Effects of calcium and amino acids on gut hormone secretions and bone turnover in healthy males

The intervention in this study consists of a 150 min intraduodenal infusion of a calcium or control solution, combined with L-tryptophan (0.1 kcal/min) from t=75-150 min. Participants enrolled into the study will receive, in randomized, double-blind fashion (i) 500 mg CaCl2, combined with L-tryptophan (0.1 kcal/min) from t=75-150 min (ii) 1000 mg CaCl2, combined with L-tryptophan (0.1 kcal/min) from t=75-150 min (iii) Saline (control), combined with L-tryptophan (0.1 kcal/min) from t=75-150 min each occurring at separate visits. Each visit will last 4.5-6 hrs in duration and will be separated by 3-7 days. Visits will be carried out in the Clinical Research Facility of the Discipline of Medicine, the University of Adelaide by staff members trained in the required techniques, and will be conducted on an individual basis. Participants will be asked to consume a standardised dinner meal (Beef lasagne; total energy content: 602kcal; McCain Food, Wendouree, Victoria, Australia) the night before each visit by no later than 6 pm, After fasting for 14 hrs overnight and refraining from exercise and alcohol for 24 hrs, participants will arrive at the laboratory at 8 am. Upon arrival, participants will be intubated with a 17-channel manometric catheter that will be inserted through an anaesthetised nostril and allowed to pass through the stomach and into the duodenum by peristalsis. The manometric catheter consists of 16 side holes spaced at 1.5 cm intervals, measuring pressures in the antrum, pylorus, and duodenum (APD pressures). An additional channel (with the side hole positioned approx 14 cm distal to the pylorus when the catheter is in position) is used for intraduodenal infusions. The correct positioning of the catheter will be maintained by continuous measurement of the transmucosal potential difference (TMPD) between the most distal antral channel and the most proximal duodenal channel. All manometric channels will be perfused with degassed, distilled water, except for the two TMPD channels, which will be perfused with degassed 0.9% saline, at 0.15 ml/min. An intravenous cannula will be placed into a forearm vein for regular blood sampling to measure blood glucose, plasma hormone (e.g. gastrin, CCK, GIP, GLP-1, and potentially other, including yet to be identified, gut hormones) and bone marker (CTX, PTH) concentrations. Once the catheter has been positioned correctly, fasting motility will be monitored continuously, and immediately after the end of phase III activity of the fasting migrating motor complex (MMC), during a period of motor quiescence (i.e. at t = -15 to -1 min), two 9-mL venous blood samples (baseline) will be taken (at t = -15 and -5 min), and the participant will complete a visual analogue scale questionnaire (VAS) to assess appetite-related perceptions (fullness, hunger, etc.) and GI symptoms (nausea and bloating). At t = 0 min (during phase I of the MMC), one of three infusions (i) saline or ii) calcium (500 mg) or iii) calcium (1000mg) will commence. At t = 75 min an intraduodenal infusion of L-tryptophan (same rate on all three study days) will be added for 75 mins. Antropyloroduodenal (APD) pressures will be measured continually for 150 min (t = 0-150 min). Vital signs (blood pressure, heart rate) will be measured at regular time intervals using a commercially available sphygmomanometer/blood pressure meter. At t = 150 min, the manometric assembly will be removed and participants will be presented with a standardised cold, buffet-style meal, which is used to assess energy intake. Participants will be allowed 30 min to freely consume food until they are comfortably full. At t = 180 min, a final blood sample and VAS questionnaire will be collected. The intravenous cannula will then be removed and participants will be allowed to leave the laboratory. A total of 135 mL of blood will be taken on each study day (study total of 413 mL, including screening test).