The analysis of cortisol in human scalp hair: examination of fundamental methodological questions concerning incorporation and elimination pathways

J. Grass

Abstract

The analy­sis of cor­ti­sol in human scalp hair is in­creas­ingly rec­og­nized as a promis­ing new en­docrine mea­sure of long-term sys­temic cor­ti­sol se­cre­tion in the con­text of chronic stress re­search. Hair cor­ti­sol con­cen­tra­tions (hairF) are as­sumed to re­sult from con­tin­u­ous in­cor­po­ra­tion of cor­ti­sol into slowly grow­ing hair and to thus re­flect in­te­grated cor­ti­sol lev­els over the time pe­riod of hair growth. Due to their ret­ro­spec­tive and long-term na­ture, hairF should be intra-in­di­vid­u­ally sta­ble and un­af­fected by sit­u­a­tional vari­abil­ity. Over the past years, cu­mu­la­tive ev­i­dence has sup­ported sev­eral of the gen­eral as­sump­tions un­der­ly­ing hair cor­ti­sol analy­sis, such as the over­all va­lid­ity, high test-retest re­li­a­bil­ity of hairF as well as ro­bust­ness to sev­eral po­ten­tial so­ciode­mo­graphic or hair-re­lated co­vari­ates. How­ever, more re­search on fun­da­men­tal method­olog­i­cal ques­tions of this rel­a­tively new method is war­ranted. Im­por­tantly, the exact mech­a­nisms of steroid hor­mone in­cor­po­ra­tion into hair are still not sat­is­fac­to­rily un­der­stood. Ex­ter­nal in­cor­po­ra­tion of cor­ti­sol, orig­i­nat­ing from sweat, is con­tin­u­ally dis­cussed as a po­ten­tially rel­e­vant source of hairF. Fur­ther, there are still con­cerns about whether hairF are in­deed not al­tered by acute sit­u­a­tional in­flu­ences. To ad­dress these is­sues, one focus of the pre­sent re­search pro­gram was to ex­plore the pos­si­bil­ity that cor­ti­sol from sweat con­tributes to hairF and that hairF may be sub­ject to acute sweat-in­duc­ing in­ter­ven­tions. For this, the first part of this the­sis in­cludes the val­i­da­tion of an an­a­lyt­i­cal pro­ce­dure to mea­sure cor­ti­sol con­cen­tra­tions in sweat (sweatF) and de­ter­mi­na­tion of sweatF in nat­ural sweat sam­ples (study I) as well as ex­am­i­na­tions on ef­fects of in­cu­bat­ing human scalp hair in a hy­dro­cor­ti­sone-con­tain­ing ar­ti­fi­cial sweat so­lu­tion on hairF (stud­ies II & III). Re­sults show that cor­ti­sol is pre­sent in human sweat in con­cen­tra­tions that are strongly re­lated to those in saliva. Fur­ther, the find­ings con­sis­tently re­veal higher hairF after ex­pos­ing hair to ar­ti­fi­cial sweat under in vitro and in vivo con­di­tions, re­spec­tively. Com­bined, these find­ings show ini­tial ev­i­dence for the as­sump­tion that cor­ti­sol in sweat may be a rel­e­vant source of hairF. In the sec­ond part of this re­search pro­gram, two in­de­pen­dent ex­per­i­men­tal in vivo stud­ies were con­ducted that ex­plored whether hairF are sub­ject to acute vari­a­tion, po­ten­tially re­lated to cor­ti­sol in­cor­po­ra­tion from sweat. For this, healthy in­di­vid­u­als ei­ther un­der­went a tread­mill run in which sweat­ing to­gether with a sys­temic cor­ti­sol re­ac­tion was pro­voked (study IV) or took part in a sauna bathing chal­lenge in which sweat pro­duc­tion was in­duced with­out sys­temic cor­ti­sol re­ac­tiv­ity (study V). Find­ings show that hairF as well as hair cor­ti­sone con­cen­tra­tions (hairE) are not al­tered by sin­gle sweat-in­duc­ing in­ter­ven­tions and un­re­lated to acute sali­vary cor­ti­sol re­ac­tiv­ity. In ad­di­tion, find­ings in­di­cate that hairF and hairE were not sub­ject to di­ur­nal vari­a­tion. Thus, the out­come of this work pro­vides sup­port for the ‘clas­si­cal’ model of hair cor­ti­sol, which holds that hairF rep­re­sent a sta­ble ret­ro­spec­tive marker of sys­temic cor­ti­sol pro­duc­tion. Fur­ther, the find­ings sub­stan­ti­ate the po­ten­tial value of hairE for ob­tain­ing in­for­ma­tion on long-term sys­temic cor­ti­sol lev­els. Be­sides in­cor­po­ra­tion path­ways, ex­am­i­na­tions on cor­ti­sol elim­i­na­tion mech­a­nisms from hair are es­sen­tial to un­der­stand the com­plex na­ture of hairF. Here, ul­tra­vi­o­let/sun­light ra­di­a­tion con­sti­tutes a po­ten­tially im­por­tant fac­tor that has re­ceived lit­tle at­ten­tion in hair cor­ti­sol analy­sis so far. The third part of the pre­sent doc­toral the­sis thus con­ducted a first com­pre­hen­sive in­ves­ti­ga­tion into this topic. Three stud­ies ex­am­ined the sta­bil­ity of cor­ti­sol mol­e­cules to ra­di­ant en­ergy (study VI) and ef­fects of a sin­gle short-term ar­ti­fi­cial light ir­ra­di­a­tion (study VII) and long-term nat­u­ral­is­tic sun­light ra­di­a­tion (study VIII) on hairF and hairE in human scalp hair sam­ples. Re­sults con­sis­tently re­veal that longer light ex­po­sure re­sulted in a de­crease of hairF and hairE under in vitro con­di­tions. How­ever, the mag­ni­tude of this ef­fect under real-life test­ing con­di­tions needs to be de­ter­mined in fu­ture re­search. Notwith­stand­ing, our data strongly sug­gest that hair sam­ples for steroid analy­ses need to be stored in a dark en­vi­ron­ment. In sum, over­all find­ings of the cur­rent the­sis pro­vide in­sights into basic method­olog­i­cal as­pects of hair cor­ti­sol analy­sis. The re­search proves the ro­bust­ness of hairF (and hairE) to acute sit­u­a­tional in­flu­ences and thus fur­ther high­lights the po­ten­tial of hairF as an intra-in­di­vid­u­ally sta­ble mea­sure pri­mar­ily re­flect­ing long-term sys­temic cor­ti­sol lev­els. How­ever, the in­ves­ti­ga­tions also point to a pos­si­ble con­found­ing in­flu­ence of in­di­vid­ual per­spi­ra­tion on hairF, po­ten­tially re­lated to ex­ter­nal de­po­si­tion of cor­ti­sol from sweat into hair. Fur­ther, the pre­sent the­sis shows ini­tial ev­i­dence for in­di­vid­ual-level sun­light ex­po­sure as a pos­si­ble con­found in hair steroid analy­sis.