This is a randomized, double-blind, placebo-controlled, cross-over experimental design. Participants will be asked to complete 5 visits to the laboratory; one informed consent/screening visit and 4 study visits. On study visit days, participants will report to the laboratory following a 12-hour fast, and 24-hour abstention from exercise, caffeine, and alcohol. During the study visits, participants will be instrumented for skin temperature (12-sites: calf, shin, front of the thigh, back of the thigh, chest, upper back, forehead, lower back, abdomen, forearm, hand, and foot; iButtons) core temperature (rectal thermistor), indices of shivering (surface mechanomyography using triaxial accelerometers at 3 anatomic sites (chest, upper back, and thigh) and the bedside shivering assessment scale), brachial artery blood pressure (brachial artery auscultation), and heart rate (3-lead ECG). Thermal perceptions will be assessed using Likert scales for thermal discomfort and thermal sensation. Participants will be shirtless (sports bra for women) and will wear shorts throughout the study. After 20 minutes of quiet resting in the supine position, 5 minutes of baseline measurements will be taken, thermal perceptions and an infrared thermography image of the supraclavicular fossa will be obtained as an indicator of brown adipose tissue activation (6), and a whole blood sample will be obtained.

Participants will then ingest mirabegron (100 mg, 150 mg, or 200 mg) or placebo. Thirty minutes after mirabegron or placebo has been ingested, participants will enter the whole-body indirect calorimeter; the internal temperature of the calorimeter will be set to 20 degrees C (68 degrees F). This will elicit a mild cold stress over the 6 hours of observation in the whole-body indirect calorimeter. The whole-body indirect calorimeter provides an accurate and continuous measure of thermogenesis (in the form of energy expenditure) that can be used over long periods of time in a stable environment. The whole-body calorimeter is designed to perform human studies that are up to 48 hours in duration and contains a private washroom. While in the whole-body indirect calorimeter, participants will be instructed to be sedentary and will be allowed to watch television or read. In order to stay consistent between study visits, participants will be instructed to spend the same amount of time watching television or reading for each study visit. Every 30 minutes, we will record skin temperatures, core temperature, blood pressure, and heart rate. Thermogenesis will be determined by having participants lie supine for 30 minutes and resting energy expenditure will be calculated using the abbreviated Weir formula every hour (5). Thermal perceptions and an infrared thermography image will also be obtained at these time points. At approximately the midpoint of the 6-hour measurement period, participants will have the opportunity to use the restroom and an additional urine sample will be collected at this time. At the end of the 6-hour measurement period, participants will exit the whole-body indirect calorimeter and final measurements of skin temperature, core temperature, heart rate, and blood pressure will be obtained as well as a final infrared thermography image and a whole blood sample. After the experiment is complete, participants will be given a mylar blanket to re-warm themselves prior to leaving the laboratory.

Participants will be asked to return to the laboratory after 10-14 days to repeat the experiment until all 4 dosing conditions (placebo, 100 mg, 150 mg, and 200 mg of mirabegron) have been tested. Each participant's order of assignment to placebo or drug dosing over the 4 study visits will be randomly assigned.