The study investigates the prevalence and effects of metabolically active brown adipose tissue (BAT) in healthy adult humans, particularly focusing on the impact of cold exposure and adiposity. Using positron emission tomography (PET) combined with X-ray computed tomography (CT), researchers examined 56 healthy volunteers (31 male and 25 female) aged 23–65 years. The results show that under cold conditions (19°C), 17 out of 32 younger subjects (aged 23–35 years) and 2 out of 24 elderly subjects (aged 38–65 years) exhibited significant FDG uptake in the supraclavicular and paraspinal regions, which was absent in warm conditions (27°C). Histological examinations confirmed the presence of brown adipocytes in these regions. Cold-activated FDG uptake was higher in winter compared to summer and was inversely related to BMI, total fat, and visceral fat areas estimated from CT images. The findings suggest a high incidence of metabolically active BAT in adult humans, indicating its role in regulating body temperature and adiposity.The study investigates the prevalence and effects of metabolically active brown adipose tissue (BAT) in healthy adult humans, particularly focusing on the impact of cold exposure and adiposity. Using positron emission tomography (PET) combined with X-ray computed tomography (CT), researchers examined 56 healthy volunteers (31 male and 25 female) aged 23–65 years. The results show that under cold conditions (19°C), 17 out of 32 younger subjects (aged 23–35 years) and 2 out of 24 elderly subjects (aged 38–65 years) exhibited significant FDG uptake in the supraclavicular and paraspinal regions, which was absent in warm conditions (27°C). Histological examinations confirmed the presence of brown adipocytes in these regions. Cold-activated FDG uptake was higher in winter compared to summer and was inversely related to BMI, total fat, and visceral fat areas estimated from CT images. The findings suggest a high incidence of metabolically active BAT in adult humans, indicating its role in regulating body temperature and adiposity.