Seborrheic dermatitis is not contagious and is typically restricted to certain areas of the skin . Thus, one could argue that there is something happening at those specific areas of skin.
The first clue comes from the areas of skin which are most commonly affected. These are almost always sebum rich areas with a large amount of sebaceous glands. This includes areas such as:
- Side of nostrils
- Behind the ears
- Front of the chest
- Area between shoulder blades
Everyone is different
Sebum rich areas do differ from individual to individual and some individuals experience seborrheic dermatitis on other areas of the skin, but the ones mentioned here are the most common.
And since the unifying feature of skin affected by seborrheic dermatitis is the sebum, one can argue that the cause of this skin condition is directly linked to sebum.
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Shifting Focus From Malassezia to Sebum Composition
Malassezia fungi are considered superficial commensals of the skin. This simply means that they are a common part of healthy skin microbiota, but also have the ability to provoke inflammatory reactions .
Malassezia yeasts feed on lipids which make up a large portion of the sebum our skin produces . This feeding process is known to result in a gradual release of oleic free fatty acids. And in some individuals, the presence of these fatty can cause the symptoms of seborrheic dermatitis .
So technically, it’s not the malassezia themselves that are the cause of seborrheic dermatitis, but the oleic free fatty acids which they release. And since malassezia is present even on the skin of healthy individuals, it may make sense to focus on the composition of sebum (it’s food source) instead of focusing strictly on the malassezia.
A Closer Look at the Role of Sebum
The composition of sebum mainly consists of triglycerides, wax esters, squalene, cholesterol, anti-microbial peptides and anti-microbial histones . The oily nature of sebum makes it a target for a fairly large number of oil-dependent bacteria and fungi, such as propionibacterium acnes and malassezia yeasts.
To defend against bacteria and fungi, the skin relies on the breakdown of triglycerides to anti-microbial free fatty acids  and an adequate supply of anti-microbial peptides and histones . As a result, healthy sebum is essential for proper protection.
The Issue With an Altered Sebum Composition
An altered sebum composition may essential be missing the components required for healthy skin. And unprotected fatty acids can become an excellent food source for various bacteria and fungi, such as malassezia.
More specifically, unprotected sebum may be allowing malassezia to feed on the skin’s first line of defenses, removing the protective layer and leaving behind fatty acids which are considered irritants . So, not only is your skin’s first line of defenses compromised, it is also turned into a unfavorable composition (by the malassezia) which causes irritation.
The skin then has to react by initiating the second line of defenses, which rely on inflammation and the activation of more complex immune processes (adaptive immune response). At the same time, the skin likely attempts to compensate for the broken sebum layer by producing yet even more sebum. Unfortunately, instead of aiding repair it may unintentionally create a vicious cycle favorable for the malassezia.
Sebum Composition in Seborrheic Dermatitis
Since sebum plays such an integral role in the progression of seborrheic dermatitis, isolating specific difference in sebum composition (between healthy individuals versus those affected) may hold significant value. Sadly, very few studies exist which examine the sebum composition specifically in the case of seborrheic dermatitis.
Nevertheless, these few studies do indicate some specific differences exist:
- The iodine number and squalene content are lowered in seborrheic dermatitis, while the cholesterol content is raised .
- Individuals who suffer from seborrheic dermatitis have decreased levels of vitamin E, glutathione, and specific polyunsaturated fatty acids .
- Infants affected by seborrheic dermatitis show an impaired function of the enzyme delta-6-desaturase , which is required for breakdown of certain essential fatty acids .
Overall, it appears the skin is simply having issues producing the correct composition of sebum required for its protection (not over-production).
Many Potential Reasons for Altered Sebum Composition
There are a variety of possible explanations for an imbalanced sebum composition. Some of the most common include:
- Issues in metabolism of consumed lipids
- Insufficient vitamin E levels required for adequate protection of fatty acids
- Issues of fatty acid biosynthesis at the skin level
And this is just scratching the surface.
This section made the connection between seborrheic dermatitis and sebum composition. These are some of the key takeaways:
- Seborrheic dermatitis is not contagious and is most commonly restricted to sebum rich areas of skin.
- Malassezia fungi feed on certain fatty acids found in sebum and this processes is often blamed for causing seborrheic dermatitis.
- Malassezia are superficial commensals, meaning they are a common part of healthy skin, but have also been shown to provoke inflammatory reactions.
- The skin relies on certain components of sebum, specifically free fatty acids and peptides, to defend against various bacteria and fungi.
- Sebum that lacks key protective components can serve as an excellent food source for malassezia and results in the release of irritating oleic free fatty acids
- Heightened malassezia activity, may promote further secretion of compromised sebum and contribute to a viciously cycle of inflammation
- Some differences in the sebum composition of seborrheic dermatitis affected skin have been documented, but no large scale studies exist.
- There are many possible reasons for the documented differences in sebum composition, however, it is difficult to single out any specifics.