The intervention in this study consists of a 75-min intraduodenal infusion of an isotonic solution containing either of calcium chloride (CaCl2) or control (saline).

Participants enrolled into the study will receive, in randomised, double-blind fashion (i) Saline (control), (ii) 500 mg CaCl2, or (iii) 1000 mg CaCl2 in three 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.

Each participant will be required to consume a standardised dinner meal (beef lasagne; total energy content: 602 kcal; McCain Food, Wendouree, Victoria, Australia) on the night before each visit by 7 pm. After fasting overnight for ~ 13.5 hours and refraining from exercise and alcohol during the previous 24 hours, the participant will arrive in the laboratory at 8.30 am. After confirming adherence to the study requirements, vital signs ( blood pressure, heart rate and body temperature) will be measured to establish baseline values for safety, and they will be monitored throughout the study. Then, a manometric catheter will be inserted through an anaesthetised nostril and allowed to pass through the stomach and into the duodenum by peristalsis. The infusion port will be positioned ~ 14 cm distal to the pylorus. The correct positioning of the catheter will be monitored continuously by measurement of the transmucosal potential difference in the stomach (~ -40 mV) and the duodenum (~ 0 mV). Once the catheter has been positioned correctly, an intravenous cannula will be placed into a forearm vein for regular blood sampling. At time = -30 minute, a baseline blood sample (7 ml), visual analogue scale (VAS) questionnaire to assess gastrointestinal (GI) symptoms, and breath sample to assess gastric emptying will be collected. Then the infusion of one of the study treatments, i.e. i) saline, ii) 500 mg CaCl2 or iii) 1000 mg CaCl2), will commence and continue for 75 minutes (times = -30 to 45 minutes). At time = -1 minute, participants will consume, within 1 minute, a mixed-nutrient drink (350 mL, containing 500 kcal, 74 g carbohydrates) labelled with 100 mg 13C-acetate for measurement of gastric emptying by non-invasive breath sampling at regular intervals, for subsequent analysis of 13CO2 levels. Blood samples for the measurement of plasma glucose and hormone concentrations, and VAS questionnaires, will be collected before and after the mixed-nutrient drink. At time = 240 minute, after collecting final blood and breath samples and VAS questionnaire, the intravenous cannula will be removed and the participant will be served a light lunch, after which they will be allowed to leave the laboratory. A total of 98 mL of blood (14 sampling time points, 7 mL each) will be taken on each study day (study total of 310 mL, including screening and glucose tolerance test samples).

Inclusion Criteria:

• Males with type 2 diabetes mellitus (T2DM). Diagnosis of T2DM will be based on WHO criteria. Only males will be included to avoid the confounding effects of the menstrual cycle on gastric emptying.
• BMI: 28-38 kg/m².
• HbA1c >=6.5 - <=7.9% at screening.
• Blood glucose medications will be required to be withheld for 48 hours prior to each study day.
• Weight-stable (i.e. <5% fluctuation) at study entry, which will be ascertained by a stable body weight in the preceding 3 months.

Exclusion Criteria:

• Significant GI symptoms, or history of GI disease or surgery
• Current gallbladder or pancreatic disease
• Cardiovascular or respiratory diseases
• Any other illnesses (except type 2 diabetes) as assessed by the investigator - (including chronic illnesses not explicitly listed above)
• Use of prescribed or non-prescribed medications (including vitamins and herbal supplements) which may affect energy metabolism, GI function, bodyweight or appetite (e.g. domperidone, cisapride, anticholinergic drugs (e.g. atropine), metoclopramide, erythromycin, hyoscine, orlistat, green tea extracts, Astragalus, St Johns Wort etc.)
• Lactose intolerance/other food allergy(ies)
• Individuals with low ferritin levels (<30 ng/mL), or who have donated blood in the 12 weeks prior to taking part in the study
• High performance athletes
• Current intake of > 2 standard drinks on > 5 days per week (>140g/week)
• Current smokers of tobacco (cigarettes, cigars, pipes, sheesha, chewing, vaping etc.)
• Current use of recreational drugs, e.g. marijuana
• Current intake of any illicit substance
• Vegetarians
• Inability to tolerate nasoduodenal tube
• Inability to comprehend study protocol
• HbA1c <6% or >7.9%
• Estimated glomerular filtration rate <45 ml/min
• Any patient whose medication cannot be withheld for 48 hours for medical reasons

The datasets generated during and/or analysed during the current study are not publicly available due to the ethical statement and informed consent that require privacy of data.

In order to blind both investigator and participant, the treatments will be prepared on the morning of each study day, by a research officer who will have no involvement in data analysis.