It's Michelle of Lab Muffin Beauty Science here again with more skin care
related chemistry nerdery. Today we're going to be discussing a misconception
that's quite common in skincare communities.
It's about fatty acids and oils. Ihis might sound really abstract and
theoretical but it's fundamental to understanding why some claims about oils
aren't as valid as they sound. For example you might see lauric acid is 15
times better at killing acne bacteria than benzoyl peroxide. Coconut oil is 50
percent lauric acid, so coconut oil is going to be fantastic at killing acne
bacteria. Or oleic acid disrupts your skin barrier. Since argan oil contains
about 50% oleic acid it will disrupt your skin. But chemistry and biology say
otherwise. Let's chat about fatty acids and triglycerides.
An acid in organic chemistry is a group that contains a carbon double bonded to
an oxygen atom and an OH group. This hydrogen can ionise which is what makes
it an acid. Fatty acids have this acid group connected to a long non-polar
hydrogen and carbon chain. This chain makes the chemical feel greasy and
fatty, hence the name fatty acid. There are a range of fatty acids. Yhere are
saturated fatty acids which contain no double bonds like lauric, myristic
stearic and palmitic acids. There are monounsaturated fatty acids that have one
double bond like oleic and sapienic acid, and polyunsaturated fatty acids
that have multiple double bonds like linoleic and linolenic acid. Some
unsaturated fatty acids are called omega fatty acids. The omega refers to where
the double bonds are.
A triglyceride also known as a triacylglycerol looks like this. It has one
glycerin molecule connected to three fatty acids via what are called ester
linkages. Triglycerides are the main components of the common fats and oils
that you'll know: the vast majority of plant oils like olive oil, sunflower oil,
coconut oil, but also fats like beef fat and lard.
If you zoom into the ester linkages in a triglyceride you'll notice that the COOH
acid groups aren't there anymore. There technically aren't any fatty acids in
the triglyceride at all! That's because the acids have undergone a chemical
reaction to make the triglyceride so they're no longer acids. So what happens
is this OH on the fatty acid comes off and a hydrogen on the glycerin also
comes off, and they connect like that. So you can see the acid no long has that H+
that can come off, and so it's no longer an acid. It's a little bit like how
sodium metal and green chlorine gas undergo a chemical reaction to give
sodium chloride, which is white and salt and delicious, not green or metallic or
gas-like at all. But chemical reactions can be reversed
under the right conditions. That's why you'll see lots of nutritional advice
saying that fish oil is a good source of essential omega-3 fatty acids, even
though the actual fish oil contains only triglycerides.
Lipase enzymes from your saliva, stomach and pancreas break up the triglycerides
and turn them back into free fatty acids. This breakdown reaction is called
hydrolysis. Soap makers also talk about oils in terms of the percentages of
fatty acids contained in them, because the main step in soap making involves
performing a chemical reaction with lye to break up the covalent bonds in the
triglycerides to free the fatty acids. But plant oils contain very little free
fatty acids. In general you'll have less than 1% free fatty acids in the oil that
haven't been either transformed into the triglyceride or have broken off the
triglyceride. Too many free fatty acids actually makes the oil rancid which is
when it smells and tastes off. For example coconut
oil can't be sold if it contains more than 0.5% free fatty acids. Virgin
coconut oil has an average of 0.1 percent free fatty acids. Cold-pressed
unrefined rose hip oil has about 2% free fatty acids. So when we put oils on our
faces we're applying very little free fatty acids. So the question is: do we
have lipases on the surface of our skin that can break up these
triglycerides? If yes then using oils is the same as using the free fatty acids,
but if not then it's like going to a restaurant and ordering a steak and
being given the steak while it's still attached to the cow. It turns out that
the answer is quite complicated. I've actually been wondering about this
and reading about it on and off for about three years, and I've been talking
to my nerd friend/cosmetic chemist Stephen about it a few times
and I finally sat down and read through hundreds of references before coming to
a conclusion that I'm somewhat confident with. I've included the most relevant
references here, but if you're really interested in the science and the
chemistry behind it all I'd encourage you to do a proper literature review
yourself.
There are enzymes called lipases in your skin that can break up
triglycerides. Some of these are made by humans but a ton of these are made by
microorganisms living in your skin. Most of these live under the surface of
your skin in your pores. Humans produce an oil called sebum in the sebaceous
glands under the skin. When it's at the bottom of the pore near the sebaceous
gland it contains a high amount of triglycerides
and almost no free fatty acids, but by the time it gets to the skin's surface
around 1/4 of this has broken up to give 15% free fatty acids. Scientists in the
70s did some experiments and found that P acnes, the acne bacteria, is the main
microbe that produces these lipase enzymes that break down the
triglycerides, but other species can produce lipase enzymes too. For example
Malassezia fungus, staph and corynebacterium. The vast majority of these
organisms and their lipase enzymes sit inside the pores. There's also a lipase
that your body makes in the living epidermis of your skin that then gets
released into the stratum corneum with the rest of the lipids. So there are
lipases and theoretically they can break down the triglycerides to give free
fatty acids - but how much and how fast? There aren't that many studies that we
can look at to see how much and how fast triglycerides get broken down to give
the free fatty acids when they're applied to the surface of your skin, so
here's what I've managed to find. In the 1950s there was a radioactivity study
where radioactive tripalmitin which is the triglyceride of palmitic
acid was applied to someone's back for two and a half hours then wiped off
and analysed. 6% of the radioactive tripalmitin was broken up versus
approximately 30% of the triglycerides in regular sebum. So why was this figure
so low? The scientists who did this experiment suggested that it was likely
that the oil on the surface couldn't get into the pore whether lipase enzymes
were, and even if it did it would probably get pushed out by the new sebum
that was coming out of the sebaceous glands. 6% in 2.5 hours sounds like
quite a lot especially when you think about how long oil stays on your skin
overnight, but only a really tiny amount of the
tripalmitin was used in this experiment. It was 0.5 milligrams which converts to
0.56 microliters. One drop of water is about 50 microliters so this is
one hundredth of a drop, which was applied over 300 square centimeters
which is about this much skin. If you use more oil then it's even less likely that
it'll get into the pore and so you'll have an even lower percentage. Another
study used mixtures of oleic acid as the free fatty acid and glycerol
trioleate which is 3 oleic acid molecules bound up, bonded to glycerin.
