It is a common observation that hair that was once a deep brown or even black begins to develop warm, reddish, or auburn undertones as the years pass. This shift in color often occurs long before the hair shows significant streaks of gray or white. The change is a direct result of changing biological processes within the hair follicle itself. Understanding this phenomenon requires examining the specific pigments that give hair its color.
How Hair Gets Its Color
The natural color of human hair is determined by the specific type and amount of pigment, known as melanin, produced by specialized cells called melanocytes located within the hair follicle. There are two primary forms of melanin that combine to create the entire spectrum of human hair colors. Eumelanin is responsible for the darker shades, specifically brown and black tones.
Pheomelanin provides the lighter, warmer shades, including yellow and red. A person’s natural color is dictated by the precise ratio of these two pigments. Hair that appears brown or black contains a large amount of eumelanin, which effectively masks the underlying presence of pheomelanin.
The Age-Related Shift in Melanin Production
The reason older hair often takes on a reddish hue lies in the differential rate at which the production of these two pigments declines with age. As the body ages, the pigment-producing melanocytes become less efficient in their function. The production of the darker pigment, eumelanin, tends to decrease faster and more dramatically than the production of pheomelanin.
This creates a slow shift in the pigment ratio within the hair shaft. As the dominant, darker eumelanin production wanes, the underlying red and yellow tones from the more stable pheomelanin are no longer masked. The hair begins to appear auburn or reddish-brown as the relative concentration of pheomelanin becomes more visually prominent. This process is distinct from complete graying, which happens when the melanocytes stop producing any pigment entirely.
External and Internal Factors Affecting Pigment Stability
Beyond the natural decline in pigment production, external and internal elements can accelerate or influence how visibly red the hair becomes.
UV Exposure
One major external factor is chronic exposure to ultraviolet (UV) radiation from the sun. Eumelanin, the dark pigment, is designed to absorb and scatter UV light, but this protective action causes the pigment to degrade over time. The structure of eumelanin is more susceptible to photodegradation than pheomelanin under UV exposure. This preferential breakdown of the darker pigment leaves the red-toned pheomelanin remaining in the hair shaft, effectively bleaching the hair to a warmer color. This accelerated color loss is particularly noticeable in the older, longer portions of the hair that have endured more years of sun exposure.
Hormonal and Stress Factors
Internal changes, particularly hormonal shifts associated with perimenopause and menopause, can also modulate hair follicle health and pigment delivery. While not directly causing the differential pigment decline, these systemic changes can influence the overall hair growth cycle and the environment in which melanocytes operate. High levels of stress hormones, such as cortisol, may also contribute to the damage of pigment-producing cells, potentially accelerating the visible shift to warmer tones.
When to Consult a Professional
While the gradual shift to red undertones is a normal sign of biological aging, any sudden or dramatic change in hair color warrants attention. Abrupt color loss or significant graying, especially when accompanied by excessive hair loss or texture changes, may suggest an underlying health issue. Nutritional deficiencies, such as low levels of iron or specific B vitamins, can sometimes interfere with the pigmentation process. Thyroid hormone imbalances are also known to affect hair follicle function. If the color change is rapid, patchy, or concerning, consulting a dermatologist or a general practitioner is prudent to rule out treatable conditions like vitamin deficiencies or thyroid dysfunction.