometer isn't reliable in measuring water levels underneath petrolatum.

In Group two, Acetamidoethoxyethanol performed better than Glycerin. This rather unknown humectant surprised us as we haven't heard much of it before. Propanediol also showed great results even if the humidity was only 24%. Some requested Ectoin, and it led to a decrease in hydration compared to the rest of the humectants. One thing to note here is that Corneometry measures hydration only with respect to water and not other solutes. It's also possible (especially with the structure of Urea and Ectoine) that Ectoine and Urea replace water rather than hold it. If this is the case, then the Corneometer would not be able to measure this change in hydration/humectancy.

Group three results are quite interesting. Glycerin still proves to be the best among the group. It is followed by Algae extract and Tremella extract. Interestingly, the Algae extract is actually a 20% solution of Algae extract in Glycerin. Is the increase in hydration only caused by Glycerin? If Algae extract had an effect, it should be at least as hydrating as Glycerin, but that is not the case here, so we're thinking that Glycerin might be carrying the weight here. The same seems to be true for Tremella extract in Propanediol. Tremella extract also showed great humectancy, but since we didn't compare it with pure Propanediol, we can't be certain.

The other interesting result here is that not all glycols are the same. Propylene Glycol and Heptanediol seems to be neutral in terms of hydration. Pentylene Glycol (which is supposed to be the most hydrating glycol) didn't show significant results. Hexanediol went the other way and dehydrated the skin (probably why the manufacturers don't push it as much).

The most interesting results from this experiment are the film-formers. Because they form viscoelastic layers on top of the skin, we expect them to increase hydration in the Stratum Corneum. Because we know that the film-formers have varying Molecular weights, we hypothesize that they would perform differently. Surprisingly, all of them resulted in decreased hydration. Past studies have shown that HA, depending on the molecular weight can act as a barrier loosener. However, even those with high molecular weight also led to lower hydration levels. This potentially gives credence to the fact that some humectants can dehydrate your skin at low humidity IF used alone.

We were hoping to get more meaningful differences between the molecular weights, but the study doesn't have enough power to illuminate those differences. So what can we conclude with this study?

Our main conclusions are:

1. Glycerin is an excellent humectant even at lower humidity.
2. Acetamidoethoxyethanol is another humectant worth looking into. We could do more studies to solidify its hydrating effects and potential synergy with Glycerin.
3. Some humectants could dehydrate the skin in low humidity, but we cannot rule out that they can replace water to promote skin elasticity.
4. We definitely need more data to elucidate the effect of Hyaluronic acids and film-formers on the skin, but it is safe to say that they do not lead to a significant increase in hydration when used alone at low humidity when compared to Glycerin.

Limitations:

1. Sample Size: The study was performed using five participants. Once again, all five participants reside in the same geographical location and work in the same environment. Therefore, all participants are exposed to the same environmental stressors and will have similar skin conditions at the time of testing. All three of our humectant studies were performed during the winter season, a time when the temperature and relative humidity are low, so it would be interesting to test humectants' hydration abilities on participants who have been consistently exposed to opposite environmental conditions.
2. Application Surface Area: Although the amount of product applied was controlled using a micropipette, the surface area of the test site was not identical for each humectant and between participants. We tried our best to draw a similar surface area, but the subjects' forearm's varying "available" areas are not the same. One person did the application of the products to keep everything even. However, for future studies, the size of the test sites should be better controlled and more reflective of how much product would be spread out over the face for daily use.
3. Environment Changes: For each humectant study, every participant was acclimated to the test room for a minimum of 30 minutes before Corneometer measurements were taken. Participants remained in the same room for the duration of the studies and took special care to ensure that they did not do anything which could affect the applied humectants. However, the climate-controlled environment for the first study differed from that of the last two humectant studies. The first humectant study was performed in an environment where the temperature and relative humidity were measured to be 22.8°C and 46%, respectively. For the remaining two studies, the temperatures were measured to be about 1°C lower (21.8°C and 21.6°C) and the relative humidities were in the 20% range (24% and 21%). As the relative humidity for the first study is almost double of that for the last two, the results obtained from the first study may have been different if the study was performed at a lower relative humidity like the last two. Therefore, it would be best to ensure that all studies are performed in the same environment to minimize any variance in results from environmental changes.
4. Time Between Measurements: Skin hydration measurements were taken 30, 60, and 120 minutes after application of humectants. Although the time between application and skin hydration measurements was tracked, due to the number of participants and humectants measurements took longer than anticipated. Therefore, some measurements would have been taken later than originally scheduled, introducing more variance for the measurements between participants. The difference at each scheduled time point varied up to 20 minutes and the humectants may have had more time to be absorbed by the skin, thereby, increasing or decreasing hydration for some participants. The timer for each measurement was started after occlusion of the last humectant, but the humectant and occlusion application process for each participant took about 15 to 20 minutes total. Thus, the humectants which were applied early on would have already been absorbed by the time the last test site was occluded and measurements would have occurred up to 20 minutes later than they should have.
5. Humectants in Solutions: For the first two studies (natural humectants and synthetic humectants), 5% humectant solutions were prepared and applied. For the final study where film-former humectants were tested, the test solutions were prepared to contain either 1% or 5% humectants. The difference in percentages tested introduces a variance in the results and does not allow for a direct comparison of all humectants. For example, different molecular weights of hyaluronic acid were tested, and 1% solutions were prepared for each. However, a 5% solution of sodium hyaluronate crosspolymer was prepared instead. Therefore, sodium hyaluronate crosspolymer's hydration ability cannot be directly compared to that of the various molecular weight hyaluronic acid solutions.