【Doctor commentary】The reason why women are less likely to experience hair loss compared to men

The role of hair includes important functions for maintaining life, such as regulating body temperature and protecting the skin. Human hair is widely distributed on the skin throughout the body, except for certain areas like the palms of the hands, soles of the feet, and some parts of the genitals. On average, adults have around 100,000 to 150,000 hairs growing on their bodies.

To maintain the hair cycle, there are hair follicle stem cells present in the part called the hair follicle. These cells support the growth of hair.

Hair goes through a process of regeneration and regression called the hair cycle. During the growth phase, hair proliferates, while in the regression phase, cell growth stops and regression occurs. For human scalp hair, the growth phase lasts about 2 to 6 years, the regression phase is about 2 to 3 weeks, and the resting phase is about 3 to 4 months. In terms of the entire scalp, the proportions of the hair follicles during each period are roughly 85-95% in the growth phase, 1% in the regression phase, and 10% in the resting phase.

Hair maintains its density by undergoing physiological shedding and regrowth according to the normal hair cycle. Even for the same type of hair, the growth phase for eyelashes is shorter, about 30-45 days, while the resting phase is around 100 days. The ratio of growth to resting phase for vellus hair is approximately 30-50%, and its growth phase is about 50 days. The hair cycle can vary depending on the area and environment.

In acquired hair loss conditions, disruptions to the hair cycle lead to thinning or patchy hair. Alopecia areata is an autoimmune disorder that causes distinct round or oval bald patches with a diameter of 2-3 cm, suddenly appearing without preceding lesions in areas where hair was present, including the scalp.

Male pattern baldness (androgenetic alopecia or AGA) occurs in the frontal and vertex regions of the scalp. Testosterone is converted to dihydrotestosterone (DHT) by type II 5α-reductase. DHT binds to androgen receptors in male-pattern hair follicles, leading to shortening of the growth phase and the transformation of terminal hair into vellus-like hair. A similar condition can also occur in women.

In females, it is common to see diffuse thinning of hair over a relatively broad area on the crown of the head, leaving the frontal hairline intact. While females also have more 5α-reductase and androgen receptors in the frontal area compared to the occipital area, they possess more aromatase, which converts testosterone to estradiol, than males. This could explain why females are less prone to hair loss compared to males.

Dr. Munenori Matsuzawa, Chief Medical Officer, Aoyama Medical Clinic.

References：
1) Cotsarelis G, Sun TT, Lavker RM: Label-retaining cells reside in the bulge area of pilosebaceous unit: implica- tions for follicular stem cells, hair cycle, and skin car- cinogenesis, Cell, 1990: 61: 1329-1337.

2) Ito M. Yang Z, Andl T, et al: Wnt-dependent de novo hair follicle regeneration in adult mouse skin after wounding, Nature, 2007:447:316-320.