Effect of Ambient Temperature on Blood Glucose and Insulin Absorption in Adults With Type 1 Diabetes

Background and Rationale

Climate change is increasing the frequency of extreme weather events, including heat waves and cold snaps. People with type 1 diabetes face higher risks of illness and death during these extreme temperature periods compared to people without diabetes. However, the reasons for this increased risk are not well understood.

When insulin is injected under the skin, it forms a small reservoir from which it gradually enters the bloodstream. Temperature may affect how quickly this insulin is absorbed. Warmer temperatures increase blood flow to the skin, which could speed up insulin absorption and potentially cause blood sugar to drop too low (hypoglycemia). Colder temperatures might slow insulin absorption, leading to higher blood sugar levels (hyperglycemia).

Previous research from the 1980s showed that heating or cooling the injection site affects insulin absorption, but these studies were conducted before modern rapid-acting insulins and diabetes technologies like insulin pumps and continuous glucose monitors were available. No recent studies have examined how whole-body temperature exposure affects insulin absorption and blood sugar control in people with type 1 diabetes.

Study Design Overview

This is a randomized, crossover clinical trial where each participant will experience all three temperature conditions in random order. The study includes:

* 1 preliminary visit for screening and measurements * 3 experimental visits (cold, neutral, hot conditions) * Total participation time: approximately 14 hours over up to 4 months

Study Population

The study will enroll 30 adults aged 18-45 years with type 1 diabetes for at least 2 years. Participants will be equally divided by:

* Sex: 50% male, 50% female * Insulin delivery method: 10 using multiple daily injections, 10 using insulin pumps, 10 using hybrid closed-loop systems

Preliminary Visit Procedures

During the first visit at the Montreal Clinical Research Institute (IRCM), participants will:

* Review and sign informed consent documents * Complete medical history and lifestyle questionnaires * Fill out a heat exposure questionnaire based on Health Canada guidelines * Complete the GENESIS PRAXY questionnaire to document gender-related factors * Undergo body measurements including height, weight, waist circumference, and body composition scan * Have resting heart rate and blood pressure measured * Receive training on continuous glucose monitor use (if not already using one)

Experimental Visit Procedures

Each of the three experimental visits will take place at Centre ÉPIC and follow the same protocol in different temperature conditions:

Pre-Exposure Phase (60 minutes)

* Participants arrive 2 hours after eating a standardized snack (100g Greek yogurt) with their usual insulin dose * Body weight measurement * Installation of rectal temperature probe (self-inserted privately) * Placement of intravenous catheter for blood sampling * Attachment of skin temperature sensors, blood pressure cuff, and heart rate monitor * 60-minute rest period at room temperature to establish baseline measurements

Temperature Exposure Phase (120 minutes)

Participants will sit in an environmental chamber set to one of three conditions:

* Cold: 10°C (50°F) - causes shivering and increases energy expenditure by approximately 80% * Neutral: 23°C (73°F) - comfortable room temperature * Hot/Humid: 36°C (97°F) with 65% humidity - simulates a hot summer day in Montreal

Measurements During Exposure

Continuous monitoring:

* Heart rate via chest strap monitor * Core body temperature via rectal probe * Skin temperature at four sites (arm, chest, thigh, calf) * Blood sugar levels via continuous glucose monitor (checked every 15 minutes)

Intermittent measurements:

* Blood pressure every 10 minutes * Blood samples at baseline, 60 minutes, and 120 minutes for analysis of glucose, insulin, glucagon, and other metabolic markers * Comfort and sensation ratings every 10 minutes using visual scales for thermal comfort, thermal sensation, and thirst

Safety Monitoring

Throughout each visit, research staff will continuously monitor participants for:

* Signs of hypoglycemia (low blood sugar) - treated immediately with glucose if needed * Core body temperature limits (stopped if ≤35.5°C during cold or ≥39.5°C during heat exposure) * Blood pressure limits (stopped if \>180/110 mmHg or \<90/60 mmHg) * Participant comfort and willingness to continue

Data Collection

24-Hour Activity Logs

Participants will record their food intake, insulin doses, sleep patterns, and physical activity for:

* 48 hours before the preliminary visit (baseline lifestyle) * 24 hours before each experimental visit * 24 hours after each experimental visit

Continuous Glucose Monitoring

A Dexcom G7 sensor will be installed 48-72 hours before each visit to capture:

* Glucose patterns during temperature exposure * 24-hour glucose control before and after each visit * Time spent in target range (3.9-10.0 mmol/L) * Time spent above target (\>10.0 mmol/L) * Time spent below target (\<3.9 mmol/L)

Laboratory Analyses

Blood samples will be analyzed for:

* Plasma glucose and insulin levels * Glucagon (hormone that raises blood sugar) * Free fatty acids and glycerol (markers of fat metabolism) * Other metabolic hormones

Special Considerations

For Female Participants Study visits will be scheduled during the follicular phase of the menstrual cycle (or placebo pill phase for those using oral contraceptives) since hormonal changes can affect blood sugar levels.

• Change in plasma insulin concentration during temperature exposure [Time frame: Measured at baseline, 60 minutes, and 120 minutes during each 2-hour environmental chamber exposure.]
• 24-hour time in glucose range [Time frame: 24 hours before and 24 hours after each temperature exposure visit.]
• 24-hour time above glucose range [Time frame: 24 hours before and 24 hours after each temperature exposure visit.]
• 24-hour time below glucose range [Time frame: 24 hours before and 24 hours after each temperature exposure visit.]
• Continuous glucose monitor accuracy [Time frame: During each 2-hour temperature exposure.]