What are the causes of hair loss?

Hair loss is part ofa stage in the hair cycle. It is adaily phenomenon but entirely normal. Indeed, each hair has its own life cycle composed of three phases.

• The Anagen Phase (2 - 6 years): This is the growth phase of the hair during which there is proliferation of the matrix cells that form the inner sheath of the root, the cortex, and the medulla of the hair shaft. The synthesis and pigmentation of the hair shaft only occur during this phase. Its duration determines the length of the hair, generally 1 cm per month.

• The catagen phase (2 - 3 weeks): The first sign of the catagen phase is the cessation of melanin production, the pigment responsible for the colour of our hair, in the hair bulb. The hair stops growing but remains attached to its hair follicle.

• The telogen phase (2 - 3 months): Following the catagen phase, the follicles enter a resting period, known as the telogen phase. The hair shaft eventually detaches from its follicle, which has already begun to produce a new hair beneath the skin.

Each hair has an asynchronous life cycle. Some may be in a growth phase, while others are about to fall out. This is why we naturally lose between 50 and 150 hairs per day. Their life cycles are dictated by hormonal or nutritional variations. However, certain factors can cause an abnormal hair loss.

To date, we have identified numerous forms of hair loss that are hereditary. For example, theandrogenetic alopecia is due to genetic factors where the scalp follicles react abnormally to the action of testosterone. Indeed, testosterone is converted under the action of the 5-alpha reductase enzyme into dihydrotestosterone (DHT). The latter is known to shorten the lifespan of hair by accelerating the hair cycle and thus promoting hair loss. There are also other hereditary forms of hair loss.

One of the reasons that can cause hair loss is hormonal changes. This issue is thus quite common during pregnancy and after childbirth. Similarly, around the age of 50, women experience menopause. This stage is characterised by the cessation of reproductive hormones produced by the ovaries.

As a result, the menopause can trigger a hormonal imbalance which is characterised by a deficiency in oestrogen concentration and an increase in androgen levels. Indeed, the latter leads to an acceleration of the hair cycle and the hair follicle renewal capacity will prematurely deplete. This upheaval will then cause hair loss, also known as "androgenic alopecia".

Hyperandrogenism, which indicates an excess of androgen, is often associated with hair loss. Men are most affected as androgens, the primary hormones involved in hair loss, are male hormones. Hyperandrogenism can occur as a result of congenital adrenal hyperplasia, a genetic disorder that involves a mutation in some of the enzymes that metabolise steroid hormones, or an adrenal-origin tumour. Women are also affected by hyperandrogenism, which can occur as a result of polycystic ovary syndrome or an ovarian tumour.

Stress or anxiety can lead to hair loss. An emotional shock can trigger a chain reaction in the body, leading to hair loss. Indeed, during a period of stress, a locally released substance P can directly inhibit the growth of keratinocytes in hair follicles, or it can induce their apoptosis and inhibit their proliferation via the release of hair growth inhibitory cytokines, such as TNF-α, IL-154 and IL-55. TNF-α is an inflammation mediator that triggers the transition of hair into the telogen phase, their shedding phase. Depression caused by stress can also prolong the disruption of hormones. Thus, people suffering from depression can lose a lot of hair over a long period.

Hair loss can also be attributed to dietary deficiencies. Indeed, diet has a direct impact on the health and appearance of the hair.

• Iron Deficiency.

  Iron is a cofactor of the ribonucleotide reductase, an enzyme that limits the speed of DNA synthesis. The cells of the hair follicle matrix are among the cells that divide the fastest in the body and can be extremely sensitive, even to a minor decrease in iron availability, thus leading to a decrease in hair growth. Nevertheless, further studies need to be conducted to elucidate its mode of action and to assert that an iron deficiency can be responsible for hair loss.

• Zinc deficiency.

  Clinical studies have shown that out of 312 patients diagnosed with Alopecia Areata (AA), male hair loss, female hair loss, or Telogen Effluvium (TE), their serum zinc levels had significantly decreased, unlike a control group. Indeed, zinc plays a role in the regulation of hair growth, as it acts as a catagen inhibitor through its inhibitory action on apoptosis-related endonucleases. Therefore, a zinc deficiency can promote hair loss.

Hair loss is a daily occurrence, but you may have noticed that it is more pronounced in the arrival of autumn.Indeed, during the summer, we observe a decrease in the production of melatonin, which is inhibited in the presence of light. Melatonin is involved in the hair cycle by prolonging the anagen phase.

Clinical studies have shown that in women suffering from androgenetic alopecia, a solution of melatonin applied to their scalp was able to increase their hair growth by extending the anagen phase compared to control groups. Thus, in summer, melatonin is inhibited and the hair enters the catagen phase and then the telogen phase. This process can take a few months, which explains why our hair only falls out at the beginning of autumn.

• RANDALL V. A. & al. Seasonal changes in human hair growth. British Journal of Dermatology (1991).

• ARCK P. C. & al. Indications for a brain-hair follicle axis: inhibition of keratinocyte proliferation and up-regulation of keratinocyte apoptosis in telogen hair follicles by stress and substance P. The FASEB Journal (2001).

• HADSHIEW I. M. & al. Burden of hair loss: Stress and the underestimated psychosocial impact of telogen effluvium and androgenetic alopecia. Journal of Investigative Dermatology (2004).

• OLSEN E. A. & al. Iron deficiency in female pattern hair loss, chronic telogen effluvium, and control groups. Journal of the American Academy of Dermatology (2010).

• KIL M. S. & al. Analysis of serum zinc and copper concentrations in hair loss. Annals of Dermatology (2013).

• LIN R. L. & al. Systemic causes of hair loss. Annals of Medicine (2016).