Correlates of cortisol in human hair: implications for epidemiologic studies on health effects of chronic stress.

“It is not stress that kills us; it is our reaction to it.” - Hans Selye (stress research pioneer) Despite its extensive study, psychosocial stress (hereafter called stress) remains challenging to describe. Investigators acknowledge stress as a process whereby a stimulus imposes demands or overwhelms an organism’s ability to cope, causing physiological and psychological responses[ 1 – 3 ]. The magnitude, duration and quality of these responses are contingent upon the individual´s interpretation of the stressor and how well the individual adapts or copes with the stressor[ 3 ]. Two neuroendocrine pathways are activated in stress response: one, the sympathetic-adrenal-medullary axis is activated rapidly–its activation has been described elsewhere[ 1 ]. The other, the hypothalamic-pituitary-adrenal (HPA) axis, is activated less swiftly: first, the hypothalamus discharges corticotrophin-releasing hormone, stimulating the dispensation of adrenocorticotropin-releasing hormone by the pituitary into systemic circulation, which then triggers the release of cortisol by the adrenal cortex[ 1 ]. Cortisol has emerged as an important objective biological measure of stress. Cortisol helps maintain homeostasis in the body by aiding metabolism and immune response[ 4 – 6 ]. The human body secretes approximately 10mg of cortisol daily; however, excess or insufficient amounts of cortisol are synthesized and released into systemic circulation under stressful conditions[ 1 , 7 – 9 ]. Although acute alterations in cortisol concentrations may not immediately cause disease, long-term alterations of the HPA axis are associated with adverse health outcomes. For example, high concentrations of cortisol have been associated with Cushing’s disease[ 10 ], diabetes complications[ 11 ], adverse perinatal outcomes[ 12 ], and myocardial infarction[ 13 ]. Low cortisol concentrations have been associated with Addison’s disease[ 14 ], endometriosis[ 15 ], chronic pelvic pain[ 15 , 16 ], and psychiatric disorders including post-traumatic stress disorder (PTSD)[ 17 ]. Saliva, blood, and urine, the predominant matrices from which cortisol is obtained, have been useful in establishing the diurnal profile of circulating cortisol and in elucidating the relationship between aberrations in diurnal cortisol patterns and various disorders. But these matrices have several limitations. First, they only inform about short-term cortisol concentrations, i.e. cortisol released within a few hours or one day[ 18 ]. Second, cortisol concentrations obtained using these specimens are easily influenced by a host of factors including study procedures[ 19 ], time of the day[ 20 ] and food consumption[ 21 ]. Third, the process of specimen collection can be invasive, particularly for blood, which in itself may induce stress and corresponding cortisol levels. Fourth, to deal with variability and the need to obtain approximate measurements of chronic cortisol concentrations, researchers need to take repeated measurements during 24 hours for several days[ 22 ]. This need for repeat sampling is expensive, burdensome for study participants and likely to increase incomplete sample collection and loss to follow-up. Given these limitations, investigators have searched for matrices that can ensure better, less invasive measurements of long-term cortisol concentrations. The most recent innovation is the isolation of cortisol from hair. To the best of our knowledge, Raul et al were the first to examine cortisol concentrations in human hair[ 23 ]. Previously, three investigative teams[ 24 – 26 ] provided evidence of hair as a matrix for various glucocorticoids—for example, hair was used to detect exposure to corticosteroids, amphetamines and anabolic steroids in athletes[ 26 ]. Hair allows for retrospective assessment of long-term cortisol concentrations because it grows over weeks, months and years (e.g., 18 month hair cortisol concentrations were assessed in [ 27 , 28 ]). Collecting hair is less invasive than obtaining blood and hair can be stored easily ( Table 1 ). The proposed mechanisms by which