What is Sebum?
Sebum is a naturally occuring substance that is produced by the sebaceous glands of mammals. The primary function of sebum is to moisturize, lubricate and protect skin and hair. Healthy sebum production is essential for the integrity and normal function of the skin as a protective organ. Normal human skin is composed of three primary layers: the stratum corneum, the epidermis, and the dermis. The outermost layer of the skin, the stratum corneum, functions as the primary barrier to the external environment, preventing water loss and the invasion of microorganisms. Sebum is secreted to the stratum corneum from the sebaceous glands and helps maintain an effective, hydrophobic barrier. The composition and rate of sebum production varies from individual to individual, and can even vary depending where on the body the sebum is produced. While normal sebum levels help maintain healthy skin, abnormalities in sebum biology can lead to a variety of diseases. Acne vulgaris is most often associated with sebum over-production, which can increase the incidence of clogged pores, stimulate inflammation, provide nutrients for bacterial growth, and influence other factors central to the pathogenesis of acne vulgaris.
The Composition of Sebum
Sebum is a complex mixture of naturally produced fats, oils, waxes, cholesterols and other molecules. It should be noted that the fats and oils present in sebum do not originate directly from the fats and oils consumed in the diet. Rather, sebum is manufactured and stored by the sebaceous glands under the direction of a highly specialized biological process. Nearly all mammals (but not whales and dolphins) produce some sort of sebum, and each animal has its own unique blend. In addition to sebaceous glands and sebum, there are other structures in the skin that produce a sebum-like mixture called “epidermal lipids”. In humans, sebum is composed primarily of gycerides, free fatty acids, wax esters and squalenes. Glycerides are more commonly known as “fats” and are molecules of two or three free fatty acids connected together by a glycerol backbone. Free fatty acids are the building blocks of glycerides and other molecules. They are composed of a polar head group and a non-polar (aliphatic tail). Wax esters are molecules composed of fatty acids linked to fatty alcohols by an ester bond. Jojoba oil is composed primarily of wax esters. Squalenes are hydrophobic chains of carbon atoms that serve as the basic building block for naturally occuring steroids and other types of signal molecules. Shark liver oil contains high levels of squalenes.
The Role of Sebum in Acne Vulgaris
Several research studies have shown a direct correlation between increased sebum secretion and an increased incidence of acne symptoms. However, there are several possible explanations for how sebum production may contribute to acne, and there is not necessarily unanimous agreement between experts when it comes to explaining this relationship. The most common explanation is that increased sebum production leads to increased follicular plugs, which clog pores and cause acne. Sebum can also function as a nutritional source for acne-causing bacteria that live in the follicle, such as Propionibacterium acnes. The accumulation of sebum within plugged follicles provides ideal growing conditions for P. acnes bacteria and can lead to high bacterial populations in the skin. People with sebaceous gland disorders (e.g. sebaceous hyperplasia) that lead to very high sebum levels, often have severe forms of acne and other skin diseases. Research has also shown that sebum itself can lead to increased inflammation. Byproducts of the sebum metabolism, in some cases, can lead to the accumulation of molecules that trigger an inflammatory immune response. It is possible that this mechanism is at work in some individuals with inflammatory acne. In those cases, decreasing sebum production will improve symptoms. Research has also shown that people with high sebum levels tend to produce sebum that has a different composition than the sebum from people with normal levels. Apparently, people with acne tend to have decreased levels of free fatty acids, but increased levels of glycerides and squalene. Some scientists have proposed that these compositional changes play a role in the development of acne.
The Regulation of Sebum Production
The proliferation of sebaceous glands and production of sebum is directly regulated by a complex system of hormones and other cellular signals. Going deeper, these hormonal signals are controlled by an even more complex balance that includes genetics, environmental conditions, metabolic conditions, stress, diet, injury and many other factors. Despite this extraordinary complexity, scientists have begun to unravel the central relationships in sebaceous gland biology. As a result, several of the central players have been identified and are currently being investigated. The major regulators of sebacsous gland activity include:
Androgens: Androgens are male sex hormones, like testosterone. It is well known that acne usually develops in males during adolescence, when levels of androgen hormones in the body are their highest. Androgens drive the development of many male characteristics, like muscle and body hair growth. They also stimulate the proliferation of sebaceous glands, particularly those located on the face, chest and upper back. Individuals with excessively high levels of androgen hormones tend to have higher levels of sebaceous gland proliferation, sebum production and acne vulgaris. Women with elevated androgen levels tend to have higher levels of acne and hirsuitism (excess body hair growth). The effect of androgens on sebaceous gland activity is also why the use of anabolic steroids, which increase androgen levels, can cause acne symptoms.
