Hair, Cortisol & Mummies: How your brain’s stress response can leave marks on your body that last a millennium

Cortisol ‘tagging’ hair strands. Image by Sam Mestern, Composite illustration. Map released by British Library under public domain.

Around 1,000 – 2,000 years ago, an individual in the Nazca region of Peru succumbed to an illness that had been afflicting them for months. Due to poor record-keeping, we are not sure exactly when this individual - identified as CAH493 - lived, nor do we know exactly where he was found. But what researchers can reconstruct may surprise you. We know, for example, in the months preceding his death, the individual experienced mounting physiological stress – likely the rumblings of the illness that would take his life. Two months before his death, stress levels were peaking, they were ill. Very ill. One month prior, things took a turn for the worse - CAH493 experienced a sudden plummet in stress levels. Researchers theorize that this may be the result of failing organs – the disease had progressed too far, and the individual could no longer mount a proper stress response. But, with no written record of this individual, how do we know so much about him more than 1,000 years following his death?

As it turns out, researchers at Western University can utilize hormones found in human hair to reconstruct a ‘timeline’ of experienced stress in the months leading up to death. The above example comes from the study “Integrating cortisol and isotopic analyses of archeological hair: Reconstructing individual experiences of health and stress”, the result of a collaboration between the Longstaffe and Van Uum labs at Western University. In this study, researchers took samples from mummified remains in the Nazca region of Peru. In addition to assessing stress hormones found in hair, the researchers measured levels of carbon and nitrogen in the hair, which differ in response to a changing diet. Combining dietary and stress measures from hair composition allowed researchers to correlate dietary changes with stress levels, which may inform them as to whether the stress is resulting from famine or otherwise. In total, hair samples were gathered from 14 individuals (ranging from 1,000 to 2,000 years old), and researchers then took on the task of measuring stress in all of these samples. Surprisingly, this record of stress can be maintained in hair many centuries after death.

This timeline reconstruction was accomplished by measuring levels of the stress hormone ‘cortisol’ incorporated into human hair. Cortisol represents the body’s hormonal response to stress. In the event of a stressor. like a final exam or a tight project deadline at work, cortisol blood levels will rise. As hair grows, its soaks up whatever bodily levels of cortisol are present. Moreover, cortisol does not spread within the hair strand2. This means that a hair strand can act as metaphorical ‘tree-rings’ for stress levels. As levels peak – say as a result of illness – hair grown at that time will have a strong presence of cortisol compared to further down the hair strand.

Cortisol release is the endpoint of the brain’s stress signaling cascade, known as the hypothalamic-pituitary-adrenal axis (HPA). A small region located at the base of the brain, called the hypothalamus, serves as the central integrator of the brain stress response. Signals from across the brain inform the hypothalamus about changes to the body’s physiological (illness, violence, food availability), or psychological balance. In response to a stressor, the hypothalamus initiates the hormonal stress response, eventually resulting in the release of cortisol. Cortisol wears many hats in order to prepare the body for challenges; it regulates immune function, blood pressure, and metabolism. Importantly, because excessive stress is harmful, cortisol also provides negative feedback on the HPA axis. In essence, cortisol keeps itself (and in-turn the stress response) in check, and it will turn off ‘the tap’ (the HPA axis) once levels of cortisol have risen high enough. Loss of this crucial feedback may result in a system that runs haywire, cascading into un-remitting un-controlled stress response. Loss of negative feedback has been implicated in illnesses like depression. All-in-all, body levels of cortisol may be correlated with physiological and/or psychological stress.

In this way, hair cortisol represents a potential biological marker of the brain's stress response — a semi-permanent chronicle of the nervous system state.

With this in mind, primary author Emily Webb and colleagues segmented the hair into 1cm chunks. Each segment represents approximately one month of growth, and therefore a snapshot of cortisol over that same period. The researchers also measured levels of carbon and nitrogen in the hair to characterize the individual’s shifting diet and nutrition. In some individuals, the researchers found differences in cortisol associated with changes in diet. The researchers proposed that these individuals’ shifting diet & cortisol were the result of uprooting and moving elsewhere. In CAH493’s case, the shift in nutrition was not consistent with starvation or diet overhaul, but with an immune response, which is typical when the body attempts to fight things like the flu. Cortisol is also a potent suppressor of the immune system, and CAH493’s mounting cortisol response was likely a result of heightened immune signals in the hypothalamus. In other words, the brain was releasing cortisol as an attempt to slow down the immune response. Other individuals experienced an elevation in cortisol levels at specific points, independent of changes in diet. Webb et al. propose this is likely the result of violence or another psychological stressor present in this person’s life shortly before death.

Recently, the Van Uum lab collaborated on a hair-cortisol study regarding current-day youths exposed to high levels of trauma. In their study “Hair cortisol concentrations in war-affected adolescents: A prospective intervention trial” Rana Dajani and colleagues sampled hair cortisol from Jordanian and Syrian youths. They found that individuals that presented high levels of insecurity expressed high levels of cortisol. Those exposed to multiple traumas displayed abnormal cortisol levels, indicative of a breakdown of proper stress responses7. Importantly, following an intervention, the researchers found a normalization of cortisol levels for all participants. These findings underline the importance of mental health interventions in erasing or reducing the potential harm that long-term high cortisol levels may leave on the body.

Overall, the discussed study highlights a novel method of reconstructing a timeline of stress. Since this study was published, other researchers have investigated timeline reconstruction. For example, a 2019 study replicated the Peruvian technique in Egyptian mummies dated to 50-450CE. Moreover, a 2017 study reconstructed stress timelines in whales by analyzing their baleen, which is a part of their filter-feeding system that is similar in composition to human hair. These studies give us insight into how stress and diet are reflected throughout the body and give us a fascinating look into how science can reconstruct portraits of people’s lives from microscopic remnants of the past.