Diurnal Cortisol Slope and Nighttime Blood Pressure: A Study in European Americans and African Americans.

Objectives: African Americans (AAs) have higher nighttime blood pressure (BP) than European Americans (EAs). Stress has been suggested to play a role in this difference, but the mechanism is not well-understood. Flatter diurnal cortisol slope (DCS) is a well-known biological marker of stress. The objectives of this study were to: 1) examine ethnic differences in DCS; 2) evaluate the association between DCS and nighttime BP; and 3) determine the extent to which ethnic differences in nighttime BP can be explained by ethnic differences in DCS.

Methods: A total of 510 participants (age range: 14-35 years; 49.6% AAs, 54.5% females) provided four salivary cortisol samples at bedtime, wakeup, 30-minutes post-wakeup, and 60-minutes post-wakeup. Additionally, participants wore an ambulatory BP monitor for 24 hours. DCS was calculated as the average of the three morning samples minus the bedtime measurement.

Results: After adjustment for age, sex, BMI, and smoking, AAs had blunted DCS (P=.018) and higher nighttime systolic BP (SBP) and diastolic BP (DBP) (Ps<.001) compared with EAs. The DCS was inversely related to nighttime SBP and this relationship did not depend on ethnicity. The ethnic difference of nighttime SBP was significantly attenuated upon addition of DCS to the model. Mediation test showed that 9.5% of ethnic difference in nighttime SBP could be explained by DCS (P=.039).

Conclusion: This study confirms ethnic differences in DCS and nighttime BP and further demonstrates that the ethnic differences in DCS can, at least partially, explain the ethnic differences found in nighttime BP.