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If you’re a skincare nerd, you may know that different skincare oils have different fatty acid profiles – that is, they differ in terms of which fatty acids they contain e.g. linoleic, linolenic, oleic and lauric acid.
The fatty acids have interesting properties. For example, lauric acid is strongly antibacterial, and works better than benzoyl peroxide against Propionibacterium acnes bacteria. Oleic acid is a barrier disruptor, while linoleic acid can fade pigment. So it sounds like skincare oils could be interesting active ingredients or irritants, alongside moisturisation!
But there’s a catch: in oils, these fatty acids aren’t actually acids at all. They’ve undergone a chemical reaction to form new substances called triglycerides. These triglycerides can be broken down to form the free fatty acids again under the right conditions: during digestion, and in the process of saponification in soap-making. But does this happen on the surface of your skin?
I’ve been wondering about this for a while, and I’ve had a few chats on and off with fellow nerd Stephen of Kind of Stephen over the years but we never really came to any firm conclusions. I finally sat down recently and dug up all of the papers I could find on fatty acids and combed through them all to try to make some sense of it (there were literally about 400 papers that I sorted through, which is why I couldn’t bring myself to do it earlier… I probably didn’t need to go through so many, but I like to be extra thorough to convince myself thrice over before trying to convince anyone else!).
A disclaimer: I don’t recommend trying to sift through the literature to “discover” new things in research-heavy areas (e.g. acne and other skin disorders) because rest assured, researchers who know a lot more about this stuff and have a ton more training have already done this and published it in a review, and if you manage to find something new it’s probably not supported by the research. But whether skincare oils break down into free fatty acids is not the sort of topic that gets funded, so I feel safe(ish) making my own conclusion here, although I welcome review!
Related post: Are unsaturated oils bad for your skin?
Here’s the video, in which I go through the chemistry of fatty acids and triglycerides, the key studies, and explain the (sort of, kind of, tentatively convincing) conclusion I came to: triglycerides in oils are not broken down to give significant amounts of free fatty acids on the skin’s surface. In other words, we can’t rely on oils for the properties of their free fatty acids except when the FFAs are effective at very low concentrations.
Studies on Oil Hydrolysis to Free Fatty Acids on the Skin
The studies I talk about in depth in the video:
Nicolaides N, Wells GC, On the biogenesis of the free fatty acids in human skin surface fat (open access), J Invest Dermatol 1957, 29, 423–433.
- 0.5 mg of tripalmitin was applied over 300 cm2 skin on the back of a single individual
- 2 hours later, 6% was hydrolysed to palmitic acid
Mack Correa, MC et al., Molecular interactions of plant oil components with stratum corneum lipids correlate with clinical measures of skin barrier function (open access), Exp Dermatol 2014, 23, 39–44.
- Mixtures of free oleic acid and glyceryl trioleate were applied to skin
- TEWL and penetration were proportional to oleic acid content after 24 h
- There was no change with glyceryl trioleate compared to the control
Conti A et al., Seasonal influences on stratum corneum ceramide 1 fatty acids and the influence of topical essential fatty acids, Int J Cosmet Sci 1996, 18, 1–12. DOI: 10.1111/j.1467-2494.1996.tb00131.x
- Cream with linoleic-rich oils and placebo cream applied on skin for 4 weeks
- Lipids in stratum corneum were analysed
- Less free fatty acids were present in the treated area compared to placebo, but levels of ceramide 1 linoleate were nearly doubled, while ceramide 1 oleate levels were lower
Other relevant studies
Some of the other relevant papers that I came across in my search:
Shalita AR, Genesis of free fatty acids (open access), J Invest Dermatol 1974, 62, 332–335.
- Review of experimental evidence for bacterial lipases hydrolysing triglycerides in sebum – the key species is P acnes
Anderson RL et al., Individual and site variation in composition of facial surface lipids (open access), J Invest Dermatol 1972, 58, 369–372.
- Triglyceride hydrolysis happens faster in nose follicles than the nose surface than the forehead surface
- There was a lot of variation between individuals
- Less oleic and linoleic acid in sebum at surface compared to in triglycerides at sebaceous gland
Bomar L et al., Corynebacterium accolens releases antipneumococcal free fatty acids from human nostril and skin surface triacylglycerols (open access), mBio 2016, 7, e01725-15.
- Study on bacteria in nose that hydrolyse triglycerides, and discusses how colonisation varies between different people
Fierer N et al., The influence of sex, handedness, and washing on the diversity of hand surface bacteria (open access), Proc Natl Acad Sci USA 2008, 105, 17994–17999.
- Gender differences in bacteria on skin surface
Hall GS, Mackintosh CA, Hoffman PN, The dispersal of bacteria and skin scales from the body after showering and after application of a skin lotion (open access), J Hyg 1986, 97, 289–298.
- Impact of skincare products on skin surface bacteria
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