Although it varies infinitely from one person to the next, hair colour is due to only two types of melanin, a pigment produced in the hair root. Over time, the production of this pigment naturally slows down, leading to hair whitening.
- Melanin, a natural pigment
Whether our hair is brown, blond or red, it is coloured by a natural pigment called melanin, which also colours our skin. This protein pigment results from the transformation of an amino acid, tyrosine, by an enzyme called tyrosinase. It is produced by large star-shaped cells called melanocytes, located in the hair root. The melanin granules are then transferred to the keratinocytes, which make up the hair shaft. The latter is coloured as soon as it appears, and will stay coloured until it falls out naturally 3 to 5 years later. Yet melanin only represents about 1% of the total weight of hair, even in those with darker pigmentation.
Each person’s hair colour is determined by their DNA, but external factors (sun, salt, etc.) can contribute to lightening it by oxidising the melanin.
- Two types of melanin for an infinite palette
Human hair comes in a huge variety of colours, ranging from blond that is almost white, to darkest black. And yet, there are only two different types of melanin: the first (eumelanin, brown to red) is responsible for brown, dark and black hair, whereas the second (phaeomelanin, red to yellow) colours blond and red hair. The ratio of these two types of melanin and the overall quantity of pigments present determine hair colour.
- When hair turns white
As hairs go through their renewal cycles, the hair melanocytes gradually decline before disappearing completely, unlike skin melanocytes, which are protected by an enzyme. Thus the melanin granules produced by the melanocytes get smaller and smaller and their quantities gradually decrease. Recent studies have proven that the appearance of white hairs is also linked to an interruption in communication between the melanocytes and the keratinocytes, which prevents the former from being able to transfer their melanin granules to the latter.
A progressive whitening of the hair then occurs, also known as canities. Heredity plays a major role in this phenomenon: it usually starts to occur at age 40, but can also happen much earlier in some people. Stress, smoking, excessive exposure to sunlight, nutritional deficiencies and certain illnesses can also speed up hair whitening.
Grey hairs, on the other hand, are merely an optical illusion, the result of white hairs mixing with hair that is still coloured.