cortisol is incorporated into hair is beyond the scope of this review. However, we refer interested readers to a highly relevant review on this topic[ 29 ]. In efforts to assess the validity of hair cortisol as a biological marker of chronic stress, investigators have examined the correlations of hair cortisol concentrations (HCC) with cortisol concentrations from repeated samplings of other matrices. Van Holland et al found that HCC was moderately correlated with mean salivary cortisol concentrations taken on three days (r=0.41, p=0.03, samples were taken at six time points on each day)[ 30 ]. Similarly, Vanaelst et al found that HCC was significantly correlated with area under the curve for salivary cortisol collected over two consecutive days (samples were taken at four time points on each day)[ 31 ]. Statistically significant correlation between 24-hour urinary cortisol concentrations and HCC has been reported[ 32 ], though no statistically significant correlations were found between HCC and cortisol in one-time samples of morning blood serum[ 32 ] and blood serum collected after an overnight fast[ 31 ]. Additionally, investigators have examined correlations of HCC with scores on the perceived stress scale (PSS), a widely used self-reported measure of chronic stress over a 4-week period. Overall, results have been mixed with some investigators reporting correlation coefficients of <0.10 in studies of young adults[ 33 ] and in a racially diverse adult sample[ 34 ]; and correlations of 0.2 in adrenal insufficiency patients[ 35 ], 0.24 in chronic pain patients and controls[ 36 ] and 0.47 in pregnant women[ 37 ]. On balance, available evidence suggests positive associations of HCC with PSS scores and repeated measures of cortisol from other matrices. A previous review from 2012[ 29 ] found limited data on relevant correlates of HCC. Yet, as this is a rapidly developing field, we performed a systematic review of all relevant literature to shed light on correlates of HCC, broadly encompassing factors that may be important determinants, confounders, effect modifiers, interactions and mediators that could influence the relationship between HCC and covariates of interest in epidemiologic studies. Firstly, it is inevitably of principal interest to understand how hair-related factors (e.g., natural hair color, frequency of hair wash) might affect cortisol concentrations in hair. Second, it is important to examine whether hair cortisol concentrations reflect an individual´s subjective experience of chronic psychological stress and related morbidities or disorders. Lastly, the relationships between cortisol concentrations in hair and important socio-demographic and lifestyle factors should be assessed. Our systematic review summarizes evidence from existing literature on the correlates and determinants of HCC. The findings will be used to inform the design, analysis and interpretation of clinical and population-based studies that may use HCC as measures of chronic hypo or hyper-activation of the HPA axis.

Pertinent publications on hair cortisol, published before September 2013, were identified from PubMed, WorldCat, and Web of Science using keywords, “cortisol” “hair” “confounders” “chronic” “stress” “correlates” and “determinants.” We also perused references of identified publications to find other relevant papers. We included papers published in English and in peer-reviewed journals. Human studies were the primary consideration in this systematic review. For studies to be included, the investigators had to have specified the length and weight of hair analyzed, the cortisol extraction methods, participant recruitment procedures, and comprehensible quantitative figures on the primary relationships of interest. Lastly, hair must have been taken from the posterior vertex region. Hair from the posterior vertex region is the standard for hair cortisol analysis[ 38 ]. It has been shown to have higher cortisol content and lower inter-individual variability compared to hair sampled from other regions[ 39 ]. We grouped the findings according to the following themes: hair-specific characteristics, stress-related correlates, lifestyle and behavioral factors.

