Salivary testosterone changes during oral glucose tolerance tests in overweight and obese men - Postprandial or circadian variation?

Background: Serum total testosterone (T) decreases postprandially. Postprandial salivary testosterone (SalT) responses, however, have not been studied. We report on the effect of glucose ingestion on fasting SalT concentrations. Objective: To investigate the effect of oral glucose ingestion on fasting SalT. Methods: Salivary and blood samples were collected between 09.00 and 09.30 and 2 hours after a 75g oral glucose load in 32 men with mean (standard deviation) age of 52 (5.7) years and body mass index of 32.6 (5.56) kg/m(2). Free T and bioavailable testosterone (BAT) were calculated using the Vermeulen equation. Results: Two hours following oral glucose, there was a decrease in fasting mean (standard deviation) SalT [178.2 (56.6) versus 146.0 (42.2) pmol/L; P = .0003], serum cortisol [332 (105.0) versus 239 (75.3) nmol/L; P = <0.0001], prolactin [193 (75.0) versus 127 (55.9) mIU/L; P = <0.0001] and TSH [1.60 (0.801) versus 1.16 (0.584) mIU/L; P = <0.0001]. Plasma glucose increased [6.2 (0.72) versus 8.1 (3.71) mmol/L; P = .0029]. Serum total T, SHBG, albumin, Free T, BAT, gonadotrophins and FT4 remained unchanged. Conclusions: SalT decreased postprandially. A concomitant decrease in serum cortisol, prolactin and TSH reflecting diurnal variation offers an alternative explanation for the decrease in SalT independent of food consumption. Further studies are required to determine whether morning temporal changes in SalT are related to food consumption or circadian rhythm or both.