Intermittent Cold Exposure and Brown Adipose Tissue Hyperplasia

Twelve healthy adults aged 18-40 years old (6 female, 6 male) will: * Attend baseline visits in both warm and cold conditions for blood sampling, measurement of energy expenditure, skin and core temperature, muscle activity. This will be followed by dynamic total-body 18F-FDG PET/CT scan during cold exposure. * Undergo biopsies of BAT (supraclavicular), skeletal muscle (vastus lateralis), and subcutaneous WAT (periumbilical) under local anesthesia. * Complete 10 consecutive days of cold acclimation (2 hours per day) using water-perfused cooling blankets. * Return for follow-up assessments, including repeat blood sampling, energy expenditure, temperature measurements, PET/CT imaging, and tissue biopsies to evaluate post-acclimation changes.