After elimination of duplicates, abstracts and papers that did not meet the inclusion criteria, 39 studies were chosen for inclusion in this review ( Figure 1 ). Studies were conducted in multiple countries: Brazil, Canada, China, France, Germany, Israel, The Netherlands, Switzerland, Uganda, and the United States ( Supplement Table ). Approximately 33% of the studies were from Germany, published by pioneers who integrated hair cortisol measurement in their clinical and population-based studies[ 40 – 52 ]. In the sections that follow, we provide summary results of studies that assessed HCC in relation to hair-specific characteristics, stress-related correlates, lifestyle and behavioral factors. Natural hair color appears not to be associated with HCC in humans. In their early study, Raul et al reported no association between natural hair color and HCC[ 23 ]. Other investigators have confirmed these findings[ 27 , 32 , 41 , 42 , 53 ]. On balance, it appears reasonable to conclude that natural hair color is not a correlate or determinant of HCC. Investigators have speculated that the frequency and temperature at which hair is washed may be inversely associated with HCC. However, most[ 27 , 43 , 46 , 51 , 53 ], though not all[ 39 ] studies on this topic indicated no association between HCC and hair washing. Li et al found that in a subsample of three participants, mean HCC decreased from 30.5±9.6 pg/mg to 6.5±5.9 pg/mg in hair immersed in shampoo solution for four hours[ 39 ]. Furthermore, in a subsample of eight participants, cortisol concentrations progressively declined after immersion of hair in hot water at 40°C, 65°C, and 80°C ( Table 2 )[ 39 ]. As with frequency of hair washing, findings on the relationship between hair treatment and HCC have been inconsistent. Some investigators[ 27 , 32 , 51 , 54 ] but not all[ 33 , 46 , 55 ] have documented substantial influences of hair dye and other hair treatments on HCC. Manenschijn et al found that treated hair had lower HCC compared with untreated hair while use of hair product (e.g., spray, mousse, gel and wax) on the day of hair sample collection was not significantly associated with mean HCC[ 27 ]. In aggregate, available data suggest modest influence of hair dyes and other treatments on HCC. Hence, investigators may consider collecting information on these behaviors in future large-scale epidemiologic studies that rely on HCC as a biomarker of chronic stress. HCC has been shown to decrease as one moves distally from the scalp[ 42 , 44 , 49 , 56 – 58 ]. This attenuation in cortisol concentration has been attributed to exposure to water, sunlight and other elements. Of note, Steudte et al reported that HCC decreased 17.4% from the proximal 3-cm segment to the second 3-cm segment, then decreased 18.3% from the second 3-cm segment to the third[ 49 ]. Manenschijn et al , however, found no difference in HCC among six consecutive 3-cm segments of hair from healthy women ( Table 2 )[ 27 ]. Other hair-related factors that have been studied include hair texture and ultraviolet (UV) irradiation. While HCC appears to not be influenced by curls, hair curvature, or waves[ 42 , 43 ], investigators have reported that 9-hour UV irradiation, compared with no exposure, is associated with statistically significant reductions in HCC ( Table 2 )[ 39 ]. More studies are needed to further examine the effect of hair texture and UV irradiation on HCC. Taken together, while the available data suggest that natural hair color and texture are not associated with HCC, data are still scarce or inconclusive on the relationship between the various hair treatments and HCC. Thus, investigators may consider collecting information on these behaviors along with precisely defining hair segment for collection and analysis in future large-scale epidemiologic studies that rely on HCC as a biomarker of chronic stress. Socio-economic factors such as income, educational level, occupation, and neighborhood characteristics are important determinants of health and are closely related with psychosocial stress[ 59 ]. Results of studies exploring the association between these factors and cortisol concentrations in saliva, urine and blood serum have been mixed[ 60 ]. To date, only a few studies have examined the relationship between HCC and socioeconomic factors. In a sample of 333 children from 23 neighborhoods in Vancouver, Canada, Vaghri et al observed that maternal and paternal education were both inversely correlated with HCC (r=−0.18, p=0.001). However, in a subsample of 275 preschoolers, annual family income was not significantly correlated with HCC (r=−0.07, p=0.235) ( Table 2 )[ 61 ]. Chen et al observed no relationship between HCC and educational level