101 Facts About Ageing #59: Hair Loss

As Daniel Patrick Moynihan, an American sociologist, politician, and diplomat once said: “Everyone is entitled to his own opinion, but not his own facts”. And we wholeheartedly agree. A shared set of facts is the first step to building a better world with longevity for all. In that spirit, we are creating a series that covers 101 indisputable facts about ageing, health and longevity.

Age-related hair loss typically occurs in males, though up to two thirds of women may also experience some level of hair loss after the menopause. Age-related hair loss occurs primarily on the head, and usually manifests as a gradual thinning. In men this usually begins on the forehead, causing the hairline to recede, while in women it is more common for hair loss to be diffuse, and is usually first noticed as a broadening of the central parting of the hair. Some people can also lose hair in circular or patchy bald spots.

Why does this happen? While it may seem like hair grows continuously, individual hairs actually grow in a cycle made up of three phases. The cycle begins with a growth phase, and ends with a resting phase during which the hair falls out so that a new hair may take its place. The duration of these phases vary throughout the body. On the scalp, for example, the growth phase lasts 2-8 years, while most other hair only grows for a year and a half at most. This is why your eyebrows will never grow to be longer than the hair on your head.

Hair loss during ageing happens because the growth phase becomes shorter, while the resting phase lasts longer. This means that fewer hairs are growing at any given time. Hair follicles also shrink and eventually die, preventing any new hair from being grown at all. In 95% of males, these changes are caused by androgenic alopecia, the result of an androgen (male sex hormone) called dihydrotestosterone, or DHT. DHT is produced from testosterone, and can be likened to a more potent version of that hormone. DHT actually stimulates the growth of body hair, but for unknown reasons, has the opposite effect in hair follicles of the scalp, causing them to atrophy. The sensitivity of hair follicles to DHT is genetic, and tends to increase with age, making body hair grow longer while scalp hair is reduced.

Although produced in much smaller quantities than in males, females still need some testosterone and DHT. The relationship between these hormones and female hair loss isn’t well understood, but it may be related to the decline in female sex hormones following the menopause. For example aromatase, an enzyme produced by hair follicles that converts androgens to oestrogens, appears to decline with age in women. Oestrogens also extend the growth phase of the hair, which is why women sometimes lose hair at a higher rate following pregnancy. Oestrogen elongates the growth phase during the pregnancy period, resulting in a large number of hairs falling out at once when oestrogen returns to normal.