The Muscle Monitor: Early Skeletal Muscle Indicators of Insulin Resistance and Cardiometabolic Risk

The project is composed of 1) a screening visit to determine study eligibility, 2) a main test day in the lab, 3) 10 days of physical activity tracking and continuous glucose monitoring in free-living conditions and 4) a scan visit including a whole-body MRI scan and lower leg pQCT scan.

For the main lab visit the participants will arrive in the morning after an overnight fast. This visit includes measurements of anthropometrics, resting metabolic rate, resting heart rate + heart rate variability, arterial stiffness, intima media thickness, as well as blood pressure obtained in the supine position. In addition, a fasting blood sample will be obtained followed by a 2-h glucose tolerance test with concomitant questionnaires provided in writing on basic demographics, physical activity level, sleep, stress and mental health. Two thigh muscle biopsies and a subcutaneous adipose tissue fat sample from the abdominal region will be obtained, while maximal voluntary knee-extensor contraction torque and rate of force development will be measured. Lastly, cycling-based assessments of maximal fat oxidation rate, peak power and maximal oxygen uptake will be assessed using indirect calorimetry including capillary lactate samples.

The scan visit will consist of an MRI whole-body scan, soleus and gastrocnemius 1H-MRS and a pQCT bone scan of the lower limb at 4% and 66% of the total bone length.

The objective measurements of physical activity levels and glucose-handling capacity will be performed for 10 days in free-living conditions using feasibly worn sensors (continuous glucose monitor and thigh-worn accelerometer). During this time, a food dairy needs to be filled in during two week days and one weekend day to estimate the habitual food intake.

A standardized evening meal will be provided prior to the laboratory-based test day and a standardized lunch meal served during the testing day.

Inclusion criteria

• Sex: Males and females
• Age: 20-30 years
• BMI <35
• Healthy (no diagnosed chronic disease)

Exclusion criteria

• Chronic disease deemed to affect study outcomes
• Disease that increase haemorrhage risk
• Daily intake of medication that could confound study outcomes
• Regular smokers
• Active pregnancy
• Immobilization (inactivity due to injury or illness, e.g. cast or brace) for more than a week in the month prior to the study or for more than 4 weeks in the past 6 months prior to the study
• Very high structured physical exercise level (>10 h/week).
• Participants not willing to adhere to standardized meal prescriptions included in the study protocols will also be excluded (due to e.g. allergies or specific dietary preferences)