This study suggests that measurement of cumulated cortisol in hair can be used as a biological marker of exposure to major life stressors in a retrospective fashion, as hair grows approximately 1 centimetre per month [38
]. Experience of serious life events seem to be more important in raising cortisol levels in hair compared to perceived stress as measured with PSS, which was weakly negatively correlated (r = -0.061). Two out of four outliers with extremely high cortisol levels reported severe psychological problems during the last three months, which raises the question whether this new biomarker also may reflect severe psychological illness, although resent studies has shown decreased hair cortisol concentrations in patients diagnosed with generalized anxiety disorder [39
There are some reports during recent years where this novel method to measure cortisol in hair has been applied [29
]. Although our laboratory methodology is in concordance with these, we have in this report managed to reduce the amount of human hair necessarily for reliable analysis to 5 mg instead of previously reported 25 to 45 mg [33
]. This is of great value as we plan further studies in very young children.
The participants in this study were all young adults and university students, and could be considered as a relatively healthy cohort. Nevertheless, the measurement of cortisol concentrations in hair extracts retrospectively detected stress exposure i.e. experiences of serious life events. Cortisol levels thus seems to be related to serious life events even among young healthy adults, and not only in groups of ill adults who have some sort of known stressor compared to a control group. In one way this approach is a harder task than to be able to spot a difference between an exposed experimental group and a control group. The variable Serious Life Event is something that has happened or not, and it is usual to simply ask for such an event with a Yes/No question. However, one should be aware that the way in which a serious life event is perceived is a rather subjective factor. The influence of serious life events on physical and mental health has a long research tradition for over forty years from the early reports by Holmes and Rahe [40
]. Their initial publication 1967 that tried to rate life events was criticized on methodological grounds concerning the usefulness of generalized magnitude ratings of life events. Later this quite simple measurement that is used in this report has been widely used in numerous publications [41
]. When stressors like negative life events, and also chronic strains and traumas are measured comprehensively, their damaging impacts on health are substantial [42
]. In a recent study, hair samples of PTSD patients had higher cortisol levels than those of traumatized controls [43
], which support our findings that serious (traumatic) life events raises hair cortisol levels although traumatic load was assessed with a checklist of traumatic event types, not coded as yes or no. In this study there was also a positive correlation between number of different traumatic event types and hair cortisol levels.
We note that a surprisingly high number of students reported serious life events, 20 of 95 subjects. In this age group, possible explanation could be the illness/death of grandparents but also the divorcing parents.
In our report, the perceived stress measurement PSS [36
] was weakly negatively correlated to increased cortisol levels but in the multivariate analyses the indicator of serious life events outperformed this indicator. Also, the PSS scale has been validated for a recall period of 4 weeks, but was used to assess 3 months of stress in the current study, which is a limitation. According to our results, the experience of serious life events seems to be the key factor and more important in raising cortisol levels in hair than perceived stress. There is always a risk for recall and reporting bias for retrospective self-reports [44
]. Possibly, this is more conceivable for retrospective self-reports of perceived stress than for retrospective reports of actual life events. Nevertheless, a key point is that we do not really know the concordance between retrospective measured subjective and biological stress reactions.
A possible limitation in this study is that there is a risk of confounders like adrenal diseases not known to the participants, even though this is rather unlikely considering the rarity of the diseases. Almost one third of the participants (29%) reported some kind of longstanding medication but the use of pharmaceuticals were not correlated to levels of cortisol in hair in the present study. However, external contamination of the hair through e.g. use of cortisone-containing medications is unlikely but cannot fully be excluded. Coloured or bleached hair did not affect the concentration of cortisol in our analysis, although possible removal of some cortisol by repeated washing should also be considered, shown by Sauvé et al 2007 [29
]. One could argue that the likelihood of removal of some cortisol from hair by repeated hair shampooing/washing would be low considering that studies have shown that washing with a solvent in the assay process seems to be an unnecessary process since it removes only small parts of the of the total hormone extracted [23
]. But there are also studies showing that there is a "wash out" effect [31
There are a hypothesis that stress- induced cortisol secretion may contribute to central fat and there by be a link between stress and risk for disease [45
]. BMI was not asked for in this study, although it was not the aim of this study we can comment that the subjects were healthy young adults studying Health Sciences and no one (judged by eye sight) had class 1 obesity (BMI >30) or above. One student was very underweight; this was also the student who we later found out to have anorexia nervosa. This case also had extremely high levels of cortisol in hair, and was omitted from the analysis, as stated earlier.
Present biological cortisol markers like saliva, urine and blood suffer from the drawback of only covering spot time periods of up to 24 hours. Therefore, there is a need in stress research for biological markers reflecting long term and extended exposure to stress. Cortisol in hair has the potential of becoming a new indicator for stress exposure of major life stressors over periods of months and could even be used in clinical settings. An obvious advantage of analysing cortisol in human hair is the ease of sample collection and transportation as well as being non-invasive. It is also easy to store samples since hair does not decompose like other body fluids or tissues.
In public health research, a reliable and compelling marker of stress that is easily managed, even in larger population studies, would be an incredible asset in addition to psychological and psychometric tests to measure major stressors.