Habitual Caffeine Use is Associated with Heightened Cortisol Reactivity to Lab-Based Stress in Two Samples.

OBJECTIVE: Habitual caffeine consumption protects against depression, but through unclear mechanisms. Although habitual caffeine use predicts cortisol release to other acute stressors (e.g., exercise), this is less examined with lab-based psychosocial stress in healthy adults. Further, caffeine-induced cortisol increases may mask theory-predicted cortisol blunting to robust stress in people with elevated depression risk. In two samples, we tested whether acute (same-day) and habitual caffeine use would predict greater cortisol reactivity to lab-based stress, and whether caffeine would "mask" the effect of a depression risk factor, trait rumination, on blunted cortisol reactivity.

METHOD: In Sample 1, N = 128 emerging adults completed one of three Trier Social Stress Test conditions: non-evaluative Control, ambiguously evaluative Intermediate, or explicit Negative Evaluative. In Sample 2, N = 148 emerging adults completed either a Control or Negative Evaluative condition.

RESULTS: In both samples, multilevel growth curve modeling indicated habitual ( t = -1.99, p = .048; t = -2.73, p = .007, Samples 1 and 2 respectively), but not acute caffeine use predicted heightened cortisol reactivity as a function of condition. In Sample 1, the relationship between condition, rumination, and blunted cortisol was evident only in caffeine non-users, which differed from users ( t = 2.82, p = .005), but in Sample 2 the predicted blunting pattern was evident regardless of caffeine use.

CONCLUSION: This provides evidence that habitual caffeine use is associated with greater cortisol release under psychosocial lab-based stress and may mask the influence of psychosocial variables; future research should examine whether habitual caffeine-induced cortisol release has behaviorally activating effects that protect against depression.