The intervention in this study consists of 75-minute intraduodenal infusions of isosmotic solutions containing either saline (control), L-tryptophan, or calcium combined with L-tryptophan. Participants enrolled into the study will receive, in a randomised, double-blind fashion either (i) saline (control), (ii) L-tryptophan at a rate of 0.1 kcal/minute, (iii) combination of L-tryptophan (0.1 kcal/minute) + 500 mg calcium, or (iv) combination of L-tryptophan (0.1 kcal/minute) + 1000 mg calcium in four separate sessions, each of which will be separated by at least 4 (and up to 10) days. Each study session will be 4-6 hours. Studies will be carried out in the Clinical Research Facility of the Adelaide Medical School, the University of Adelaide, by staff and students trained in the required techniques.
Healthy, lean males (only men will be included in the study to avoid the confounding effects of the menstrual cycle on gastric emptying).
BMI: 19-25 kg/m2,
Weight-stable (i.e. <5% fluctuation) at study entry, which will be ascertained by a stable body weight in the preceding 3 months.
Significant GI symptoms, or history of GI disease or surgery
Current gallbladder or pancreatic disease
Cardiovascular or respiratory diseases
Any other illnesses as assessed by the investigator (including chronic illnesses not explicitly listed above)
Crossover
L-tryptophan
Other interventionL-tryptophan, an aromatic amino acid and one of the building blocks of protein, is a part of our daily diet. The load of L-tryptophan (0.1 kcal/minute) is based on our previous study, in which L-tryptophan represented a submaximal load.
Combination of L-tryptophan + Ca-500
Other interventionCalcium, an essential mineral and a key component of dairy products, is a regular part of our daily diet. In this condition, both L-tryptophan and calcium will be administered as 'active'. Recent studies have shown that calcium in a dose of 500 mg enhances the effects of L-tryptophan to stimulate gut functions and reduce energy intake. This dose of calcium will be considered 'lower dose'.
Combination of L-tryptophan + Ca-1000
Other interventionIn this condition, both L-tryptophan and calcium will be administered. In our study calcium in a dose of 1000 mg enhances the effects of L-tryptophan to stimulate gut functions and reduce energy intake. This dose of calcium will be considered 'higher dose'.
Control
Other interventionAn isotonic solution containing 2.5 g of NaCl, dissolved in 225 mL of distilled water.
Plasma glucose concentrations
Plasma glucose concentrations (mmol/L) will be assessed using the glucose oxidase method.
Gastric emptying
13C-acetate breath test, measuring 13CO2 levels in end-expiratory breath samples.
Plasma concentrations of glucoregulatory hormones e.g. glucagon-like peptide (GLP-1), glucose-dependent insulinotropic polypeptide (GIP), C-peptide, glucagon, insulin and cholecystokinin (CCK)
Plasma concentrations of hormones e.g. GLP-1, GIP and CCK (pmol/L) will be determined using radioimmunoassays, and plasma C-peptide, glucagon and insulin concentrations (pmol/L) will be measured using ELISA immunoassays.
GI symptoms (nausea and bloating) will be assessed as a composite secondary outcome.
These will be measured using 100mm VAS questionnaires.
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