in a study of adults in Nanjing, China[ 62 ]. With regard to employment-related factors, shiftwork is of increasing importance in health studies, particularly because chronic shiftwork alters the circadian rhythm of cortisol production leading to HPA dysfunction[ 63 ]. In a small but important early study, investigators observed elevated mean HCC in participants who had a fast-forward rotating shift schedule compared with those who only worked during the day (47.32 vs. 29.72 pg/mg, p<0.001)[ 64 ]. However, there appeared to be an effect of age on the relationship between HCC and shiftwork ( Table 2 )[ 64 ]. Unemployment has also been examined in relation to HCC, however with mixed results. Dettenborn et al found higher HCC in unemployed individuals compared to those employed[ 41 ]. Results of studies on the relationship between age and cortisol in other matrices have been quite mixed with investigators reporting both positive[ 66 , 67 ] and negative associations[ 68 ]. With regard to the hair cortisol literature, some[ 23 , 27 , 34 , 43 , 46 , 53 , 54 , 61 , 62 , 65 , 69 , 70 ], but not all[ 42 , 51 ] investigators have reported no relationship between age and HCC. However, inferences from most studies documenting no associations are hindered by limited variability in the age of study participants. A study of 360 individuals aged 1 to 91 years suggests a complex non-linear relationship between age and HCC[ 42 ]. Across the age spectrum, the authors found that HCC were elevated in children <10 years old and in adults aged 50–91 years[ 42 ]. Furthermore, HCC was inversely related with age (in months) of children ≤5 years (r=−0.428, p=0.023). In a smaller study with a comparable age range (2–90 years old), Raul et al found no association of age with HCC[ 23 ]. Available data suggest a complex relationship of HCC with age. Studies of individuals across broader age spectrums are needed to more thoroughly explore the relationship between age and HCC. Findings on the relationship between sex and cortisol concentrations in other matrices have been inconsistent with some investigators finding higher concentrations in men[ 71 ] and others finding higher cortisol concentrations in women[ 66 ]. HCC studies on this topic have also been mixed with some investigators finding associations between HCC and sex[ 34 , 42 , 44 , 46 , 69 , 72 ] and others finding none[ 23 , 27 , 33 , 51 – 54 , 61 , 62 , 65 , 70 , 73 ]. Dettenborn et al found higher HCC among males than among females in two age groups: adults aged 18–49 years and children 49 years nor in individuals aged 10–17 years ( Table 2 )[ 42 ]. Race and ethnicity are important indicators of exposure to social stressors[ 74 ]. Equally vital, they are determinants of hair texture and hair growth rate[ 75 ]. Yet race and ethnicity have not been adequately studied in HCC studies. The average rate of hair growth used in most hair cortisol studies and other hair analysis is 1-cm/month, however, hair growth is known to vary between 0.7cm and 3.6cm per month[ 76 ]. Loussouarn et al observed lower hair density and hair growth rate in Africans compared with Caucasians[ 75 ]. These findings are consistent with racial differences in hair growth characteristics. In a study by O’Brien et al , participants were classified into two groups: minorities (African-Cuban, Afro-Cuban, Asian, Brazilian, Indian, Latino-Hispanic and Pacific Islander) and non-minorities (European or white American), and investigators observed no relationship between racial category and HCC[ 34 ]. However, the authors found a significant interaction of race and socio-economic status (SES) on HCC with minorities having elevated HCC at low and high SES[ 34 ]. Additional studies are needed to ascertain variations in hair growth rate among diverse populations. Considering inconsistencies in findings concerning HCC in relation to gender and given the importance of race and ethnicity as important indicators of social stressors, hair texture, and hair growth rate, investigators should account for sex, race and ethnicity in the design, analysis, and interpretation of hair cortisol studies. Despite some inconsistencies, findings from studies examining cortisol concentrations in saliva, blood, and urine generally point to a relationship between HPA dysregulation, psychiatric disorders, and aberrations in cortisol concentrations or diurnal cycles[ 17 , 56 , 77 , 78 ]. A meta-analysis of 47 studies that examined salivary, blood plasma/serum, and urinary cortisol showed that compared with no-trauma controls, PTSD patients had suppressed morning cortisol concentrations and afternoon/evening cortisol concentrations[ 17 ]. Studying the association of HCC with stressful events and psychological