The Effect of Acute, Passive Heating on Glucose Tolerance in Individuals With Type 2 Diabetes Mellitus: a Randomised, Balanced Crossover, Control Trial

Visit 1 (consent, screening and familiarisation) During visit 1, participants will give their informed consent, followed by a health screening questionnaire. In addition to the health screening questionnaire, medical history and a blood sample will be collected and analysed for a full blood count, glycated haemoglobin (HbA1c), liver and kidney function. Finally, a resting electrocardiogram (ECG) will also be recorded and then examined for irregularities, where a clinical decision will be made on further participation to the study by consultants at QA (Queen Alexandra) hospital. Participants will then be shown the rest of the equipment and taken through the procedure for the next 3 visits and, if the participant is happy to continue the study, the next visit will be organised.

Visit 2, 3 and 4 Participants will arrive at the laboratory at ~9 am for conditions 1 and 2 and 8 am for condition 3. Prior to a 15 min resting period (supine) before any measures are taken participants will be asked to insert a rectal thermistor (participants will be given clear instructions using the investigator's SoPs (standard operating procedure)). Condition 1, 2 and 3 will be balanced and participants randomly allocated to begin the study in either visit 2, 3 or 4 using a blinded member of the team.

For all visits (see figure 2), participants will lie in a semi recumbent position in minimal clothing (bathing shorts and a t-shirt) for the entirety of the visit. Initially, participants will be cannulated (Versatus winged and ported IV cannula, Terumo, Japan) and blood samples drawn for analysis of osmolality (Lithium Heparin (LH) tubes BD (Becton, Dickinson and Company), USA) plasma [glucose] (Fluoride/Oxalate tubes, BD, USA), [insulin] (Ethylenediaminetetraacetic acid (EDTA) K2, BD, USA), and [eHSP70 (extracellular heat shock protein 70)] (EDTA K2, BD, USA) at baseline and every 30 min of each experimental visit. Following cannulation an 180 min OGTT (75g) (Rapilose OGTT solution, Penlan healthcare, Japan) will commence in a thermoneutral room (~ 23oC). A maximum of 18 mL of blood being drawn at each time point (max 126 mL per visit). To maintain the patency of the cannula and to reduce the risk of infection, after every sample is taken, 5 mL of saline will be flushed through the cannula. Then before every sample is taken, 2.5 mL of blood will be drawn out of the cannula to ensure any remaining saline will not interfere with the samples and data interpretation (additional 17.5mL per visit). During the OGTT, HR (heart rate) (via electrocardiogram) will be measured continuously, whilst blood pressure (M5-1, Omron, Japan), deep body temperature (rectal probe) and resting metabolic rate (indirect calorimetry) (Quark CPET (cardiopulmonary exercise test), Cosmed, Italy) will be assessed every 30 min.

Condition 2 will employ identical procedures to condition 1, except thirty minutes into the OGTT, the participant will be immersed into an immersion tank (~39oC) for 60 min. Water temperature will be manipulated as required to achieve and maintain a target Trec at 38.5 oC using water between 37.5 and 39oC, and then participants will be removed horizontally back into the thermoneutral room for the reminder of the OGTT. Participants will be towel dried and given a towelled robe to wear. Condition 3 will employ identical procedures to condition 2, with the exception that the heating via immersion will start as soon as the participant is instrumented (and following a 15 min rest period) and the OGTT will commence 30 min after the 60 min immersion time for a further 180 min (see figure 2 for a schematic).