Jr. Thuma et al., CIRCADIAN-RHYTHM OF CORTISOL CONFOUNDS CORTISOL RESPONSES TO EXERCISE- IMPLICATIONS FOR FUTURE-RESEARCH, Journal of applied physiology, 78(5), 1995, pp. 1657-1664
To investigate whether measurements of cortisol responses to exercise
are confounded by neglect of the hormone's circadian rhythm, we measur
ed the serum and salivary cortisol responses of eight women to 40 min
of 70% maximal oxygen consumption treadmill exercise beginning at 0800
and 2000. Responses were calculated relative to the usually employed
preexercise concentrations and also to concentrations at the same time
s of another day while subjects were at rest. Compared with areas unde
r response curves (AUCs) calculated relative to their circadian baseli
nes, AUCs for serum and salivary cortisol calculated by reference to p
reexercise concentrations were underestimated (serum, P < 0.001; saliv
ary, P < 0.01) by 93 and 84% in the morning and by 37 and 35% in the e
vening, respectively. Calculated by the usual preexercise baseline met
hod, rises in serum and salivary cortisol were similarly underestimate
d. More accurately calculated relative to their circadian baselines, s
erum and salivary cortisol AUCs were similar (P = 0.63 and P = 0.37, r
espectively) in the morning and evening, as were their rises (P = 0.23
and P = 0.70, respectively). In future investigations of the existenc
e and magnitude of cortisol responses, those responses must be calcula
ted relative to the hormone's circadian baseline.