symptoms or disorders is particularly important for establishing the validity of hair cortisol as a biomarker of chronic stress. Using hair samples that corresponded to 1 to 2 months after the 2008 Wenchuan earthquake, Luo et al determined that HCC were elevated in PTSD and traumatized controls (TC) compared with non-traumatized controls (NTC) ( Table 2 ). Furthermore, for an additional 2 to 4 months after the earthquake, TC had the most elevated average HCC compared with the other two groups ( Table 2 )[ 56 ]. Steudte et al also observed higher HCC in individuals with PTSD compared to TC (60% of PTSD patients and 22.2% of TC experienced traumatic events in the past year)[ 48 ]. However, in a separate study, Steudte et al observed lower HCC in PTSD patients and TC compared to NTC (75% of participants had experienced their most traumatic event >5 years ago) and no difference in HCC of PTSD patients and TC[ 50 ]. Furthermore, they found no associations between salivary cortisol levels and traumatization or PTSD[ 50 ]. On balance, available data indicate that trauma exposure and PTSD are associated with aberrant HCC, and that while cortisol levels may rise shortly after trauma, they may decrease over time. Like trauma, serious adverse life events, frequently precursors to impaired mental health, have also been associated with HCC[ 33 , 79 ]. Depression has also been studied in relation to HCC, however the results have been mixed. One study found that depressed patients had elevated HCC compared with healthy controls[ 40 ]. In another study of cardiac rehabilitation patients, investigators observed no significant difference in HCC between depressed and nondepressed subjects[ 55 ]. The authors suggested that the absence of an association may be due to already elevated stress and cortisol levels in the cardiac rehabilitation population[ 55 ]. Investigations of the relationship between HCC and scores on the Beck Depression Index (BDI) have found both negative associations[ 80 ] and no associations[ 50 , 80 ]. Saleem et al found no difference in prevalence of depression between normal HCC and high HCC subjects [ 65 ]. Other psychiatric conditions that have been studied in hair cortisol research include bipolar disorder and generalized anxiety disorder (GAD). Manenschijn et al found no overall difference between mean HCC of bipolar disorder patients, most of whom were receiving pharmacological treatment, compared with healthy controls[ 73 ]. However in a subsample, it was observed that individuals with older age of onset (≥30 years) had higher HCC compared with those with early onset (<30 years old) ( Table 2 ). The authors suggested that stressful life events and HPA axis dysregulation may play a role in later development of bipolar disorder while early onset may be related to genetics or fluctuations in sex hormones[ 73 ]. Finally, Steudte et al examined cortisol concentrations for GAD patients and controls. While they found no difference in area under the diurnal curve for 24-hour salivary cortisol concentrations between the two groups, GAD patients had lower HCC compared with controls ( Table 2 )[ 49 ]. The investigators suggested that the experience of participating in a study might have contributed to acute fluctuations in salivary cortisol concentrations such that no differences in salivary cortisol concentrations were detectable. They also reasoned that HCC might be a more accurate measure of long-term HPA axis activity as it is not easily susceptible to acute influences[ 49 ]. Finally, in a sample of young school children, HCC significantly increased after school entry, but only in children who scored high on the fearfulness subscale of the Child Behavior Questionnaire[ 53 ]. Collectively, findings suggest that trauma exposure, psychiatric disorders, serious averse life events and symptoms of negative moods and emotions may be associated with HCC. More studies are needed to firmly establish HCC as a clear biomarker of chronic stress, and to elucidate the temporal relationship of HCC deviations with psychiatric disorders while jointly considering the mediating effects of stressors and stress response. Cushing’s syndrome, Addison’s disease and other conditions affecting the hypothalamus, pituitary or adrenal cortex are associated with aberrations in cortisol concentrations or diurnal patterns in saliva, blood serum/plasma and urine[ 81 ]. Studies examining HCC and adrenocorticoidal conditions have been consistent with these findings. In a study of adrenal insufficiency (AI) patients receiving hydrocortisone replacement therapy and household partner controls, AI patients had higher HCC than controls. HCC was also positively correlated with glucocorticoid dose[ 