Effects of amino acids on gut functions in healthy lean individuals

Each participant will be studied on four occasions, separated by at least 3 (and up to 7) days. On each occasion, participants will receive, in randomised, double-blind fashion, 90-min intraduodenal infusions of the following treatments: (i) control solution (saline), (ii) L-tryptophan at 0.15 kcal/min (3.3 g over 90 min), (iii) combination of L-tryptophan at 0.15 kcal/min (3.3 g) with L-leucine at 0.22 kcal/min (5 g), or (iv) combination of L-tryptophan at 0.15 kcal/min (3.3 g) with L-leucine at 0.45 kcal/min (10 g). On study days, participants will arrive in the Clinical Research Facility, and will be intubated with a custom-built, soft, silicon manometric catheter (outer diameter: 4 mm; Dentsleeve International, Mui Scientific, Mississauga, Ontario, Canada) that will be inserted through an anaesthetised nostril and allowed to pass through the stomach and into the duodenum by peristalsis. An intravenous cannula will also be placed into a forearm vein for regular blood sampling. 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, during a period of motor quiescence (i.e. at t = -10 to 0 min), a 10-ml blood sample (baseline) will be taken, 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), the positive and negative affect schedule-short form (PANAS-SF) questionnaire to assess mood, the N-back test and word recall memory test to assess memory, and the Rapid Visual Information Processing (RVIP) test to measure sustained attention. Estimated times required to complete the questionnaires and tests are ~2 min for VAS and PANAS-SF, 2 min for N-back and word recall memory, and 6 min for RVIP. At t = 0 min, the intraduodenal infusion will commence. APD pressures will be measured continually over the 90-min period. Blood samples and VAS ratings will be collected every 15 min from t = 15 to 90 min. PANAS-SF questionnaires will be completed every 30 min from t = 30 to 90 min. The N-back, word recall memory and RVIP tests will be completed at the end of the infusion, from t = 80 min. At t = 90 min, the manometric assembly will be removed and participants will be presented with a cold, buffet-style meal to assess energy intake. The meal consists of 4 slices (~ 120 g) of whole-meal bread, 4 slices (~ 120 g) of white bread, 100 g sliced ham, 100 g sliced chicken, 85 g sliced cheddar cheese, 100 g lettuce, 100 g sliced tomato, 100 g sliced cucumber, 22 g mayonnaise, 20 g margarine, 1 apple (~ 170 g), 1 banana (~ 190 g), 175 g strawberry yogurt, 100 g chocolate custard, 120 g fruit salad, 375 mL iced coffee, 300 mL orange juice, and 600 mL water. Participants will be allowed 30 min to freely consume food until they are comfortably full. The amount of food (g) consumed will be quantified by weighing each food item of the buffet-meal before and after it was presented to the participant. Energy intake (kcal) will be calculated using commercially available software (Foodworks version 8, Xyris Software, Highgate Hill, Queensland, Australia). At t = 120 min (i.e. at the end of the meal) and at t = 180 min, further blood samples will be taken, and questionnaires administered. The intravenous cannula will then be removed and participants will be allowed to leave the laboratory. Participants will be asked to complete a 24-h dietary record (~2 pm to 2 pm) after each visit to monitor their subsequent energy intake.