Almost all biochemical and physiological parameters are circadian rhythmic (Reilly, Atkinson & Waterhouse, 1997; Hayes, Bickerstaff & Baker, 2010). The circadian pacemaker regulates the prominent 24 h variation in biological functions, including the synthesis and release of testosterone and cortisol (Mrosovsky, 2003). Testosterone has both anabolic and anticatabolic effects on muscle tissue (Hayes et al., 2010) as well as associated effects on sexual maturation. Cortisol is a steroid hormone released by the adrenocortical glands under hypothalamic and pituitary control defining the hypothalamo-pituitary-adrenal (HPA) axis. The HPA axis plays a vital role in chronic adaptation to endurance training and acute response to exercise. Cortisol exerts catabolic effects on muscle tissue (Florini, 1987) and has important metabolic functions, such as influencing the metabolism of lipids, proteins, and glucose. Intense physical exercise increases cortisol (Timon et al., 2009), which may inhibit protein synthesis with consequent decrease in muscle mass by its catabolic effect (de Souza Vale et al., 2009). The balance between these anabolic/catabolic hormones is often used as an indicator of overtraining (Duclos, 2008).
Both testosterone and cortisol exhibit circadian rhythmicity with peak concentrations in the morning, around the commencement of diurnal activity, and reduced concentrations in the evening and overnight (Touitou & Haus, 2000). The morning rise in cortisol accelerates metabolism (Florini, 1987) and stimulates gluconeogenesis and proteolytic activity, resulting in increased skeletal protein turnover (Dinneen, Alzaid, Miles & Rizza, 1993). The increase in testosterone at this time may be an attempt to counteract the stimulatory effect of cortisol on skeletal protein degradation (Kraemer, 1988).
Rose, Sulak, Johnson, Holaday and Kruez (1972) were some of the initial investigators to document the episodic release of cortisol and testosterone over the day. Although cortisol samples were only collected every 90 min, peaks in the early morning and at 12:00 and 16:00 h were clearly evident in diurnally active subjects. These observations are consistent with those of Krieger, Allen, Rizzo and Krieger (1971), who reported similar patterns only 12 months prior. Concentrations of testosterone were shown to be less erratic than those of cortisol, with no “peaks” documented as such. Although consistent, the magnitude of the diurnal change in testosterone levels was significantly less than cortisol. On average, diurnally active men’s testosterone concentration declined 42% from 06:00 h (awakening) to 23:00 h, compared with 92% for cortisol during this span. Slag, Ahmed, Gannon and Nuttall (1981) reported similar diurnal variations in cortisol, suggesting peaks around 12:00 and 16:00 h. However, these times coincided with meals, and a fasted group in the Slag et al. (1981) paper showed a blunted cortisol response at these times.
Interestingly, the aforementioned observations have all been made when blood sampling rather than salivary sampling. Salivary cortisol is an excellent indicator of plasma-free cortisol (Arafah, Nushiyama, Tlaygeh and Hejal, 2007) increasingly used to assess hypothalamic– pituitary–adrenal axis secretory activity and rhythm (Casals, Foj & Jesus Martinez de Osaba, 2011). For example, it is widely accepted that late-night salivary cortisol measurement is a simple and reliable way to screen patients for Cushing's syndrome (Casals et al., 2011). In fact, the Clinical Guideline Committee of the Endocrine Society recommends the use of nocturnal salivary cortisol as a first step procedure in the diagnosis of Cushing's syndrome (Nieman et al., 2008). It is preferable to blood sampling since it can be easily performed on an outpatient basis without disrupting a normal routine. In addition, the saliva collection is a non-invasive sampling procedure that avoids the stress-induced rise in adrenal secretion associated with blood sampling. Therefore, salivary cortisol measurements are increasingly used on a routine basis. However, there is a lack of knowledge regarding significant data required for correctly interpreting salivary cortisol laboratory results, such as the degree of day-to-day intra-individual variation or the degree of inter-individual variation.
The use of salivary testosterone has been reported to be reliable in comparison to serum for reflecting gonadal function and circadian patterns (Dabbs, 1990). Khan-Dawood, Choe and Dawood (1984) have shown the composition of salivary testosterone to be 78% free testosterone, while serum free testosterone was reported to be at 4%. Wang, Plymate, Nieschlag and Paulsen (1981) reported that increases in serum testosterone concentrations relate to a concomitant increase in salivary testosterone concentrations within 1 h. Vittek, Lhommedieu, Gordon, Rappaport and Southren (1985) examined the relationship between salivary and serum free testosterone versus salivary and serum total testosterone and reported significant correlations of r = 0.97 and r = 0.70-0.87 for free and total testosterone, respectively. These data indicate that salivary testosterone provides a good indication of the fluctuations in free testosterone (Vittek et al., 1985). The aim of the study was to establish baseline salivary testosterone and cortisol concentrations, and their ratio, throughout a waking diurnal cycle. These results can be compared to previously published serum testosterone and cortisol results in order to establish the most opportune training times in terms of work tolerance, recovery, and adaptation.
MATERIALS AND METHODS
Eighteen male university students age, stature, body mass and BMI of 23.2 ± 3.0 years of age, 180.9 ± 4.3 cm in height, 84.4 ± 15.9 kg in body mass and, a 25.7 ± 4.5 BMI volunteered to participate in the study. Experimental procedures were approved by the University of the West of Scotland Research Ethics Committee. The protocol was explained and subjects gave informed consent to participate in this study. All subjects were habitually physically active, and had abstained from alcohol, caffeine and exercise for 24 h preceding the investigation. Exclusion criteria included poor sleep quality, recent shift work, extreme chronotype according to the Horne-Ostberg Morningness-Eveningness Questionnaire (Horne & Ostberg, 1976) or travel across multiple time zones.
Fasted participants reported to the laboratory at ~07:45 h approximately 40 min after waking. Laboratory observations were conducted in the University of the West of Scotland’s Clinical Exercise Research Laboratory. The ~40m2 room was cleared of time reference devices and blinded, ensuring no natural light entered the room. Artificial lighting and a constant temperature was maintained.
Sample collection commenced at 08:00 h. Immediately after the first sampling, participants were provided with a standard breakfast consisting of Weetabix®, milk and orange juice (1,769 kJ, 18% protein, 9% fat, 73% carbohydrate). Participants remained in the study venue until ~20:05 h and provided a saliva sample every 60 min. At 13:05 and 18:05 h participants were provided with a standardised meal that consisted of ham, cheese and tomato wholegrain sandwiches (2,721 kJ, 24% protein, 22% fat, 54% carbohydrate per meal). Participants were permitted to drink water ad libitum and were instructed to rinse their mouths with water after eating. The meal plan had been previously used (Beaven, Ingram, Gill & Hopkins, 2010).
Intra-aural temperature was measured using a clinical-grade infrared ear thermometer (Braun, Germany). Blood pressure and heart rate was measured using an automated sphygmomanometer (Omron, the Netherlands). Rate pressure product (RPP) was calculated by multiplying systolic blood pressure and heart rate. These parameters were observed every 15 min. Participants were free to engage in sedentary activities of their choice. Physical activity and exercise were not allowed. Subjects left the laboratory only to use the toilet (also light controlled).