Everything you need to know about sebum.

Sebum is a complex fluid of lipids that is produced by the activity of the sebaceous glands, which are attached to hair follicles through the excretory duct through which sebum flows to reach the skin's surface. These glands are located between the middle and upper part of the dermis. Its production is notably under hormonal stimulation: this is why sebum is not produced uniformly throughout life. It is more significant during adolescence, pregnancy, or during the menstrual cycle, periods when hormonal activity is at its highest.

How is sebum synthesised?

The sebaceous glands secrete sebum through the rupture of mature sebaceous cells (sebocytes) into the follicular duct, a process known as "holocrine secretion". Around the periphery of the sebaceous gland are undifferentiated and proliferative sebaceous cells, characterised by a flattened appearance.

During their differentiation, these cells will gradually fill with lipid droplets and migrate towards the centre of the sebaceous gland at the maturation zone. During the maturation process, the sebocytes will increase in volume and continue to synthesise sebum.

At the end of maturation, when the cells approach the excretory duct, they disintegrate and release their lipid content through a process known as holocrine. This corresponds to the fact that the release of sebum occurs through the rupture of the sebocyte membrane. The released sebum then travels through the pilosebaceous canal to reach the surface of the skin, where it coats the hair fibres and spreads over the horny layer, mixing with the epidermal lipids and the aqueous phase (sweat) to form the hydrolipidic film.

What is sebum composed of?

The sebum, preformed within the sebaceous gland and as it is excreted, is a complex mixture of four lipid classes : squalene, glycerolipids (triglycerides), waxes, and cholesterol. Squalene and waxes are unique compounds characteristic of sebum, while the other lipid constituents are found in other parts of the body.

As it travels through the pilosebaceous canal to be delivered to the surface, this native sebum is subsequently exposed to oxygen and the enzymes of resident microorganisms, leading to a transformation of some of the lipids into esterified, oxidised and peroxidised derivatives. For instance, triglycerides are converted into monoglycerides, diglycerides, glycerols and free fatty acids. Similarly, a large portion of cholesterol is metabolised into cholesterol ester under the action of the skin flora's enzymes. Thus, the sebum produced in the sebaceous glands differs from that excreted on the skin's surface.

Even though sebum is considered an enemy by some, it is nevertheless essential for the health and beauty of the skin and hair. Among the functions attributed to it, we can mention:

• Maintaining the hydration of the epidermis: When combined with sweat, sebum forms the hydrolipidic film that covers the entire surface of the skin. It thus protects against the dehydration of the horny layer by retaining hygroscopic substances in the epidermal cells, conferred by its lipid composition and therefore by its occlusive effect. For hair, its role is to prevent them from becoming dry and brittle ;

• Mechanical Barrier: Sebum also has the responsibility of protecting the skin and hair against all external elements that can attack and weaken them, such as the sun's UV rays, weather conditions, pollution, cigarette smoke, air conditioning... It prevents the penetration of foreign substances into the skin;

• Bactericidal activity: The sebum even allows for the destruction of bacteria present on the skin and prevents any microbial contamination. During its excretion, a portion of the triglycerides present in the sebum is converted into free fatty acids under the action of bacterial hydrolases. Thus, thanks to the oleic and palmitoleic acids contained in the sebum, the skin has a slightly acidic pH that prevents the growth of "bad" bacteria. Similarly, some free fatty acids in the sebum possess antibacterial activity;

• Release of Liposoluble Antioxidants: During its excretion, sebum carries with it antioxidants, such as Vitamin E among others, to the skin surface. The presence of such constituents in the sebum helps to combat oxidation, particularly that caused by UV radiation;

• Pro- and Anti-inflammatory Activity: Specific sebaceous lipids have been identified as expressing both pro-inflammatory and anti-inflammatory properties. Among them, oleic acid and linoleic acid are thought to promote the differentiation of immune cells (macrophages). They also contribute to enhancing the ability of these cells to absorb P. acnes (a microbial agent responsible for acne), while palmitic, oleic, and stearic acids increase the immune cells' response to the bacteria. Furthermore, linoleic acid and oleic acid are also believed to have an anti-inflammatory effect on macrophages activated by inhibiting the secretion of certain pro-inflammatory molecules following an inflammatory reaction.

The production of sebum is continuous and is not controlled by neuronal mechanisms. It is on the order ofone to two grams per 24 hours. However, its production level varies from one individual to another and is the origin of different skin types. Indeed, this lipid secretion is a process controlled by various mechanisms. Here are examples of both physiological and environmental factors that can disrupt seborrhoea:

• Genetic predisposition:Variations in sebum production may have a genetic component, with the sebaceous glands showing a particular sensitivity to androgens;

• Age-related changes: During the last trimester of pregnancy, the foetus's sebaceous glands are functional. At birth, a stimulation of sebum secretion due to maternal hormones is observed and reaches its peak during the first month before slowly decreasing over the following months until it becomes almost non-existent by the sixth month. Around the age of 9 - 10 years, a new increase occurs to reach a peak at puberty due to the increase in testosterone in the blood. Sebum production then stabilises in the twenties but can increase for hormonal reasons (pregnancy, menstrual cycle, contraceptive, menopause). From the thirties onwards, a decline of about 23% per decade in men and about 32% per decade in women is observed. This decrease is more pronounced in menopausal women, who experience a drop in hormones (oestrogens and progesterones), than in men where the decline is slower;