35 ]. The authors speculated that patients with adrenal insufficiency may be over-treated and could be at risk for adverse effects of elevated cortisol concentrations. Investigators examining the relationship between Cushing’s syndrome and HCC have found significantly higher HCC in Cushing’s patients compared with healthy controls ( Table 2 )[ 27 , 28 , 54 ]. In their study of non-obese healthy controls and patients with cyclic Cushing’s syndrome, Manenschijn et al observed 86% sensitivity and 98% specificity for Cushing’s syndrome, using the upper-limit reference range for non-overweight healthy controls (75.9 pg/mg hair)[ 54 ]. These findings on Cushing’s syndrome lend support to the capability of hair as a valid matrix of chronic cortisol exposure. Results support an association between aberrations in cortisol concentrations from other matrices and cardiovascular disease[ 82 – 84 ], cardio-metabolic syndromes[ 85 ] and chronic pelvic pain[ 15 , 16 ]. Positive significant associations have been observed of high HCC with severe chronic pain[ 36 ], one year verbal memory and history of coronary artery bypass graft surgery in cardiac rehabilitation patients[ 65 ], cardiovascular disease and events[ 13 , 69 ]. The results on cardio-metabolic parameters have been mixed. HCC has not been associated with diastolic blood pressure (BP)[ 27 , 34 , 65 ], and while O’Brien et al found a significant positive correlation between HCC and systolic BP (r=0.25, p<0.01)[ 34 ], two other studies observed no significant association[ 27 , 65 ]. Stalder et al observed a positive correlation in unadjusted analysis of HCC and mean arterial pressure, but after adjustment, the correlation did not exist (r=0.047, p>0.05). They also observed mixed results between HCC and cardio-metabolic parameters ( Table 2 )[ 51 ]. In one study, mean values of various cardio-metabolic fitness parameters did not differ between normal and high HCC groups [ 65 ]. To date, HCC has not been associated with cancer, osteoporosis and chronic nonspecific lung diseases[ 69 ]. On balance, HCC appears to be associated with chronic disorders, particularly disorders that are strongly associated with stress (e.g., chronic pain and cardiovascular disease). There is a need for prospective studies to further elucidate the relationship of HCC with incident disease and disease progression. For example, epidemiologic studies that investigate the relationship between HCC and adrenocorticoidal conditions should take into account the duration for which individuals have had the condition, the duration of treatment, and the type of treatment that individuals receive as all these factors may influence the relationship between HCC and adrenocorticoidal conditions. Evidence of increased cortisol production during pregnancy is well-established[ 43 , 86 ]. Findings on the relationship between HCC and pregnancy have been consistent with a positive association. D’Anna-Hernandez et al studied HCC among pregnant women and found that HCC was significantly higher in the third trimester compared with the first, and then decreased during the first 2 to 3 months postpartum ( Table 2 )[ 87 ]. In their study of mothers of newborns 2–4 days of age, mothers of toddlers aged 3–9 months, and control women who were nulliparous and non-pregnant, Kirschbaum et al found that mean HCC among mothers of newborns 2–4 days of age (their hair samples corresponded with the third trimester) was two-fold higher than that of non-pregnant nulliparous women ( Table 2 )[ 43 ]. Krumbholz et al also found significantly elevated HCC in hair segment reflecting the last month of pregnancy and first month of delivery[ 58 ]. Given well-documented evidence of a relationship between pregnancy and cortisol increase, investigators should exclude pregnant women from hair cortisol studies unless of course when pregnancy is the focus of the research. In their study of term infants and preterm infants who were >25 weeks gestational age at birth, Yamada et al found that neonatal intensive care unit (NICU) infants had significantly higher HCC compared with healthy term (≥37 weeks) infants (mean±SD: 2.06±2.05 vs. 0.11±0.42 nmol/g, p=0.004). Total number of days on the ventilator was associated with HCC and there was no significant difference in HCC for NICU term infants and NICU preterm infants[ 88 ]. In a study of depressed patients and healthy subjects, Hinkelmann et al observed lower mean HCC in subjects who experienced childhood maltreatment[ 52 ]. However, Grassi-Oliveira et al did not observe a correlation between severity of early life stress and HCC in a sample of women seeking treatment for substance abuse[ 79 ]. Although studies on early life adversity and HCC in humans have