Diurnal variability of cortisol in the Pennsylvania adult smoking study: Exploration of association with nicotine intake

Cortisol in saliva, urine and plasma follows a diurnal rhythm typically characterized as a morning peak and a decline throughout the waking day. While often measured under controlled conditions, inter-individual differences in cortisol diurnal rhythms in free living populations are not well characterized. Cortisol levels may vary substantially between individuals and the level of variation may differ depending on the time of day. Further, associations with individual characteristics such as nicotine dependence on cortisol rhythms have not been adequately determined. We developed a Liquid Chromatography-Tandem Mass Spectrometry method to measure cortisol in saliva of 180 smokers from the Pennsylvania Adult Smoking Study. Diurnal patters of cortisol were determined by obtaining five timed samples throughout the day for a total of 900 determinations. Adherence to the protocol was estimated by asking participants to record the time of sample collection. Longitudinal linear mixed effects models were developed to measure the predictors of mean levels. Phenotypic groups were constructed based on the minimum and maximum cortisol levels. Mixed method modelling was conducted to determine the effects of phenotype and study adherence as well as reported measures of stress, nicotine dependence and cigarette smoking frequency. Nicotine metabolites were measured to accurately quantify dose of smoking intake. Results showed that there was moderate compliance to the timed protocol. Descriptive and analytic findings showed that some smokers had atypical cortisol patterns, and that the cortisol profiles based on experience of maximum and minimum cortisol levels can predict how cortisol varies throughout the day. There was no association with reported stress, cigarettes per day, and nicotine metabolites. There was no association with the Fagerstrom Test for Nicotine Dependence. The Hooked on Nicotine Checklist (HONC) score was associated with decreased cortisol levels. Overall these results show new approaches and expectations to population-based studies of cortisol.