Can stress turn hair grey? In mice yes, and greying can reverse

The folk belief that a shock can turn hair grey has a real mechanism behind it, at least in mice: a 2020 study in Nature reported that acute stress leads to hair greying through the fast depletion of the pigment-producing stem cells in the hair follicle [s1]. What that experiment does not show is that greying is a one-way door — a separate human study published in eLife the following year found that white and grey hairs can naturally regain their pigment [s2]. Both findings are more specific, and more interesting, than the anecdote they explain.

What the mouse study actually found

The Nature paper set out to test the long-repeated association between stress and greying, which the authors note had until then received little scientific validation [s1]. Working in mice, they found that acute stress depletes melanocyte stem cells — the follicle's reservoir of pigment-forming cells — and that this is what produces unpigmented hair [s1].

The value of the study is in how it ruled out the obvious explanations. Using a combination of adrenalectomy, denervation, chemogenetics, cell ablation and a knockout of the adrenergic receptor specifically in melanocyte stem cells, the team showed the stress-induced loss was independent of immune attack and independent of adrenal stress hormones [s1]. Instead, greying resulted from activation of the sympathetic nerves that innervate the melanocyte stem-cell niche [s1].

The sequence they describe is the memorable part. Under stress, those sympathetic nerves release a burst of the neurotransmitter noradrenaline, also known as norepinephrine [s1]. That burst causes the normally quiescent stem cells to proliferate rapidly, and is followed by their differentiation, migration and permanent depletion from the niche [s1]. In other words, stress does not bleach existing hair; it burns through the follicle's stem-cell supply, so that later hairs grow in without pigment.

Two caveats sit on this. It is a study in mice, and it concerns acute experimental stress, not the slow grey of ordinary ageing. And "permanent depletion" is the operative phrase — once those particular stem cells are gone from a follicle, that follicle's capacity to make pigment is gone with them [s1]. The authors also showed the reverse: transient suppression of the stem cells' proliferation prevented the stress-induced greying, which is what nails the mechanism down rather than leaving it a correlation [s1].

Why "greying is irreversible" turned out to be too strong

Greying is a hallmark of ageing that is generally believed to be irreversible, the eLife team notes at the outset — and they set out to test that belief directly in humans [s2]. Their method was to profile hair pigmentation patterns along individual hair shafts, in effect reading a single hair like a strip of film in which each segment records a slightly earlier point in time [s2].

Read that way, the hairs contradicted the assumption. The study documented white and grey hairs that naturally regain pigmentation, across sex, ethnicities, ages and body regions, which the authors say quantitatively defines the reversibility of greying in humans [s2]. Reversal, in other words, is not a freak event in one person; it showed up broadly enough to be a general property.

The molecular reading pointed the same way as the mouse work without simply restating it. Grey hairs, the study found, upregulate proteins related to energy metabolism, mitochondria and antioxidant defences [s2] — a signature consistent with greying being an active, metabolically costly state rather than a passive fading. And by combining the hair-mapping with proteomics on single hairs, the team reported greying and reversal that can occur in parallel with psychological stressors [s2]. To make sense of hairs that grey and then recover, they built a computational simulation, which suggests a threshold-based mechanism for the temporary reversibility of greying [s2].

A threshold is the useful idea to carry away. It implies a follicle can drift toward grey and, if it has not passed some point of no return, drift back — which is exactly the pattern of a hair that loses colour during a hard stretch and regains it afterwards.

The honest synthesis

Put together, the two studies support a more careful version of the cliché. Stress can drive greying through a defined nerve-and-neurotransmitter pathway, demonstrated in mice [s1]. In humans, greying is not uniformly permanent, and some grey hairs re-pigment, sometimes in step with changes in life stress [s2].

What neither study licenses is the strong claim that everyday stress is the main cause of the grey that arrives with age, or that managing stress will restore a full head of colour. The mouse mechanism is about acute stress and ends in permanent loss of specific stem cells [s1]; the human reversal study is observational and describes a phenomenon, not a treatment [s2]. There is no intervention here to buy. The finding that actually moves the field is subtler: pigment loss in a hair is a legible, sometimes reversible record of recent biology, which makes the hair itself a rare window onto how the body responds to stress over time [s2].

This article describes what the studies found and is not medical advice.

Sources

• Hyperactivation of sympathetic nerves drives depletion of melanocyte stem cells — Nature, 2020-01-22
• Quantitative mapping of human hair greying and reversal in relation to life stress — eLife, 2021-06-22