Oils are of a class of molecules that do not dissolve in water. Each oil’s chemical structure is unique, which may impact its ability to create comedones (clogged pores). Therefore, to determine which oils are comedogenic (pore-clogging), scientists must examine each oil to determine if, and how severely, it clogs pores. Through these examinations researchers found that cocoa butter and coconut butter were the most comedogenic oils, while maleated soybean oil, safflower oil, and sunflower oil were the least comedogenic oils.
Structure of an Oil
Scientists define oil as a molecule with a chemical structure that contains a glycerol molecule attached to three fatty acid molecules. Glycerol is soluble in water. However, when a glycerol molecule becomes attached to fatty acid molecules, it becomes an oil, which is not soluble in water. The difference among various oils depends on two things. First, the number of carbon atoms; secondly, and more importantly, whether the fatty acid molecules are saturated or unsaturated. An oil’s structure is classified in two ways, based on the type of fatty acid that is attached to the glycerol molecule.1,2
Saturated fatty acid molecules are tightly packed together. This type of packaging is possible only when each carbon atom in the chain is bound to four other molecules, which is the maximum number of molecules carbon can bind.
Unsaturated fatty acid molecules are more loosely packed together. This type of fatty acid is further subdivided into two classes:
Monounsaturated fatty acids contain multiple carbon atoms, but only one of them in the chain is not bound to four other molecules. This creates a kink in the carbon chain, which disrupts the tight packaging.
Polyunsaturated fatty acids contain multiple carbon atoms, more than one of which in the chain not being bound to four other molecules. This creates multiple kinks in the carbon chain, disrupting the tight packaging in multiple places.
Based on the definition of an oil, certain non-comedogenic substances which the public believes to be oils, such as jojoba oil (a wax ester) and tea tree oil (an essential oil), actually are not oils. Therefore, when examining an oil for comedogenicity, it is important to verify that it is in fact an oil by looking at its structure.
Testing the Comedogenicity of an Oil
Two methods exist to test an oil’s ability to clog pores.3,4
The rabbit ear model involves applying an oil at 10% strength to a small section of a rabbit ear daily for two weeks. After the two-week period the scientists measure the number of clogged pores and give the oil a score of 0 (no clogged pores) to 5 (many clogged pores).
The human model involves applying an oil to a small section of the upper back daily for four weeks. After four weeks the scientists measure the number of clogged pores present in the upper back area. The human model does not involve a 0–5 scale as the rabbit ear model does.
The rabbit ear score does not perfectly predict how human skin will respond to the same substance because rabbit skin is thinner and more sensitive than human skin. However, scientists are confident that if a substance does not clog pores in the thin, sensitive skin of rabbits (score 0–2), then it most likely does not clog pores in human skin. If a substance does clog pores in rabbit skin (score 3–5), then it may clog pores in human skin.
In 1989, researchers performed a study to test the comedogenicity of 200 substances, including many oils, through the rabbit ear model. The results of this study found that cocoa butter and coconut butter each scored 4. The study also identified three non-comedogenic oils, with a score of 0. These oils include maleated soybean oil, safflower oil, and sunflower oil.4
Testing Comedogenicity – A Different Approach
One study tried to determine the comedogenicity of mineral oil, when included in several product types. As in the above-mentioned study, all trials were carried out on the ears of rabbits. However, in an attempt to obtain a more objective understanding of comedogenicity, the researchers decided to abandon the grading approach. Instead, they compared the average ratio of unclogged pores to clogged pores, measured before and after exposure to formulations containing mineral oil, to the numbers of unclogged pores and clogged pores measured before and after exposure to distilled water. The products did not cause a significant difference in the number of clogged pores when compared to distilled water. They therefore were not considered to be comedogenic.5
Why Some Oils Are More Comedogenic than Others
Researchers have not yet identified one clear reason that some oils are more comedogenic than others. However, scientists have proposed several explanations. More research needs to be performed to confirm if they do indeed affect an oil’s comedogenicity.
Saturated fatty acids may be more comedogenic than unsaturated fatty acids. How saturated fatty acids may cause more clogged pores than unsaturated ones is unknown, but researchers have observed that more comedogenic oils tend to have higher amounts of saturated fatty acids. The amounts of saturated and unsaturated fatty acid of common oils are found below.6
More refined oils may be less comedogenic than less refined oils. Some researchers believe that their inability to dissolve in water prevents them from entering pores. However, pores also contain oil, and thus this hypothesis has been disputed. At any rate, according to this hypothesis, an oil must contain impurities that can enter and clog the pore in order for it to be comedogenic. Oil refining is a process that purifies an oil by removing those impurities. Therefore, more refined oils would be less comedogenic because of their fewer impurities.3
Composition of Sebum
Skin oil is called sebum. Glands within the pores produce this oil, which is then expelled onto the surface of the skin. We call it an oil because of its oily characteristics, but sebum actually is not a typical oil. It is a collection of molecules that include mainly free fatty acids. Depending on the individual, the components of sebum vary.
Research has found that people who have excessive amounts of sebum tend to have more acne lesions. However, it is not just the amount of sebum but also the concentration and type of fatty acids in the sebum that impact the development of acne. Scientists have found that the types of fatty acid in sebum differ between acne and non-acne patients. More specifically, they have found that acne patients had much lower concentrations of the fatty acid, lineolic acid, than non-acne patients. Therefore, it is likely that composition of fatty acids in sebum affects the formation of clogged pores.7,8
Diet and Changes in Sebum
We have talked about how oil on the surface of the skin may affect acne. However, there is some interesting science regarding intake of various fatty acids and how they interact with sebum.
Because the human body does not make all the fatty acids that are required for the production of sebum, some must be consumed through diet or supplementation. Scientists believe that a high-fat diet may in fact change the amount and composition of sebum and potentially lead to clogged pores. However, this belief is highly controversial, and researchers have performed independent studies that both support and refute the claim.9,10
Initial research has found that consuming more omega-6 fatty acids may increase sebum production, clogged pores, and acne, while omega-3 fatty acids may decrease inflammation, including inflammatory acne. Although more research needs to be performed before scientists are able to prove these correlations, it is clear that correctly balanced lipids are essential for skin health and that imbalances caused by poor diet can lead to disease.11,12Here are the food groups that essential fatty acids, which the body cannot produce, are found in: