Reactive hair loss: what is the link between hair loss and stress?

Hair loss is a daily 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 hair growth phase during which there is proliferation of 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): After the catagen phase, the follicles rest in a dormant phase, the telogen phase. The hair shaft eventually detaches from its follicle, which is already starting to produce a new hair beneath the skin.

Thus, within a head of hair, the hairs are not necessarily in the same phase as they each have their own independent life cycle. Some may be in a growth phase, while others are on the verge of falling out. This is why we naturally lose between 50 and 150 hairs per day. Their life cycles are dictated by hormonal or nutritional variations.

During periods of stress, many individuals have noticed an increase in hair loss compared to usual. Consequently, researchers have delved into this topic to try and understand the biological or molecular links that could explain this observation.

Studies on mice have provided the first evidence that stress plays a role in hair loss. Indeed, stress can lead to the release of substance P by the sensory nerve fibres in the skin. This locally released substance P can directly inhibit the growth of keratinocytes in hair follicles or induce their apoptosis and inhibit their proliferation through the release of hair growth inhibitory cytokines such as TNF-α and IL-154, 55 from macrophages and mast cells. The TNF-α is an inflammation mediator that triggers the transition of hair into the telogen phase, which is their shedding phase. Moreover, previous studies had already demonstrated that an intracutaneous injection of substance P could inhibit hair growth and induce premature regression of the hair follicle triggered by apoptosis. Therefore, it can be concluded that stress disrupts the hair life cycle by hastening the transition into the catagen phase and then into the telogen phase promoting hair loss.

For your information: this substance P binds to the neurokinin 1 (NK1) receptors. The results of this study have therefore led to research to find NK1 antagonists that can counteract the negative effects of stress on the hair follicle.

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).