Hair texture, curl pattern, diameter, and density define an individual’s hair type. Although many people assume their hair type is fixed from birth, it can change dramatically throughout a person’s life. These shifts are tangible alterations in the biological mechanisms responsible for hair production. Understanding the science reveals why straight hair may become wavy in adulthood or why a lifetime of curls may suddenly loosen.
The Biology That Determines Hair Type
The shape of the hair follicle, a tiny organ in the skin, is the primary determinant of hair texture. A perfectly round follicle produces straight hair because the keratin protein is extruded symmetrically. Conversely, an oval or flattened follicle causes the hair shaft to bend as it grows, resulting in wavy or curly hair. The flatter the follicle’s cross-section, the tighter the resulting curl will be.
The follicle shape is initially programmed by genetics. However, the hair follicle is not a static structure; it is a dynamic organ that undergoes a growth cycle lasting several years. Since the hair shaft is a non-living protein fiber, any change in texture or pattern must originate from an alteration in the follicle itself.
Internal Physiological Causes of Structural Change
Major life events that cause systemic shifts in the body’s chemistry are the most common drivers of long-term hair type alteration. These internal processes alter the shape or function of the hair follicle. The most significant changes are driven by hormonal fluctuations, which affect the expression of genes that regulate follicle shape.
Hormonal shifts occurring during puberty, pregnancy, or menopause are frequently associated with a noticeable change in texture. For example, the surge in estrogen and progesterone during pregnancy can prolong the hair’s growth phase and sometimes alter the follicle’s angle, causing straight hair to develop a wave or curl. Conversely, the decline in estrogen during menopause can reduce natural oil production, making hair feel drier, coarser, and sometimes causing curls to loosen.
Aging also contributes to structural changes in hair independently of major hormonal events. As the body ages, the diameter of the hair shaft often becomes smaller, leading to thinner and finer individual strands, a process called miniaturization. The follicle may also become less symmetrical or its position relative to the scalp may shift, causing hair that was once consistently straight or wavy to become frizzier or exhibit an irregular texture. Additionally, the lack of melanin in gray hair contributes to a coarser, more wiry texture, partially due to reduced natural oils and altered structural protein distribution.
Certain medications and medical conditions can interfere with the normal hair growth cycle and follicle function. Thyroid disorders, which affect keratin production, can make hair dry and brittle. Medications that target rapidly dividing cells, such as chemotherapy, can cause temporary but drastic changes in texture, often resulting in curlier hair regrowth once treatment concludes.
Environmental Factors and Temporary Shifts
While major physiological changes alter the follicle itself, many perceived hair type changes are temporary or related to external factors affecting the hair shaft’s condition. Chemical treatments like perms, relaxers, and color processing dramatically change the hair’s appearance by permanently altering the keratin structure. These processes work by breaking and reforming the disulfide bonds within the hair shaft, which determine the hair’s natural curl and strength.
This type of change is considered damage rather than a natural shift in hair type because the follicle continues to produce hair with its original genetic pattern. Chronic stress and severe nutritional deficiencies can also impact the quality of the hair shaft being produced. High levels of the stress hormone cortisol can push hair follicles prematurely into the resting phase, leading to excessive shedding (telogen effluvium) and hair that appears thinner.
Inadequate intake of nutrients like iron, zinc, and biotin, which are necessary for keratin production, can result in hair that is brittle, dry, and prone to breakage. These issues mimic a change in texture due to poor quality, but they do not alter the follicle’s shape and are often reversible once the underlying deficiency or stressor is managed. Environmental moisture, such as high humidity, causes temporary changes by allowing water molecules to interact with the hair’s hydrogen bonds, leading to frizz and increased curl definition that reverts once the climate changes.