Estrogens: Estrogens are female sex hormones. In most cases, estrogens antagonize (suppress) the effects of androgen hormones. This relationship partially explains why acne symptoms tend to change over the course of a woman’s menstrual cycles, or during and after pregnancy. Men do not usually produce estrogen hormones. Estrogens may also directly modulate sebaceous gland activity, although this relationship is not well studied.
Insulin-Like Growth Factor 1 (IGF-1): IGF-1 is a protein hormone that is produced in the liver and is similar in structure to insulin. Research has indicated that high IGF-1 levels correlate with elevated sebum production. Levels of IGF-1 tend to be highest during adolescence. Since insulin is similar in structure to IGF-1, it is possible that elevated levels of insulin could cause increased sebum production. Insulin levels are often elevated in individuals who consume a high glycemic diet (high sugar/carbohydrate), or who have Type 2 diabetes. This relationship could explain the observed correletion between high glycemic diets, obesity and increased incidence of acne vulgaris. Increased IGF-1 has also been linked to milk consumption, although these studies are not necessarily conclusive. IGF-1 hormone production is stimulated by human growth hormone (hGH).
Retinoids: Retinoids are cell signalling molecules that are derived from Vitamin A. Retinoid is the generic name for a diverse class of related molecules that play essential roles in many human biological systems, including development of the human embryo. The proliferation of sebaceous glands and the production of sebum is directly controlled by specific retinoid signal molecules. Accutane (isotretinoin, 13-cis-retinoic acid) is a retinoid that is a powerful anti-acne drug. Binding of accutane molecules to specialized receptors on the surface of sebocyte cells causes them to slow down their growth and sebum production. The balance of different retinoids and their relative abundance has a direct impact on sebaceous gland activity.
Environmental Conditions: Recent research has shown that sebum secretion levels change in response to seasonal and environmental changes. While the changes are not drastic, researchers observed that sebum secretion levels were highest during the summer. These changes may be due to the increased fluidity of sebum in warmer conditions, or something else entirely. Generally speaking, acne sufferers tend to observe an improvement in their acne symptoms during the summer, although this could be more directly related to factors such as UV light exposure or stress levels, than to sebum production. Overall, it seems likely that sebum production is modulated by environmental conditions, although it is less clear whether these normal fluctuations play a role in the development of acne or other skin conditions.
Stress: Research has indicated that there is a direct correlation between stress and increased acne symptoms. However, further research has shown that stress does not appear to increase the levels of sebum production. While it is well understood that stress can modulate levels of certain hormones, like cortisol, it does not appear that these pathways directly impact sebaceous gland activity.
Treatments for Excessive Sebum Production
The primary treatment of hyperactive sebaceous glands and excessive sebum production involves the use of retinoids. The oral retinoid Accutane (isotretinoin) is a potent anti-acne drug that can dramatically reduce sebum production. For many individuals, treatment with isotretinoin can lead to significant, long-lasting improvement in their acne symptoms. Several topical retinoids are also used to treat acne and other skin conditions. Common topical retinoids include tretinoin (Retin-A), adapalene (Differin), tazarotene (Tazorac) and isotretinoin (Isotrex). These treatments also decrease sebum production, although the effect is often less dramatic than that of oral retinoids. Retinoids are also present in some essential oils and homeopathic compounds, such as Rose Hip Seed Oil. Another course of treatment for excessive sebum production involves the use of androgen inhibitors and/or estrogen hormones. Androgen inhibitors like spironolactone (Aldactone) and cyproterone (Androcur) can partially block the effects of androgen hormones. Estrogens, commonly found in contraceptive medications, can also decrease the effect of androgen hormones. Both anti-androgen medications and birth control medications are usually reserved for use in females, because their effects can disrupt the normal function of the male hormone system. In recent times, light and laser therapies have become increasingly popular for the treatment of acne and other skin disorders. Certain light and laser treatments, like Photo Dynamic Therapy (PDT) and Diode Laser Therapy, can be used to damage the sebaceous glands themselves, which ultimately decreases sebum production at the treatment site. Certain medications, like cyclosporin, have been shown to increase sebum production.
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