Fatty Acids and the Skin Barrier

Free fatty acids (FFAs) in the form of essential fatty acids (EFAs) and non-essential fatty acids (NEFAs) compose 15% of the stratum corneum (SC) multilamellar lipid matrix. FFAs play numerous roles in the skin, but their primary function is to form lamellar layers with Ceramides and Cholesterol for the reduction of trans-epidermal water loss (TEWL). Structurally, fatty acids are composed of a straight carbon chain with a carboxyl group at one end. EFAs are distinguished by their unsaturated 18 carbon long chain, meaning that it contains double bonds. Meanwhile, the straight chain of NEFAs does not contain any double bonds (i.e. is saturated) and can range in carbon chain length. The most common fatty acids in the SC are saturated 22 to 24 carbon long NEFAs [1]. Although EFAs are less prevalent, with only 2% being present in the form of linoleic acid (LA), they have been shown to be fundamental for maintenance of barrier function [2]. 

FFAs also make up two components of the 3:1:1:1 lipid ratio for barrier repair. In most literature studies, linoleic acid is used as the EFA and palmitic acid is used as the NEFA; however, there isn’t much information available as to why these two are specifically selected. In our first version of C.R.E.A.M. we used a mixture of linoleic acid and linolenic acid for the EFA component, and stearic acid for the NEFA. However, during our C.R.E.A.M. 2.0 development we were curious as to whether changing any of these would have any effect on barrier repair. After performing barrier repair studies with different types of FFAs and various derivatives of EFAs in the 3:1:1:1 ratio, the results we obtained were quite surprising!  

Essential Fatty Acids

Essential fatty acids are considered to be essential because our body cannot synthesize them on its own, so we can only obtain them through dietary intake. EFAs can be divided into two categories: n-6 (omega-6) and n-3 (omega-3). Linoleic acid and linolenic acid are the parent fatty acids of the omega-6 and omega-3 families, respectively. Majority of literature studies center around linoleic acid’s role in maintaining the skin barrier, but there are other EFA derivatives which have also been shown to have benefits for the skin. We decided to test the following forms of linoleic acid and linolenic acid for their ability to accelerate barrier repair:

• linoleic acid and linolenic acid mixture
• alpha-linolenic acid      
• alpha-linoleic acid
• gamma-linolenic acid
• ethyl linolenate
• ethyl linoleate
• conjugated linolenic acid - ethyl ester
• conjugated linolenic acid - free fatty acid

EFA Results

Our results show that a mixture of linoleic acid and linolenic acid is best at accelerating barrier repair through TEWL reduction. At all time points following barrier disruption, this EFA mixture had the highest barrier repair ability and was able to achieve 93% repair at 8 hours. Interestingly, alpha-linolenic acid also performed well at 8 hours which is contrary to previous thoughts that it doesn’t play a significant role in the skin. Although it lagged behind at 2 and 4 hours, alpha-linolenic acid was able to achieve 75% repair at 6 hours which is only 9% lower than the EFA mixture which achieved 84% barrier repair, and at 8 hours both achieved 93% repair. Conjugated linolenic acid (CLA)-ethyl ester and CLA-free fatty acid had the lowest barrier repair ability at 6 and 8 hours. However, this does not mean that their role in the skin barrier is not as significant as the other EFAs. CLA-free fatty acid is  a natural PPAR mediator, so it plays a role in the regulation of keratinocyte proliferation, inflammation, and barrier homeostasis  [3]. It is also important to note that all EFAs were able to accelerate barrier repair when compared with the control which did not contain any EFAs. 

Non-Essential Fatty Acids

Non-essential fatty acids can be synthesized by the body and range in carbon length from short-chain (less than C6) to long-chain (C13 to C21), and even all the way up to very long-chain (C22 or more). Depending on their chain length, NEFAs play different roles in maintaining normal skin function. Short chain NEFAs can behave as antimicrobial molecules to maintain micro-ecological balance in the skin [4]. Long-chain NEFAs primarily play a role in maintaining the structure of the lipid matrix to reduce TEWL and prevent the entry of foreign substances. We decided to test the following long-chain NEFAs, as well as a very long-chain NEFA for their barrier repair abilities:

• myristic acid (C14)
• 12-hydroxystearic acid (C18)
• 10-hydroxystearic acid (C18)
• behenic acid (C22)
• palmitic acid (C16)
• stearic acid (C18)

NEFA Results

This is where the surprising part is: Our initial hypothesis was that the barrier repair ability of NEFAs is correlated with their chain length, so we expected longer chain NEFAs to perform better. However, these are not the results which we obtained. Our results show that these three NEFAs are best at accelerating barrier repair: myristic acid, 12-hydroxystearic acid (12-HSA), and 10-hydroxystearic acid (10-HSA). At 8 hours myristic acid was able to achieve 92% barrier repair, while 12-HSA and 10-HSA both achieved 90% repair. Meanwhile, the very long-chain behenic acid performed very similarly to the shorter chain palmitic acid, which disproved our original theory. At 8 hours, behenic acid repaired the barrier by 78% and palmitic acid repaired it by 76%, and we’re still working on figuring out the science behind these results. Similar to the EFA results, all NEFAs were able  to accelerate barrier repair when compared with the control which did not contain any NEFAs. 

C.R.E.A.M. 2.0 Fatty Acids   

With these results, we then proceeded to test a combination of the best performing fatty acids in order to optimize the barrier lipid ratio. C.R.E.A.M. 2.0 contains a mixture of linoleic acid and linolenic acid for the EFA component, and a combination of myristic acid, 12-hydroxystearic acid, and 10-hydroxystearic acid for ultimate barrier repair. But we feel like we’ve opened a can of worms! This is the point in our research where we realized that every component in the 3:1:1:1 ratio has an effect on barrier repair.   

References

[1] Bouwstra, J. A., Dubbelaar, F. E. R., Gooris, G. S., & Ponec, M. (2000). The Lipid Organisation in the Skin Barrier. Acta Dermato-Venereologica, 208 (Suppl.), 23-30. DOI: 10.1080/000155500750042826  

[2] Menon, G. K., Cleary, G. W., & Lane, M. E. (2012). The Structure and Function of the Stratum Corneum. International Journal of Pharmaceutics, 435 (1), 3-9. DOI: 10.1016/j.ijpharm.2012.06.005

[3] McCusker, M. M., & Grant-Kels, J. M. (2010). Healing Fats of the Skin: The Structural and Immunologic Roles of the Omega-6 and Omega-3 Fatty Acids. Clinics in Dermatology, 28, 440-451. DOI: 10.1016/j.clindermatol.2010.03.020 

[4] Yang, M., Zhou, M., & Song, L. (2020). A Review of Fatty Acids Influencing Skin Condition. Journal of Cosmetic Dermatology, 19, 3199-3204. DOI: 10.1111/jocd.13616