We've established that studies have shown that some oils can penetrate the upper layers of our stratum corneum and that this can be a good thing because it means those oils can help improve the barrier lipids we find in the stratum corneum. So let's take a moment to see if fatty acids we find in our oils can penetrate our skin and, if they do, what effect they might have on our skin.
fatty acids that aren't attached to the glycerol backbone of the triglyceride, so they're just floating around in the oils. One of the benefits of refining oils is the removal of these free fatty acids, so we end up with a tiny amount - something like 0.05% - in our oils. (Reference and reference) It's a good thing to remove them as they can contribute to "pro-oxidant" behaviour like our oils going rancid more quickly.
Check out this study on whether or not free fatty acids can penetrate our skin. The conclusion they reached was that lauric acid (C12) and oleic acid (C18:1) could penetrate the skin, with lauric acid shown to accumulate in the dermis. The authors put forth that adding lauric acid might help with penetration of fatty acids. Capric (C10) and linoleic acids (C18:2) penetrated, but not as much as the other fatty acids. This book notes that medium chain length (like capric or caprylic acids), saturated fatty acids, and unsaturated fatty acids are the most effective at penetration.
Here are some pictures of that penetration. Kinda interesting....
This book puts forth the idea that oleic acid (C18:1) disrupts the packed structure of the intercellular lipids in our stratum corneum because it has a kinked structure thanks to that cis double bond, so it's more effective at penetrating our skin than stearic acid (C18), a saturated fatty acid that doesn't have that double bond so it lines up in a straight chain.
Do we want fatty acids to penetrate our skin? Is there value in it? In this study. they noted that although capric acid (C10) penetrated the skin, it didn't cause re-arrangement of lipids. And in this book, it is noted that oleic acid may penetrate the stratum corneum, but doesn't mix with the lipids. If something penetrates but doesn't have an impact, do we care? (Just a thought...)
Here's the abstract of a study that just came out. It's quite interesting and raises a ton of questions for me. (Experimental Dermatology. Jan2014, Vol. 23 Issue 1, p39-44. 10p.) (If you want to see the whole thing, click here...)
Plant-derived oils consisting of triglycerides and small amounts of free fatty acids (FFAs) are commonly used in skincare regimens. FFAs are known to disrupt skin barrier function. The objective of this study was to mechanistically study the effects of FFAs, triglycerides and their mixtures on skin barrier function. The effects of oleic acid (OA), glyceryl trioleate (GT) and OA/GT mixtures on skin barrier were assessed in vivo through measurement of transepidermal water loss (TEWL) and fluorescein dye penetration before and after a single application. OA's effects on stratum corneum (SC) lipid order in vivo were measured with infrared spectroscopy through application of perdeuterated OA (OA-d34). Studies of the interaction of OA and GT with skin lipids included imaging the distribution of OA-d34 and GT ex vivo with IR microspectroscopy and thermodynamic analysis of mixtures in aqueous monolayers. The oil mixtures increased both TEWL and fluorescein penetration 24 h after a single application in an OA dose-dependent manner, with the highest increase from treatment with pure OA. OA-d34 penetrated into skin and disordered SC lipids. Furthermore, the ex vivo IR imaging studies showed that OA-d34 permeated to the dermal/epidermal junction while GT remained in the SC. The monolayer experiments showed preferential interspecies interactions between OA and SC lipids, while the mixing between GT and SC lipids was not thermodynamically preferred. The FFA component of plant oils may disrupt skin barrier function. The affinity between plant oil components and SC lipids likely determines the extent of their penetration and clinically measurable effects on skin barrier functions.
I did some searches and wasn't able to find anything about free fatty acids disrupting skin barrier function other than a few "it is known" kind of things, which is frustrating because you'd think something that is known would be more easily findable, but it is something to think about. I found a reference in this book on page 233 that if we add only one type or two types of of lipids found in our skin (cholesterol, free fatty acids, or ceramides) it "impedes or rather than facilitates barrier recovery". If we add all three, we see "normalized rates of barrier recovery".
So how do I interpret this? What I see this saying is that free fatty acids, like oleic acid, could have a negative impact on our skin by disrupting barrier function that could increase transepidermal water loss. However, refining those oils should bring us down to 0.05% free fatty acids, which is a really tiny amount in our oils, so I think we're just fine given that we are using so many things that will have a positive impact on skin's barrier functions, but that's only my interpretation and opinion, and I'll keep on looking for more information.
As a P.S....This study does shed light on a question posed by Sarah in which she reports that someone out there is saying that refined oils are not good for lotion making or unsafe. In fact, I think this study might show the opposite, that refined oils are the ones that are safest because they have fewer free fatty acids and will go rancid slower than refined oils.