been few, compelling results have been observed in studies of non-human primates. Two studies that examined monkeys showed that those separated from their mothers in early life had aberrant HCC when placed in new social environments compared with those that were reared by their mothers[ 89 , 90 ]. Additionally, Laudenslager et al observed that monkeys whose social groups were relocated during the perinatal period had elevated HCC compared with monkeys raised in a constant environment[ 91 ]. On balance, results from studies of humans and non-human primates suggest that early life adversity may influence cortisol concentrations, and have long-term negative effects on development. Thus, investigators should consider early life adversity in studies of HCC where necessary. There is a need for more studies to elucidate the relationship between HCC and perinatal and other early life experiences. Results of studies examining relationships between adiposity and cortisol in blood, urine, and saliva have been varied. Investigators have reported no association[ 92 ], inverse[ 93 – 95 ] and positive associations[ 96 , 97 ] of cortisol concentrations with a full range of measures meant to reflect body fatness or adiposity. In Table 2 , we provide a summary of the relatively sparse literature devoted to assessing HCC in relation to different measures of adiposity. Of note, some[ 27 , 46 , 51 , 64 ], but not all[ 34 , 73 ] available studies have found positive relationships between HCC and at least one adiposity measure including body mass index (BMI), waist circumference (WC) and waist-to-hip-ratio (WHR). Positive associations between HCC and BMI have been reported in various populations including fast-forward rotating shift workers[ 64 ], university students and adults[ 51 ]. However, other studies have found no relationship between HCC and BMI[ 27 , 65 , 69 , 73 ] or WC[ 65 ]. Still, others have shown HCC to be positively correlated with WC[ 27 , 51 , 69 ] and WHR[ 27 , 51 ] but not with hip circumference[ 27 ]. Despite some inconsistencies, available data suggest that HCC may be associated with adiposity in some, but not all populations studied to date. Studies with objective measures of adiposity and central obesity are needed to further clarify suggested associations. Studies of other matrices suggest that aberrations in cortisol concentrations are associated with alcoholism[ 98 ] and alcohol relapse[ 99 ]. Two studies have found positive associations between HCC and high alcohol intake[ 45 , 69 ]. A third study reported no associated between HCC and regular alcohol consumption (≥3 days/week), but observed positive associations between HCC and serum γ-glutamyltransferase (γGT)[ 51 ]. Overall, these findings suggest a positive relationship between high alcohol intake and HCC. More studies are needed to explore the relationship between HCC and alcohol use or abuse. Investigators should ascertain alcohol consumption among study participants and examine alcohol’s effect on HCC where necessary. Evidence suggests that nicotine alters the HPA axis and acutely increases cortisol concentrations in saliva, urine and blood plasma[ 100 – 104 ]. However, the association may only be acute[ 101 ]. Thus far, HCC has not been associated with cigarette smoking[ 42 , 44 , 46 , 51 , 69 ]. More studies are needed to provide further evidence about the relationship between cigarette-smoking status and HCC, especially since smokers report higher levels of stress than non-smokers[ 105 ]. The results of studies examining the effect of oral contraceptive use on cortisol concentrations in other matrices have been inconsistent. A summary of the relationship between oral contraceptive use and cortisol concentrations in blood plasma and saliva is given by Dettenborn et al [ 42 ]. So far, investigators have reported no significant positive or negative relationships between HCC and oral contraceptive use[ 42 , 46 , 50 ]. See Table 2 for report of findings on the association between HCC and other medications. Investigators have previously reported on the influence of moderate to high intensity physical activity to acutely raise concentrations of circulating cortisol in blood[ 106 ]. Positive associations have been observed for HCC and participation in endurance sports[ 44 ] and vigorous physical activity. However, no associations have been observed for moderate physical activity[ 70 ]. Further, Stalder et al observed no association between HCC and physical activity level as assessed by summing up 5-point frequency ratings in the areas of mild, moderate and vigorous physical activity over the preceding 6 months[ 51 ]. On balance, evidence suggests an association between HCC and vigorous physical activity.