Oleic acid as a free fatty acid is known to be a barrier disruptor. They made a
bunch of mixtures that were 0% free fatty acid and 100 percent
triglyceride; 25 percent free fatty acid; 50 percent free fatty acid and 100
percent free fatty acid. They applied one drop of each mixture to the forearms of
12 people, covered it with gauze, gave them plastic wrap so they could
cover it while they were showering, then looked at the sites the next day. First
they measured the trans epidermal water loss or TEWL which is the leakiness of
the skin to water leaving. They found that the sites with 100% triglyceride
had no change from the untreated areas of skin. The trans epidermal water loss
for the rest of the sites was directly proportional to the amount of free fatty
acid oleic acid. The scientists also measured how easily chemicals could
penetrate the skin going in using fluorescein dye. The depth of the
penetration of the dye was proportional to the amount of oleic acid as well. So
these results suggest that only very negligible tiny amounts of triglyceride
were actually broken up into the free oleic acid or at least anything that
was broken down to give oleic acid was immediately turned into something else
that didn't disrupt the skin barrier, or perhaps they just didn't end up in the
same place. Applying free oleic acid and applying oleic acid bound up in
triglycerides definitely did not have the same effect, at least within the time
frame of the experiment which was 24 hours. There was a third study which was
a longer-term study. In this study they made a cream which contained linoleic
acid-rich oils which were sunflower and borage seed oil.
There was also a plain placebo cream and both of these cranes were applied to dry
skin in winter. They were applied twice daily for four weeks on the inside forearm
of eleven different people. Each person applied the active cream to one arm and
the placebo plain cream to the other arm, then afterwards the scientists analysed
the lipids in the stratum corneum. They looked at the ceramides, free fatty acids
and cholesterol content after four weeks. They found that there was actually less
free fatty acids in the treated area compared to the plain cream but they
found that the levels of ceramide 1 linoleate which are ceramides with
linoleic acid attached were nearly doubled on the treated part compared to
the plain cream, while the same type of ceramide but with oleic acid attached
was much lower. It's also worth noting that in sebum in the free fatty acid
portion there's actually less unsaturated acids like linoleic acid
and oleic acid than in the original triglycerides. So what this suggests is
that some free fatty acids, linoleic acid and oleic acid, are selectively removed
after triglycerides are hydrolyzed or broken down, and used to make other
substances like ceramides.
So what does this mean? From these studies we can see that triglycerides
applied on skin aren't broken down to give you free fatty acids on your skin.
This is important because in pharmacology the dose is very important.
For example if you take one ibuprofen painkiller tablet it's less effective
than taking two. Taking one ibuprofen pain tablet might get rid of your
headache, but taking tiny portions of one tablet over three days won't be
noticeable. It's even more complicated because breaking down the triglycerides
depends on microorganisms and different people have different microorganisms on
their skin. For example men have more microbes on their skin than women and
other factors like climate and genetics will make a big difference too. Different
parts of your face and body also have different microorganisms. One study found
that sebum triglycerides were hydrolysed more quickly at the nose than
on the forehead. Taking antibiotics will change your skin's microbes obviously,
but so will other things like using different cleansers, creams and
antibacterial products. So in conclusion we shouldn't rely on oils for the
properties of their free fatty acids, unless they've been shown to be
effective at very low concentrations, although having them in an oil and
having them in a solution of alcohol, which is how they usually test the
activity of these free fatty acids, will have very different effects on how well
they penetrate your skin. But this doesn't mean that all oils are useless.
They can still be very beneficial for example oils are often excellent
moisturizers. They can be occlusive and slow down how quickly water leaves your
skin and make it dehydrated, plus they work as emollients that soften your skin.
They are also very good bases for oil soluble ingredients and sometimes they
naturally have beneficial ingredients in them. For example rosehip oil contains a
few forms of vitamin A mostly beta carotene but there's also a tiny amount
of retinoic acid or tretinoin in there as well. They can also potentially be
good sources of fatty acids for making ceramides in your skin as shown in the
study on sunflower and borage seed oils. So in summary oils contain fatty acids
bound up in triglycerides, not free fatty acids. Applying skin care oils won't give
you the same effect as applying the free fatty acids. That's
all from me today! If you like my videos I'd love it if you could subscribe to my
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