Why can white hairs get dark again?
I have learned (probably in high school?) that hairs turning white is caused by the part of the folicle which produces the pigment dying and being replaced by an air bubble. This sounds very irreversible.
I have long dark brown hair, and since I turned 25, I have had a few white hairs here and there. It is not unusual to find hairs which have a dark tip but are white at the root, which fits the above theory.
But I also have hairs which are the opposite: the lower ten centimeters are pure white, but at some point, there is an abrupt transition to dark brown, and the hair is dark again up to the root. This is all a natural phenomenon, I haven't used dyes or bleaches.
So what makes white hair white, if it can turn back to dark? Why would a single hair turn back?
The pigmentation of hairs is achieved by the follicular melanocytes (specialized pigment cells) at the base of the hair shaft. These cells produce the pigment which is subsequently transported into the cells which produce the hair and integrated into the hair matrix.
Besides genetic reasons, there are two major ways of losing this pigmentation. First, as a part of the normal hair cycle, the hair grows for a while until it falls out and the hair follicle regresses. In this process the cells in the hair follicle die, including those which made the hair as well as of the melanocytes. In this step of the hair cycle the hair follicle changes its morphology. When a new hair cycle starts, the pigment cells in the hair follicle are replenished from a stem cell population called melanocyte stem cells. These cells are located in the hair bulge (marked in red in the figure below). The cells then start to proliferate to multiply their number. A few of these cells stay stem cells (to be able to repopulate the pigment cells after the next hair cycle.) Others differentiate into melanocytes which subsequently migrate towards the bottom of the hair bulb (shown in green in the figure).
Over time, the number of stem cells gets lower, and at some point there are none left. This leads to a situation where the pigment cells are not replenished at the beginning of a new hair cycle and the hair stays unpigmented. As this is a continuous process, the pigmentation gets weaker before it is completely lost. As far as this process is understood to date, this depigmentation is irreversible.
There are two very interesting papers about this topic:
- Melanocyte Stem Cell Maintenance and Hair Graying
- Melanocyte stem cells: biology and current aspects.
The other reason for losing pigmentation (temporarily or permanent) is the generation of so called "reactive oxygen species" (Hydrogenperoxide) in the cells of the hair follicle, which then destroy the pigment. This is basically the same reaction which is used to bleach hairs, but in this instance from inside. This reaction can have numerous causes, among them are vitiligo and stress. The reason for the build-up of the hydrogenperoxides is that enzymes, which usually break them down before they can cause harm are either mutated or downregulated. If the level of these enzymes (the catalases) goes back to normal, pigmentation is also getting back to normal. This has been tested experimentally with people who suffer from the pigmentation disease vitiligo.
@CRags If you lack tyrosine and cysteine, probably yes. Though I haven't read any reports on people changing their pigmentation levels upon a famine.
When you say "This includes dying of the cells" are you saying the hairs are "dyed" or they "died?"
Hair colour is maintained by a pigment called Melanin which also affects skin colour too. When the melanin content in your hair decreases, it turns grey and eventually white. Multiple factors affect melanin levels in your body.
4) Cell Stress
You can read about them in a little more detail here. I cannot specifically pinpoint if you have deficiency in a particular mineral or the reason for your hair whitening is hereditary from your question. It could be a singular reason or a combination of different problems. Some solutions to treat hair can be found in this link (and plenty more if you Google it). If that doesn't work, I would suggest you meet a certified Dermatologist specializing in hair treatment.
A search of the Internet shows many anecdotal cases of people reporting their grey or white hair returning to their normal color. Interestingly, many of these ancedotal cases begin by stating that their hair had turned grey or white at a young age (early 20s) before returning to it's natural color. Finding a scientific explanation does not seem as easy.
Dr. Robert Graves reported in 1847 a case of a military field officer that had contracted dysentery, fever and other diseases. Prior to his illinesses, the officer's hair had turned white. After a few years of receiving treatment for his illiness, his hair had returned to its natural color. This is consistent with a study by Bublin and Thompson (1992) which found that various drugs can cause sudden hair color changes. (The abstract does not say whether this includes a change from white to natural color.) Schaffrali et al. 2002) noted two people that had a disease called porphyria cutanea tarda regained their hair color. Therefore, stress from disease or drugs may cause hair color changes.
Flesch (1949) found that the fur of black rats could be turned white when fed a diet deficient in copper. Restoring copper to the diet turned the fur color back to its normal color. Arthur (1965) found that excess molybdenum in the diet of guinea pigs can inhibit copper, causing their hair to turn white. These results suggest that diet can also play a role in color loss and gain in mammals. Vitamin B12 deficiency may also have affect hair color.
These studies show that regaining hair pigmentation has many possible causes. Of course, I have no way of knowing whether any of these apply to you, nor am I qualified to make such judgements. But, hopefully, some of this information will help you to understand some of the possible reasons for hair color change in humans.
In the case of drug induced pigmentation changes, there are two possibilities. Either the drug kill the melanocytes which produce the pigment and they are later replenished as I explained in my answer or that the drugs interfere with the signalling pathways which regulate the pigmentation. The case of the diet lacking copper is relatively easy to explain. The most important enzyme in the pigmentation process is Tyrosinase, as it is catalyzing the first step of the synthesis of melanin. This enzyme has copper in its active center, without copper it is not functional.