The study of correlates of hair cortisol concentrations is still at an early stage. Yet, available data obtained through this systematic review are suggestive of the validity of this biomarker. Importantly, our findings suggest that hair cortisol may be associated with early life adversity, stress-related psychiatric conditions, and medical conditions indicating chronic activation of the HPA axis or high stress levels. Further research is needed to underpin the validity of the biomarker as well as to obtain deeper understanding of its potentially different associations to reactive psychiatric conditions versus psychiatric disorders with stronger hereditary etiology. The literature suggests that some factors (e.g., natural hair color, cigarette smoking, oral contraceptive use, and medication use) may not have important influences on HCC. Other factors, such as sex, adiposity, hair treatments, and substance abuse, may be potential determinants of HCC and thus investigators may want to include these measures in future studies. For a number of other factors (e.g., age, physical activity and hair texture) available evidence are insufficient, particularly since most existing hair cortisol studies assess the effect of socio-demographic and hair-specific behaviors secondary to some other primary outcomes. As such, studies were not usually conducted in ways that maximized opportunities for comprehensive assessments of these secondary covariates[ 42 ]. Over the last decade, hair as a measure of cortisol has emerged as a promising biomarker of chronic stress and alterations of the HPA axis. Hair cortisol concentrations have shown favorable intra-individual variability in some studies. In one study, Stalder et al observed a relatively strong correlation between HCC after an average 375.3 day interval (r=0.78, p<0.0001, adjusted for perceived stress scores and number of competitions per year)[ 47 ]. Three repeated measurements at two month intervals showed correlation coefficients ranging from 0.53–0.79 (p<0.001)[ 47 ]. These findings support the reliable intra-individual stability of HCC. Many gaps in understanding the fidelity and determinants of hair cortisol remain. One major limitation of current hair cortisol research is that most studies have been cross-sectional. Little is known about how HCC change across the human life-course; about the degree with which HCC changes across preclinical and clinical manifestations of disease; how HCC change with pharmacological and non-pharmacological treatments of conditions including mood and anxiety disorders; and how time since last episode and severity of illness influence HCC. Other under-studied areas include the influences of diet, hair loss, and seasonal variations on HCC and the extent to which different methods of laboratory analysis affect HCC observed and reported in studies[ 75 , 76 , 107 ]. To date, there have been few studies of large representative populations and few studies have included ethnically, racially and economically diverse populations. Studies conducted in increasingly diverse populations will enrich the literature and provide investigators with much needed information that can be used to design, analyze and interpret HCC in clinical and population based studies. Our assessment of correlates of HCC suggests that hair cortisol may indeed constitute an important biomarker of HPA-axis dysregulation and chronic psychosocial stress. Investigators who elect to integrate this emerging biological marker into their studies should be strategic about accounting for covariates that may confound or modify associations of primary interests. Results of hair cortisol studies should be interpreted with consideration of appropriate factors including study size and segments of hair studied. The assessment of cortisol in hair presents unique strengths—and challenges that if overcome may revolutionize the study of the potential health effects of chronic stress in large-scale epidemiologic studies.