• Hormonal Issues: These are the primary culprits in regulating sebum production. Indeed, three components of sebocyte function (differentiation and proliferation of sebocytes, and lipid synthesis) are controlled by hormones. Androgens (dihydrotestosterone), hydrocortisone, and growth hormone (insulin or IGF) promote the growth of sebaceous cells and lipogenesis, while oestrogens and progesterone inhibit excessive activity of the sebaceous glands by exerting an antagonistic effect to androgens from a certain dosage;

• Diet too high in sugar : Studies have shown that foods with a high glycemic index (white bread, potatoes, processed foods, fruit juices, etc...), dairy products, as well as meat (due to the presence of a specific amino acid, leucine) are risk factors involved in the phenomena of excessive sebum secretion. Indeed, by increasing insulin levels, they stimulate the secretion of a growth hormone which, in turn, increases the levels of androgens in the blood;

• Circadian Rhythm: A study has highlighted the existence of a circadian rhythm in the secretion of sebum in rats, with a decrease in the activity of the sebaceous glands during the night and a peak towards the end of the morning;

• Body Temperature: A study has revealed that temperature fluctuations could influence sebum excretion. Indeed, an increase/decrease of 1°C in skin temperature could cause an increase/decrease of approximately 10% in sebum secretion. This may reflect a change in the rate of sebum excretion on the skin's surface, likely due to a change in sebum viscosity known to vary with temperature;

• Drug Interactions: Vitamin B12, anxiolytics, lithium, corticosteroids, certain antituberculosis drugs, and halogens (iodine, bromine) can lead to hyperseborrhea.

There are other endogenous factors that can disrupt sebum secretion: exposure to the sun's UV rays, stress, cigarette smoke, pollution, etc...

When there is an overproduction of sebum, skin disorders can be observed. Indeed, there is a blockage of the pilosebaceous follicles, which clogs the pores of the skin or the roots of the hair. The results are: Oily hair, blocked and dilated pores, the appearance of imperfections such as blackheads, comedones or acne spots and folliculitis.

Conversely, insufficient sebum production also has direct consequences on the health and quality of the skin and hair. It is notably the cause of dry skin, dry and brittle hair, and peeling skin. Indeed, this low amount of sebum leads to an increased insensible water loss, but also an alteration of the keratin and the hair cuticle. This weakening of the barrier function also maintains an inflammatory state and makes the skin more prone to redness and irritation.

• SHUSTER S. & al. Circadian rhythm in sebum excretion. British Journal of Dermatology (1970).

• SHUSTER S. & al. The effect of local temperature variations on the sebum excretion rate. British Journal of Dermatology (1970).

• LAURENT R. & al. Sebum levels during the first year of life. British Journal of Dermatology (1980).

• DOWNING D. T. & al. Age-related changes in sebaceous wax ester secretion rates in men and women. Journal of Investigative Dermatology (1985).

• JACOBSEN E. Age-related changes in sebaceous wax ester secretion rates in men and women. Journal of Investigative Dermatology (1985).

• LAPIERE C. & al. Patterns of follicular sebum excretion rate during lifetime. Archives of Dermatological Research (1987).

• DOWNING D. T. & al. Changes in the relative amounts of endogenous and exogenous fatty acids in sebaceous lipids during early adolescence. Journal of Investigative Dermatology (1989).

• KLIGMAN A. M. & al. Rhythm of sebum excretion during the menstrual cycle. Dermatologica (1991).

• ORTONNE J. P. & al. Circadian variations in the number of actively secreting sebaceous follicles and androgen circadian rhythms. Chronobiology International (1993).

• CILETTI N. & al. Mechanisms of androgen induction of sebocyte differentiation. Dermatology (1998).

• PACKER L. & al. Sebaceous gland secretion is a major physiologic route of vitamin E delivery to skin. Journal of Investigative Dermatology (1999).

• CHUNG K. & al. Sebum output as a factor contributing to the size of facial pores. British Journal of Dermatology (2006).

• VARIGOS G. A . & al. A low-glycemic-load diet improves symptoms in acne vulgaris patients: a randomized controlled trial. The American Journal of Clinical Nutrition (2007).

• MASTROFRANCESCO A. & al. Sebaceous gland lipids. Dermato-endrocrinology (2009).

• MATSUNAGA K. & al. Fatty acid compositions of triglycerides and free fatty acids in sebum depend on amount of triglycerides, and do not differ in presence or absence of acne vulgaris. Journal of Dermatology (2014).

• SEITE S. Effect of air pollution on sebum Rate and acne: how to manage acneic skin in a polluted environment. The Journal of Cutaneous Medicine (2017).

• TÖROCSIK D. & al. Sebaceous-immunobiology is orchestrated by sebum lipids. Dermato-endocrinology (2017).

• TÖROCSIK D. & al. Sebum lipids influence macrophage polarization and activation. British Journal of Dermatology (2017).

• KIM E. & al. Influence of exposure to summer environments on skin properties. Journal of the European Academy of Dermatology and Venereology (2019).

• TAN J. & al. Effects of diet on acne and its response to treatment. American Journal of Clinical Dermatology (2021).

• MICALI M. D. & al. Diet and acne: review of the evidence from 2009 to 2020. International Journal of Dermatology (2021).